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MG MGB Technical - Removing Heater Control Knobs

Okay, I accept that I am a mechanical imbecile, but I just can't seem to be able to remove the lower heater control knob on my 1972 MGB GT. The reason I need to get the knob off is so I can disconnect the control cable and thereby remove the heater matrix. Once this is achieved I can properly clean the thing up as well as tarting up the surrounding engine bay area. By the way, I have read about inserting the sharp object (pin, wire etc.) into the appropriate hole but have had no success.
My patience is running out and I feel like racing for a heavy blunt instrument! Any advice would be most welcome.
Happy New Year,
Robin.
R Hewat

Robin
there is a 3mm dia hole in the underside of the knob where a dowel on a peice of spring steel engages, just push the dowel in with a 2mm drill whilst pulling the knob.
when the knob starts to move remove the drill, you may need to spray a little WD40 into the hole.

Regards Pete
PJ Thompson

Robin, they can be terribly stuck after being undisturbed on their spindles for years. I tried everything including repeated doses of WD 40, patiently returning to reapply and patiently wiggling some more over a period of several months. In the end I had to ruin my original knobs to get them off.
This shouldn't damage the control spindles, but the shame for me was that (as usual), the reproduction knobs I bought were visibly different to the originals: shiny black plastic, slightly larger diameter than the old ones, smudged lettering and just a friction fit onto the spindle; there was no locking pin in the new ones. So the new ones look wrong and even feel wrong. (They wobble a bit).
I hope that you can get your control knobs off, or if not, that the replacement ones you find are better than the ones I bought (from a UK retailer).
T Aczel

You don't need to remove the control knob to remove the heater core. The control cable can be disconnected at the heater. On the RHS of the tunnel there is the heat control flap. Unscrew the hinge and remove. This exposes the cable's connection to the heater. Undo the lock screw and the cable can be disconnected. Refitting is more of a challenge but can be done with a bit of patience and needle nose pliers.

Herb
Herb Adler

Presumably you have done one, both are exactly the same, so the second one is obviously stuck.

I replaced mine and got the correct items, so they are available. You can generally tell by the part number, from reputable suppliers anyway, as if it is a copy item they append a suffix letter like Z to the original part number, and these might be iffy.
PaulH Solihull

Robin-
Since you're going to pull the heater box out, you may find the following to be useful-

One of the tasks that most MGB owners eventually confront is the overhauling of the heater box. The heating system on the MGB may be an antiquated design, but it is adequate if kept in good operating condition. Unfortunately, reconditioning the system is a time-consuming and labor-intensive task, so they are almost always in a state of severe neglect. It is not unusual for the heater core to be so loaded with sediment that it barely functions, if at all. Reversing the flow within the closed system by switching the hoses in an attempt to purge the heater core is a very bad idea as that procedure would force the sediment trapped inside of the heater core into the rest of the system, there to do all kinds of mischief in the coolant passages of the engine, coolant pump, radiator, thermostat, and heater valve. However, you can backflush the heater core with the hose disconnected and hope for the best. Simply pouring a descaler into the system can be risky as it is often the "kiss of death" for an old, internally corroded heater core, resulting in leakage of the engine coolant into the cockpit. If you want to get its performance up to what its designers intended it to be, then you must pull the heater box out and do a rebuild. While you are going through this ordeal, keep telling yourself that the entire process is just part of the initiation ritual into the ranks of an elite group of MG owners who have "been there, done that". Consider it to be the MGB owner's equivalent of a quaint and colorful puberty rite.

Note that, when looking into the engine compartment from the front, there is the back of the motor on the left, a hose connected to the tap on the cylinder head low down near the middle, and another hose at top right connected via a metal pipe to a spur from the bottom hose. Logically, and for the most efficient coolant flow, the inlet should be higher than the outlet so natural convection (the temperature of the coolant reduces as the matrix transfers heat to the air) aids pumped flow, but in fact it is the reverse, i.e., heated coolant comes out of the tap on the cylinder head, into the lower port of the matrix, flows upwards through the matrix, and out of the top port to the bottom hose to be circulated through the engine again by the pump. This is done in order to eliminate the presence of air bubbles that would interfere with the performance of the heater box.

The back of the heater casing butts up against the front of the air-box at the base of the windscreen, and there is a large circular hole connecting the two with the motor impeller sitting in the hole behind a mesh screen (except earlier cars) in the rear of the heater box. Thus when travelling along, air pressure at the base of the mesh screen will force air into the heater box, and when the motor is running, the impeller forces air into the cockpit from the heater box.

Air is blown downwards through the matrix in order to pick up heat, and thence towards the air direction flap at the base of the unit that controls air distribution. A foam strip is wrapped round the ends and sides of the matrix preventing cold air from bypassing the matrix, which is one of the several causes of poor heater performance (good air flow, but cool), as is anything partially blocking the matrix (poor air flow).

Air Direction is controlled by one of the two knobs on the dashboard - the one that says “Screen”, “Interior”, and “Off”. This moves a control cable which operates an air direction flap at the bottom of the heater box inside of the car in order to direct the air to either the vents for the windscreen, the footwell vents, or block it off altogether. Note that opening the fresh air door under the dashboard and blocking the intake in front of the windshield (windscreen) will convert it into a recirculating system.

Getting at the heater control unit can be a fiddle. First you have to remove the control knob. On the shaft of the control knob you should see a 3mm hole or depression that looks as though it might contain a set screw (grub screw). However, it does not. Instead, it is a spring-loaded pin on the control shaft. Depressing it with a 2mm drill bit while pulling on the knob should allow the control knob to slide off, but some control knobs have been known to seize on the control shaft and require simultaneous levering from opposite edges of the control knob.

In order to disconnect the cable from the screen/demist control knob, first gain access to the back of the control knob, either by removing the dashboard mounted radio / radio blanking plate on early models, or the fresh air vents, or the console assembly on later cars. With the knob off, remove the securing nut and withdraw the control unit from the rear of the dashboard or the console.

With the control unit in your hand, you should see that the outer sheath of the control cable is clamped to a fixed arm by a curved bracket that is secured with a screw, and that the inner control cable attaches to the moving part with a trunnion that fits into a hole on the end of the lever. This trunnion is also used at the other end of both control cables, i.e., at both the air directional flap and at the heater valve. I have seen two types of control cable - a spiral-wound outer sheath with a solid wire inner control cable, and a later type with a plastic outer sheath with embedded longitudinal wires, again with a solid wire inner. It works on a push-pull principle, i.e., it pushes in one direction and pulls in the opposite direction. Thus it is not like the accelerator cable in which the pedal pulls the cable, but the cable is afterward returned under the action of springs on the carburetors. I suspect that the use of the later type of outer sheath came about because with the earlier spiral-wound outer, if the other end is stiff, then pushing on the wire just causes the spirals of the outer sheath to open up like a coil spring instead of moving the valve or the air direction flap. The later sheath does not do this, and thus you can get greater force on the inner control cable when pushing.

Note that there are two different control mechanisms installed into the dashboard for heat and for air direction, so should you happen to remove them both, remember to label them and to check the 'logic' of them before going to the effort of reconnecting the control cables and reinstalling them into the dashboard. As well as different mechanisms for air and heat there are also different ones for RHD and LHD, 4-cylinder and V8 models, and for various export markets - no less than 11 different types, plus 10 different knobs, and while the later models had knobs in common, there were 4 different dials that went behind them! The controls have two locating pegs that fit into holes on the dashboard, and each control can fit in one of two positions 1800 apart, so check to be sure that the legends on the control knobs align correctly with the datum pip in the dashboard above each knob as well. The heat control goes above (dashboard-mounted) or to the left (console-mounted) of the direction control.

Air to the footwells comes out of a large opening at the bottom of the heater box, on top of the tunnel, and then travels sideways to exit into the footwells, via plain moveable flaps on early cars and fixed cowls on later cars, both mounted on the sides of the transmission tunnel. Note that early cars had a door on the footwell vents, and if this is closed, it will block off the air no matter how the heat and direction controls are set. The later fixed cowls are better at directing heat down onto the feet, with the earlier moveable flaps a lot of the air will flow horizontally and not be directed downwards. However, it must be said that with the individual manually-closeable flaps you could close the passenger one to get more air to the driver’s footwell if you so desired. In both RHD and LHD cars there should be a lever in the left-hand footwell by the right knee of any occupant. It opens a large flap behind the console which opens the bottom of the heater box and lets cold air in directly from it. It is only effective when the car is moving, not when the fan is on, and even if the heater is going full-blast makes the cabin quite cold if it is either open or the seal is defective! The lever is said to click the flap into three positions, one of them being closed, but later models have an extra 'notch' on the lever quadrant giving a third, even wider-open position, albeit needing a bigger tug and a longer movement of the lever to get to it. Note that the 2nd open position has a larger notch than the others, which means it would take more of a pull on the handle to move it from that one to the final one. If this flap remains open or is leaking, it will then easily defeat the attempts of the heater to warm the cabin on a cold day.

Air for the demister vents exits from two holes on the lower back of the heater box, pointing towards the rear of the cockpit, various tubes and hoses then taking this up to the demister vents on the dashboard. These are all push-on mounted, and as such can be easily dislodged in two or three places each, which is why you may have demisting from one vent but not the other. There should be a large rubber block-type seal (BMC Part # BHH 389) glued to the lower back face of the heater box, which is where the demister tubes are pushed into. If this rubber block-type seal is crumbling away, the demister air will leak into the footwells instead of being directed onto the windscreen, and in severe cases the demister tubes may even keep falling out. Fortunately, this essential rubber block-type seal is still available (Moss Motors Part #363-135). Note that in November 1970 the screen vents were changed to include baffles that improved the distribution of air across the screen and hence improving demisting.

By removing the footwell vents and standing on your head in the footwell, you should be able to see the air direction flap moving as the control is turned. If not, then perhaps the control cable has become detached at one end. Refitting of the direction control cable has to be done at a particular time. If yours does not work, then maybe a DPO got the heater box back in forgetting to refit the control cable and could not face getting it out again. Another problem is that there should be felt strips glued to the air direction flap, and these can become detached, blocking the airways. If the air direction flap moves and the felt strips are in place, yet the air direction does not correspond with the markings of the knob, it could then require the relatively simple job of adjusting the control cable in the clamps on the dashboard control unit. Be aware that there were several sets of knobs and controls over the years. If yours works backwards, then a DPO may have fitted the wrong one.

Air is blown downwards through the matrix wherein it absorbs heat, and from thence towards the air distribution flap at the base of the unit. A foam strip is wrapped around the ends and sides of the matrix in order to prevent the cold air from bypassing the matrix, which is one of the several causes of poor heater performance (good air flow but cool), as is anything partially blocking the matrix (poor air flow).

The air direction flap is situated in the large opening at the bottom of the heater box, running across the car, pivoting on a horizontal axis. The ends of this flap can just be seen and felt through the footwell vents. When in the “Off” position, the top half is tilted towards the back of the car so that it covers the angled plates that serve to direct air into the ports for the demister, thus blocking them off, while and the bottom half of the air direction flap is tilted towards the front of the car, also blocking the footwell vent. When in the middle “Interior” position the flap is vertical. Both the interior and demister outlets are then uncovered, but because there is more resistance to flow upwards to the windscreen, the bulk of the air exits out of the footwell vents. On “Demist” the air direction flap is tilted so that it completely blocks off the footwell exit, and thus all of the air is directed upwards through the ports for the demister.

Effectiveness of both the “Off” and “Demist” positions is aided by felt strips that are glued onto the edges of the direction flap in order to both give obstruction to airflow and to prevent rattling. These felt strips can become unglued, which not only prevents complete closure from a missing strip, but the dislodged strip can prevent normal movement of the flap, as well as block airflow to both the ports for the demister and to the footwell. On a cold day the attendant leakage can actually defeat the best efforts of the heater.

The control cable that operates the air direction flap attaches to the right-hand end of the flap, and uses the same kind of trunnion for the inner and curved clamp on the outer as for the heater tap and dashboard controls. While it might just be possible to get to the trunnion on the flap lever through the right-hand footwell, the clamp for the outer seems to be concealed under the rubber block-type seal, and in any case access is extremely limited, especially by the pedals on RHD cars. There is really no alternative to pulling the control cable out with the heater box, and upon reinstallation attaching it to the heater box first and then threading the control end down through the space in the heater shelf and out the back through the firewall into the space behind the center console.

Removal of the heater box is not as difficult as some make it out to be. First, disconnect the battery, then disconnect the wires from the fan at their snap-connecters. The Original Equipment fan motor assembly fits through the hole in the lid and can be removed and replaced from outside the unit. Remove the three screws that secure the mounting plate of the heater motor, and then remove the heater motor along with its mounting plate. This will be necessary during the rebuilding of the unit, and will make the heater box lighter and thus easier to handle during removal from the car.

Now, drain the system and remove the hoses from the heater box. Next, depending on which you have, remove either the radio speaker box or the console, and then loosen the demister tube retaining clip screws. Disconnecting the demister hoses underneath the dashboard may require removal of a few other parts, such as the under-dashboard cover panels. Remove the screen demister duct tubing and the pipe elbows from the lower part of the heater box, and then remove the pipes from the rubber block-type seal into which they were mounted. They should slide right out.

Disconnect the two heater control cables. The one under the heater box is not accessible until the heater box has been lifted up from its shelf on the firewall. Be aware that you must first disconnect the back end of the control cable from the dashboard-mounted control units, otherwise you will then get a nasty kink in the front end that will have to be straightened out prior to reassembly.

Loosen each of the control knobs by locating the small hole in its neck, press inward its spring retaining ball, and then remove. Remove the nut which holds the heater air control assembly in place. Remove the heater air control assembly, and then disconnect the control cable from it. In order to disconnect the control cable from the screen / demist control assembly, you must first gain access to the back of the control knob, either by removing the dashboard-mounted radio / radio blanking plate on early models, or the fresh air vents, or the console assembly and the under-dashboard cover panels on later cars. Pull the control assembly out from behind the dashboard in order to allow access to the connection of the control cable. Undo the inner control cable, and then release the outer cable sheath of both control cables.

Finally, remove the screws that secure the heater box to the firewall (bulkhead), and then remove the screws that secure the heater box to the shelf. One particular screw below the heater is difficult to access, requiring a very short angle drive tool. In a pinch maybe a long-nose Vice Grip will loosen it, and if you are lucky, you might be able to unscrew it the rest of the way with your fingers.

Note that the control cable under the heater box is not accessible until the heater box is lifted. Be warned that if you do not first disconnect the end of the control cable from the control assembly, you will get a nasty kink in its front end. Gently withdraw the heater box from the engine bay, pulling the attached control cable through the bulkhead behind the heater.

Lever the heater box upwards while pulling it forward at the same time, in order to clear the rear lip along the top of the engine compartment. If there are any brake pipes located along the inside of this lip, take great care to avoid damaging them as you pull the unit away. It will require a considerable amount of force in order to free the unit, but once the old seals break away, it should pull free.

Pull the control cable out with the box. Make sure that you thread the directional control cable through behind the dashboard - this has two purposes: firstly, so it won't snag on any wires as you pull the heater out and thus result in a electrical problems and, secondly, since it will come out with the heater, it is easier for the control cable to thread through the bodywork without having to "U-turn" itself from where the control unit is located. Be aware that there is a rubber gasket beneath the flange of the heater box and that it will often be difficult to remove. Normally it is just a matter of wiggling, tilting the assembly forward from the top, and just gradually working it out.

Once you have the heater box out of the car, remove the four cover retaining clips and peer inside. Should the air intake not be protected by a screen sandwiched in between the heater box and the circular air intake gasket, you should not be surprised to find a considerable quantity of leaves and other debris clogging the fins of the heater core, thus preventing adequate air circulation. Take the heater core to a local radiator shop and have it boiled out to remove the usual accumulation of crud from inside of it and have them pressure-test it. If it leaks, a new one is available from Victoria British. To do a proper rebuild on the heater box, you will need the following parts:

Heater box Base Gasket (Moss Motors Part# 282-750)
Vent Door Seal (Moss Motors Part# 363-145)
Heater Core Foam Seal (Victoria British Part# 9-9079)
Air Intake Seal (Victoria British Part# 12-5556)
Air Intake Screen (Victoria British Part# 12-085)
Heater Outlet Seal (Moss Motors Part# 363-135)

In addition, you will need thick strips of felt which you should be able to obtain from a fabric shop.

Should you still be dissatisfied with the performance of the heater, you might consider installing the uprated heater core available from the MG Owner's Club in the UK (MGOC Part # ARO1994HE). Included in the replacement matrix kit is a set of new foam seals and these must be glued in place with contact adhesive before reassembly. When fitting the base foam, note the cut out area for the flap control and the position of the hole for the control cable. Once the glue has dried, remember to make a new hole in the foam seal for the control cable to pass through. You may also wish to consider modifying the heater valve in the manner described by Bob Munchenhausen in his website article found at http://members.aol.com/bobmunch/index1.html .

If you wish to fit the MGOC upgraded heater blower motor and blower assembly, now is the time to do so, because the diameter of the uprated fan impeller is larger than the hole in the casing. Therefore, the fan motor has to be installed from outside of the casing, and then the impeller fitted on the inside of the casing. Note that this can only be done with the heater unit removed from the car.

When reassembling, pay special attention to the careful sealing of the heater box, so that airflow has to travel through the heater core instead of bypassing it. Note the thick high-density foam rubber pad which will attach to the back side of the heater box at the bottom. This is employed in order to seal the outlet side of the heater box against the bulkhead in the foot well. The rubber block-type seal also provides the mounting anchor point for the stub pipes that are used in order to connect the demister hoses. Run the damper control cable through the hole in the rubber pad, attach the inner control cable to the damper arm, anchor the outer jacket to the heater box, and then glue the rubber pad into place with silicone adhesive. Glue three thin foam rubber strips to the underside of the mounting flanges of the heater box. One other piece is a thicker foam rubber strip that is to be glued onto the back of the heater box just above the flanges in order to seal the back side of the heater box against the vertical bulkhead.

If you choose to install an uprated impeller fan and motor (MGOC Part # 37H7913HE), note that the new impeller fan is larger in diameter than the Original Equipment item, and that it will not fit through the hole in the side of the heater box. You do not have to enlarge the hole. Place the fan inside of the heater box before installing the cover, and then install the motor from the opposite side, and tighten the set screw onto the motor shaft once it is together. This raises an interesting point. For future servicing, the heater motor cannot be removed without first removing the entire heater assembly from the car. The replacement motor came with long wires and a separate mounting plate which you have to screw to the motor. Presumably this is a universal motor, and the separate mounting plate allows it to be used in various applications. When the motor is in place, and the fan is pressed down against the shoulder on the motor shaft, and the cover is installed, I found that the fan was rubbing slightly on the cover. I was wondering if there should be another gasket between the motor mount plate and the heater box, but there was no gasket and no mention of any gasket with the parts. So a little creative sheet metal bending may be necessary in order to stop the rubbing.

Now you are ready to reinstall the heater box. Attach the air control cable to the heater box, then attach the block seal, air inlet seal, and perimeter seal to the heater with rubber contact cement. Pass an 8" length of small metal tube (probably a length of steel fuel line) through the rubber grommet in the firewall and up through the rectangular hole where the heater resides. This tube can act as a guide for the control cable so that it will not get kinked as you lower the heater into place. You might consider taping a shop rag over the rear edge of the bonnet aperture so the heater box would not scratch the paint as you lower it into place. Using Wire Pull Lube (either Rotanium or Yellow 77 should work fine), smear it all over the rubber block-type seal. This material will evaporate over time and, unlike grease, will not attack the rubber. Pass the control cable through the metal tube, and then slowly lower the heater box into position. With the wire lube, a steady, firm pressure is all that it should take to get it into place. If possible, have a friend pull the control cable and tube from the inside of the car as you lower the heater into place. Once the unit is in place, just pull the metal tube off of the control cable.

Next is the unexpected tough chore, stuffing the heater assembly down into the hole in the shelf. The problem here is that thick rubber pad on the back of the heater box at the bottom. This pad is so resistant to compression as to make it very difficult to press the heater box back far enough to drop into the well. If you are fortunate, you will not kink the control cable, and it would still operate freely. Next, insert the stub pipes into that now-familiar block of rubber, reconnect the air elbows, demister vent tubes, and the control cables. Check that the dashboard controls actually work before final installation of the under-dashboard panels and anything else you may have dislocated earlier in the job. Cut the motor wires to length, strip the ends, solder on the two bullet connectors and plug them into the wiring harness.

Reinstall the fan with its motor, and be careful that you do not accidentally reverse the connections, otherwise the fan will run in a reverse direction.

Until 1977 or so (possibly earlier for North American specification cars) a single-speed fan motor was fitted, and after that came a dual-speed fan motor. The wiring diagrams seem to show a different motor being fitted with 'fast' and 'slow' windings but in fact it is a similar single-speed motor with a dropper resistor used in order to achieve the slower speed. The dropper resistor is located inside of the heater box so that it will benefit from the cooling flow of the fan. From the 3-pin connector the two colored wires go into the heater box and a single wire comes back out to one side of the motor, the black wire going directly from the 3-way connector to the other side of the motor. See Clausager pages 66 and 67 for both types.

Early cars (the Parts Catalogue indicates the Mk1 model, but Clausager indicates that it could have been later) may have had motors with wound stator and rotor the same as that of the previous MGA model (BMC Part # 17H 2288), but afterwards they had a motor (BMC Part # 37H 4707) with a permanent magnet stator. The effect of this is that whereas the early motor will always turn in the same direction regardless of polarity (like the starter motor), the later motor is polarity-sensitive. If the later motor is connected backwards (or the polarity of positive ground cars is changed and the heater fan isn't reversed as well), then the motor will run backwards which will result in a greatly reduced output but an increase in noise. This is not immediately obvious as air continues to flow in the normal direction. The early motor seems to have two white wires (black wires on an MGA) exiting from the same point on the motor connecting to a black and green / brown wire from the harness via bullet connectors. The later single-speed system has different colored wires exiting from opposite sides of the motor going to green / brown (before 1970) or green / yellow (1970-on) and black harness wires via bullet connectors. The two-speed motor (BMC Part # 37H 7913) has green / brown, green / yellow, and black wires with a 3-pin plug and socket, and thus cannot be reversed. The two colored wires go into the heater casing, and a third comes out going to one side of the motor, with the black wire going direct to the other side of the motor. The motor changed again in about November 1972 with the introduction of the V8, which needed a 'shorter' motor in order to clear the air box of the carburetor, but was fitted to all subsequent models. At this time I am unsure if the motor itself was shorter, or if the mounting flange was relocated further back, putting more of the motor inside of the heater casing and less outside. The motor changed again in September 1976 (BMC Part # 37H 6102), possibly being a more powerful motor to make the most of having the two-speed switching (half the speed of the original motor for the first speed being pretty useless, much like the dimmer rheostat for gauge lighting), for the remainder of production.

Note that from 1971 for the remainder of the production of Chrome Bumper models, the heater fan, as well as the wipers and the electric washers, were powered from the accessories position of the ignition switch via a white / green wire to an in-line fuse under the fusebox, and then via a green / pink wire. Bad connections in the solenoid – brown wire - ignition switch – white wire - fusebox - green wire - fan switch - green / brown wire or green / yellow wire - fan – black wire - ground circuit can contribute to slow running of the heater fan. In order to see how much power it is that you are losing, connect a voltmeter between the live wire of the fan motor and the brown wire at the fusebox, and then take a reading. One of the more common complaints about the MGB is the performance of its heater system is mediocre. This problem originates in the fact that the Original Equipment wires are of insufficient size to carry the necessary current to the fan motor. With the motor in operation and using the Original Equipment smaller gauge wires, Voltage falls from 12 Volts to 9.7 Volts at the fan motor and the current is only 2.6 Amps. Having done this, connect the voltmeter between the ground wire of the fan and a known good ground such as the engine, and then compare the reading with the previous one. In order to assess the effect of installing a relay, with the fan power on (on its fast setting if it is a two-speed fan motor), connect a heavy gauge wire between the brown wire at the fusebox and the live (not the ground!) wire at the fan, making sure that it is the fast speed wire for two-speed motors and not the slow-speed wire. Hooking the motor of the heater fan directly to the brown circuit at the fusebox and bypassing the Original Equipment smaller gauge wires with 12 gauge wires results in a Voltage drop of only one Volt and a higher current supply to the motor of 3.5 Amps. In order to remedy the problem of the slow running heater fan motor, install a relay into the heater fan motor circuit, and then tap the voltage from the purple (fused) side of the fusebox. The relay, along with 12 gauge wire, supplies the motor with the current that it needs, without the large voltage drop associated with the smaller gauge wires, and greatly increases the amount of air delivered by the heater fan.

With a properly-assembled system, you may often find that you hardly have to use the fan, the ram-effect at road speed being more than enough to provide for cabin heating. However, you do need it on a cold morning for clearing the fog that forms from your breath on the windscreen. It is rapidly cleared by just a few seconds of directing the air onto the windscreen and using the fan, even though there is not yet any heat in it. However, cracking the vent window (quarter-light) open just a bit will clear it quite quickly when underway and keep it clear. There are several areas which should be looked at if you are experiencing poor heater performance:
Correct operation of the thermostat.

Adjustment of the heat control valve.
Coolant flow through the heat control valve.
Coolant flow through the matrix (and the rest of the heater coolant circuit).
Airflow through the matrix.
Airflow bypassing the matrix.
Directional control of heated air.
Fan power supply and rotation.
The fresh air vents.
The cold air direction flap.

In its original mounting configuration, the heater valve of the B Series engine was secured with a pair of threaded studs, washers, lockwashers, and nuts. The studs and nuts were replaced by a simple special-length bolt (BMC Part # SH 604061) in order to both simplify and speed assembly. The tapped holes in the cylinder head are rather shallow, so should you be unfortunate enough lose the original bolts, you then will be forced to custom-fabricate them as they need to be cut to exactly the right length for both optimum thread engagement and without bottoming out. It is a bit difficult getting them to the correct length, and then you have special-length bolts, so if you lose or damage them, you will have to start afresh and make new ones. A better solution is to revert to the original use of studs and nuts, and easier too. The stud needs to be 1/4”-28 UNF, 7/8” in length for neatness and originality (but a 1” length stud will also work). It should screw in easily with fingers. If not, use a tap to clean out the threads. Double-nut the full thread stud in order to assist while screwing it in with a wrench (spanner) just tight enough to jam the thread at bottom so that it will stay put. These side studs can even be left in place during machine work for a valve job.

Usually the first you know about a clog in the drain tube for the air intake plenum is when the carpets get wet from the heater box filling up with water and reaching the level of the vents into the cabin. If you find evidence that water is seeping into the cockpit from under the dashboard, this means that the drain tube for the air intake plenum is clogged with debris. Left uncorrected, water will stand in the air intake plenum and the resultant rust can do serious damage.

If the car is on axle stands you could slide under there and see the drain tube hanging down beside the right front of the transmission. It should be hanging down just below the bottom of the car. On the end of it is a round bulb with a slit-like lip. In MG lore this is called "Tom's knob," supposedly after the engineer that designed it. Supposedly this "knob" was to prevent exhaust gases from working their way up the tube, into the air intake, and from there into the heater box, poisoning the air inside the cockpit. Unfortunately, the bulb can eventually get plugged up with debris, usually a compound of teeny bits of leaves and dust. This part is no longer available, so you're going to have to be careful with it.

Remove the cover and look in the corner. You will find a small hole that connects to the drain tube on the left side of the plenum chamber (when viewed from the front of the car). You will want to avoid damage to the paint, so gently push a doubled-over electrical wire into the hole and gently twist it in order to clear the hole out, then add water and gently push it down through the tube while twisting the wire. Get under the car, squeeze the knob so that the slit opens, and have a partner flush water under pressure directly down the hole in the plenum chamber. This is the easiest approach and sometimes doesn't work. If it does work, you will get a nasty bunch of crud on yourself.

If that method does not work, there is another that will: The drain nipple for the air intake plenum is located on the right hand side of the transmission tunnel, under the heater and just above the bellhousing. The drain tube is fitted to its outlet tube by a hose clamp that is accessible from the side of the gearbox tunnel down in the right hand footwell area. You might have to remove the center console heater control mounting or try looking behind the hot air outlet in the footwell. It is pretty hard to see and even harder to get your hand thru to refit. Be prepared for grazed knuckles. Remove the four screws that hold the flap to the tunnel. You then will see the tube that drains the air intake plenum. It has a clip holding the tube to the drain nipple. Be aware that this area has probably never been touched since the car was first assembled, the clip may be rusted, and that there has been at least one case where leaning on the clip to undo it caused the end of the spigot the hose attaches to, to break off. Undo that and reach under the car to pull out the tube. Now cleaning out the tube itself is easy. This is really the best way of cleaning the tube as sometimes there's a bend in the tube, thus it cannot be cleaned out using a wire.

I went through this joyous ritual with my second wife's car and resolved to never endure it again. I fabricated a vent screen using black stainless steel window screen and put it under the chromed intake grill. Nowadays you can buy one ready-made from Victoria British for $5.95 (Part# 12-017).
Stephen Strange

This thread was discussed between 29/12/2012 and 30/12/2012

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