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MG MGB Technical - heater air flow

I have removed the heater motor out of my 79 MGB. I cleaned it up and tested it. It works good. Well at least I should say it spins good.

I have several questions before reinstalling and connecting the new wiring.

Should the fan turn clock wise or counter clockwise? The motor had 2 wires going to it a Hot and a Ground. I do not remember witch wire went where.

Where does the heater draw air from? Is it form the chrome vent right in front of the windshield? It seems that would make it draw cold air in over the heater core. I would think it would be better it it re-circulate the air form inside the cab.

I know it used to have a resistor in line (removed by PO) I assume this is for a second slower speed? Could a variable resistor on the dash take the place of this?

Thanks
Bob
Bob Fowler

Hi

Best test is to try both ways and see what gives the best airflow. The motor will happily ruin both ways so you won't damage it, but the fan blades have a "correct" way.

I've blocked the chrome grill opening and removed the interior vents and tubes behind them (vents replaced with an MG stereo blanking plate). This makes the air re-circulate and is warmer. You could just open the vents rather than remove them, but not being one for originality, I think the blanking plate looks neater and leads less dusting.

I think you would find a dash mounted resistor would need to dump too much power to be practical. The extra power used in the fast setting is just dissipated in the resistor when on slow. The stock resistor is a fairly robust item and is in the "cooling" air flow of the heater system.

Regards
Darren
D Lewis

and of course "ruin" shoudl read "run"!
D Lewis

Bob, the heater on my 69 has a black wire for ground and a green with a strip for hot, are you wires not color coded? You might just need to clean them off to check the colors.
John H

Thanks

D Your idea should good.
SO just to be clear on what you are suggesting. After blocking the chrome vent on the outside of the car I could remove the center vents and ducts and but keep the 2 holes they attach to under the dash open? Theses are the holes right above the bottom vent door correct?

John the wire where color coated but they are long enough to go in either spot. So I do not know witch one went where.

Also I am in the middle of rewiring the car SO I am upgrading the wires to a larger gage.

Thanks
Bob
Bob Fowler

You say a variable resistor is not the way to go. Ok your explanation sound right.

Can you tell me if I am correct in the way I would wire the switch and motor?

I supply 12v to the Heater fan switch.
From 1 terminal on the switch a wire would go directly to the motor.

From the other terminal on the switch a wire would go to the resistor and then form the resistor to the same location on the motor the other wire went to.

Since the 2 wires are completely separated at the switch they do not feed back on each other.

Correct?

Thanks
Bob
Bob Fowler

Hi Bob

Regarding the vent, it sounds like you understand what I was trying to say. I blocked the chrome grill (with black duct tape stuck onto the standard grill mesh and with the chrome surround put back on). The vents and tubes taken out leave the two openings straight into the "box" under the chrome grill. When the fan is running, air is pulled through these openings from inside of the car (instead of from outside through the chrome grill), pushed through the heater matrix, and then back into the car through the windscreen demister or footwells depending on setting.

Regarding the wiring layout, you might want to double check a wiring diagram in case MG settled on an alternative, but the way you describe is what I would do if designing it myself. (But bear in mind I am a physicist and not an automotive electrical engineer!).

If you've not come across it yet, I cannot recommed Paul Hunt's web site highly enough. It has a lot about everything - including wiring. (I think there are also some other sites out that are similarly comprehensive but I've most always found whatever I need at Paul's so haven't really looked elsewhere).

regards
Darren
D Lewis

Try the wires connected either way round (without a dropper resistor), it will be immediately obvious which is giving the best air flow. It won't hurt anything having it the other way round.

The wiring diagrams imply a 2-speed motor, and there was no external dropper resistor in a 1980 model I rewired, although I have seen others claim they have one. It is possible that the dropper resistor is situated in the airbox for cooling, which is how it is in my Toyota.

You are correct in your wiring assumptions, Bob, but it doesn't matter if the two circuits *are* connected together in the 'fast' position of the switch. Any variable resistor would need to be high power to prevent overheating (although if on the dash could contribute to cabin heating ...) and burning out, and I don't really see you would gain much. All the 2-speed does is quieten it down a bit if you don't need full demisting, for example.

By design the heater draws cold air in through the external chrome grille and passes it through the heater matrix. Whilst this should give adequate heat for most places it is possible to recirculate cabin heat which is said to improve things. For this you simply need to block the external grille and open the dash vents, the heater will then draw air from the cabin through the dash vents, heat it some more, and pass it back into the cabin through the demister or footwell vents as appropriate. Bear in mind that there are at least ten aspects of the heating and ventilation system that can contribute to poor heating. In a good system you should be able to get at least what I get which is air at 140F/60C out of the demister vents (slightly less out of the footwell vents) with an ambient of 50F/10C.

Paul Hunt 2

I'd be very reluctant to block the outside vent as this provides the fresh air intake to the vehicle without which it is difficult if not impossible to keep the glass mist free. Appreciate that it would mean colder air being drawn over the matrix but the heater should be more that capable of handling this provided that the pipework, valve and matrix are clear and the cooling thermostat is correct and not stuck open.
Iain MacKintosh

Hi

I can confirm a UK 77 roadster (at least my one!) has a dropper resistor in the heater box (on the inside of the top surface on the fan side of the matrix). You can easily see it if you take the fan motor out.

I must be misunderstanding what Paul is saying since I think you would nontheless need to keep the fast and slow circuits separate at the switch to stop the straight through wire for "fast" from bypassing the dropper resitor when on "slow"...

Darren
D Lewis

If you have a three-pole 2-speed motor like the wiper motor then you *must* isolate the fast and slow connections from each other at the switch. But if using a single-speed motor with a dropper resistor then on the fast speed it doesn't matter if the switch is also connecting power to the slow speed wire as well, as it would if you used a light switch as a heater switch for example. Of course, if the two were still conencted together when you were trying to use the slow speed, then you would only ever get the fast speed, but it wouldn't do any harm as such.

Iain - ah, to keep the glass clear you open a 1/4-light ... :o)
Paul Hunt 2

Paul, yes agreed and you have to do that all the time as well as taking in fresh air from the front vent.
Iain MacKintosh

If there's one thing my roadster doesn't suffer from it's a lack of fresh air getting into the cabin!
D Lewis

A couple of points here, Moss sells a magnetic sheet I believe which can be placed on the cowling under the air intake grille. You don't need the spring nuts to keep the grille on anyway so you simply lift it, place the magnet and replace it, so I've heard. They also have a nice stainless screen I think. But aside from the benefit Iain pointed out, drawing in fresh air has a couple of advantages. To start, if you have anywhere even remotely close to a sealed cabin it will actually help prevent cold air blowing in as the fan will create a very slight positive pressure. The advantage of this may equal the advantage of recirculating the air. It also will continually purge the cabin air, helpful if you have a well sealed hard top or are wary of exhaust fumes. Personally I've always felt the amount of heat from the core was plenty adequate but the airflow never seemed to be very good. I'll admit I'm used to american cars with their HVAC systems but the fan and squirrel cage just seems wimpy. I am quite certain that it would be pretty simple to find a compact and high powered modern motor and squirrel cage which would easily bolt into the stock housing but someone else will have to look for it because I simply grafted on a large american housing and three speed fan which were at hand. Low moves as much air as the old fan did and high is really something. Well there's no kill like overkill I always say. Anyway, with the magnet blocking the intake, popping open the fresh air vent behind the dash will allow recirculation and all of the other controls continue to work as they should.

Jim
Jim Blackwood

See what happens when MG starts putting heaters in the cars. Never had these silly questions with the "T" series cars did we.
The wires to the heater motor should be colored...ie. a green and a black. Try plugging the green into the green and the black into the black, or however yours is matched up.
Or studt the fan and decide which way it blows best out the circumference of the fan wheel; mark the direction of rotation in red paint, then try the wires with the fan still out of the heater box and reinstall when you are sure which wireing setup gets you this correct rotation direction. Forget the 2 speed nonsense. What you need is a fast and faster, not a slower speed. Be thankful for small whiffs of heat on your ankle.
Wayne Hardy

The magnetic cover simply sticks to the grille rather than going under it. I'm cheap, so I got a piece of scrap from a sign shop and cut my own. As Jim mentioned, you have to open the fresh air door so the fan has a place to draw in the cabin air (at least on earlier cars without face-level vent. Don't know about later ones). That said, I rarely use it.....

He and Iain have a point about purging the cabin air. The air inside the car will by much more humid due to your breath. Blowing in fresh air from outside will help keep the humidity (and thus the fogging) down.

HTH!

Rob Edwards

Jim
I have thought of using a different motor also. I most likely will not do it right now since I do not have one on hand and the stick one works. But I was wondering on high does it blow to much air. I would think that blowing to much and to fast would not let the air heat up sufficiently.

Bob
Bob Fowler

To be honest Bob, I haven't used it enough in cold weather to say for sure but at a guess I'd say it might have the ability to reduce the temp of the air blowing on my feet from hot to warm, not necessarily a bad thing I think. But bear in mind when I said overkill, I really meant extreme overkill. The blower came out of an International Harvester Travelall, roughly equivalent to a Chevy Suburban, required a 1" offset on the front of the heater housing, and the scroll had to be cut down 3/4" to fit below the cowl lip. It looks good sitting in there and blows all the air I'd ever need, but it would be a better choice to find something that will fit right into the existing scroll. I'd guess the cage on this one is nearly 2" larger in diameter and an inch longer. Why ask why? I just did it.

Jim
Jim Blackwood

I was able to mount the motor and squirrel cage blower from a '87 Chevy Camaro in the housing on my midget. The opening for the cage had to be opened up a bit and the flange the motor mounts on had to be notched in a couple of places to clear the retaining clips. This blower wouldn't be optimum for all MG heaters as the vanes on the blower are designed for maximum effectiveness when rotating in a specific direction and it just happened to match the housing orientation on my car. I added the resistor package in the housing down stream from the blower and added a cooling duct outlet for the motor as was originally provided in the Camaro. I used the GM switch and high blower relay, but a secondary switch and relay could also be used along with the original MG switch. The cage is longer than the MG unit and fully fills the space in the heater box for maximum efficiency. I get great air flow from this unit. I plan on making a similar installation on the B heater unit I'm using in my current MGA project.
Bill Young

From the archives somewhere there is a note that using a relay at the heater for "direct" power from the battery will significantly increase airflow. If this isn't enough the blower assembly from a Fiero will fit into the B heater housing. Requires opening up the access hole a bit to get the larger diameter impeller in, and new mounting holes and captive nuts. Also, the motor uses cooling air from the heater housing just downstream of the impeller. Adding an equivalent intake is a simple mod.

Writeup I saw on this conversion was for an E Type jag, which uses a very similar blower assembly to the B. First time it was turned on it blew out piles of dirt and trash that had accumulated in the ducting.

I have the conversion essentially completed but the car isn't ready yet for installation. Powering up the fan assembly in a "hand held" mode it sure appears to move a lot more air than the original.

FWIW

Larry
Larry Hallanger

Whilst use of a relay can improve heater motor performance it is only compensating for bad connections in the admittedly long brown, white, green chain to the motor. Given the relatively low current draw of the motor a relay will make little difference if the connections in the fusebox etc. are good.

Increasing the blower performance will not do much if the problem is restricted coolant flow through the heater matrix, nor if the seals around the matrix have crumbled to dust so the air finds it easier to go round the matrix instead of through it. You can get uprated matrixes and blowers from a couple of sources in the UK, at least one person said he was very disappointed with the extra performance from the expensive conversion. 50/60C out of my standard system is more than enough to burn my feet in English winters.
Paul Hunt 2

This weekend I blocked off the outside vent and removed the center vents on the dash. I was worried about something getting sucked up into the holes I left open under the dash so I put a wire mesh in front of them. Now the air is sucked in from under the dash, channeled through the heater core and back out the defrost and floor vents.

I tested the direction of the fan by getting my hand wet, placing it over the defrost vents and having my some jump power to the fan. I found out that Clockwise puts out much more air.(Clockwise as looking at the back of the heater form the engine compartment)

In fact it puts out is a lot more that it ever did before.

Maybe before it was turning backwards or just was not getting enough power.

Thanks
Bob


Bob Fowler

If i were messing with the fans, or a rewire, I'd definitely install a relay for heater fan operation with much larger supply wire diameter, which will thyen amaze you with a significant fan output increase
vem myers

I am using a new harness form Avanced Auto- Wire. It does use much larger wire but is fed from a fuse, The fuse is fed form a power relay.

Thanks
Bob
Bob Fowler

make sure all the "edges" and gaps are sealed up. I had a variety of small (half mm or less) size gaps here and there. use duct tape or silicon is you like. i would say that doing this has quadrupled (at least!) the air flow at the windscreen.

An idea, The air intake on the MGB is large enought to easily fit an air control flap (like the one under the heater)Fresh air/recirculate.

Peter

This thread was discussed between 20/02/2006 and 03/03/2006

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