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MG MGB Technical - Overdrive doesn't engage?

The OD on my 72B has be operating without flaw since I purchased 2 years ago.

The OD worked fine last Friday. Saturday morning I head down the highway and when I pull lever back to engage, nothing happens.

I've tried wiggling the lever in different positions.

I've tried wiggling the shift lever in 4th gear to see if I can get safety switch (name ?) that is on top of transmission to engage.

It just stopped working. I've checked fluid level in transmission (20w50 Castrol) and it right on the mark.

The only thing I did in between Friday's drive when it worked and Saturday's drive when it didn't work, is that I oiled the ignition key slot as the key was getting stiff to turn. Since the key is below the OD switch lever on the steering column, I don't think that should have done anything.

comments/ideas?

Thanks.
R.W. Anderson

Are you getting power to it? Try a test meter/ lamp from the inhibitor switch and work backwards to the main switch, and it's supply.

M

Michael Beswick

Check the Oil Level in the Gear Box..
K Harris

He said he checked the oil level in the gearbox.

Yeah I would start by checking for power. If you're getting power, remove the solenoid valve assembly and check to make sure its all OK and not blocked. Apply power to it while its out and see if it clicks. If it does, its working. Then try cleaning the filter under the large rectangular plate. After that if it doesnt work, your oil pump may be stuck down and there's no hydraulic pressure building up. You might try replacing the oil in the gearbox with ATF oil to remedy the problem.
Ross Kelly

RW-
A nonfunctioning LH overdrive unit can be caused by either mechanical, hydraulic, or, most commonly, electrical problems. Sometimes it is a matter of nothing more than insufficient oil in the gearbox. Pre-1975 model transmissions have a dipstick that can be accessed by lifting the carpet on the transmission tunnel and removing a rubber access plug. The best way to add oil to these transmissions is to use some cheap plastic tubing with a funnel inserted into one end. Feed the tubing into the tiny little hole in the transmission, and then slowly pour oil through the funnel. An alternate method is to use an old-fashioned oil squirt can. Remember, the gearbox does not hold all that much oil (6 U.S. pints without Overdrive, 7 U.S. pints with Overdrive), so if you have a leaky transmission, check the oil level often. An inadequate oil level will interfere with the function of the sliding synchronizer hubs, causing the transmission to shift slowly. Post-1974 models have a side filler plug on the transmission, which is located at the proper level for a full transmission.

Unfortunately, a broken annular clutch makes no noise at all. These were only bonded on the later blue label overdrive units, instead of being riveted. The overdrive should always be disassembled and carefully inspected before installation onto the transmission. Although the clutch friction material is designed to operate in an oil-soaked environment, 26+ years of saturation has most likely taken its toll. In the case of late-model LH overdrives wherein the clutch friction material is merely bonded to the sliding member, it is not unusual to find the entire clutch material separated from the sliding member and floating intact inside the overdrive. Symptoms of this type of deterioration are slipping or “free wheeling” in overdrive. These symptoms can also occur in direct drive. Normally, the friction lining of the sliding member is usually more deteriorated that that of the annulus. This is caused by insufficient oil pressure in the overdrive circuit, causing the clutch to slip, thus overstressing the friction material. Of course, this results in a more rapid deterioration than in the case of the more positive engagement of the spring-driven direct drive. It may also be due to the type of oil that is used. Transmissions lubricated by 90W hypoid gear oil always show a more advanced state of deterioration of the friction material than those that have been lubricated by 20W/50 engine oil. Once the clutch friction material disintegrates, it is rapidly turned to muck by the moving parts within the overdrive. Because the transmission and overdrive both share the same oil, this muck eventually finds its way throughout the transmission and overdrive, jamming gear synchro mechanisms, blocking oil passages, fouling the oil pump of the overdrive, preventing the seating of the low-pressure and non-return valves, and generally wreaking all sorts of havoc. I cringe when I think of what this pulverized friction material can do to bearings.

However, the problem is much more likely to be hydraulic or electrical. Let’s tackle the hydraulics first. Drain both the overdrive and the gearbox. Clean the sump cover and the area around it so that dirt and grit do not get inside when you remove it. Overdrive units are very sensitive to dirt. Unscrew the sump cover securing screws and then remove the sump cover on the sump filter screen. Clean all metallic particles from the two magnets fitted inside the sump cover, then clean the sump cover and filter screen with carburettor cleaner. If needed, a new gasket and filter screen can be obtained from Moss Motors (Part # 466-360).

Now you are ready to tackle the relief valve. It is located in the top left hand corner. Remove the relief valve plug and sealing washer. Withdraw the relief valve assembly. In the following order, remove the filter screen, spacer tube, low-pressure valve assembly, and relief valve spring. Keep them together in that order and in their original orientation. Remove the relief valve plunger. Examine the relief valve plunger and seat for pitting, scoring, and excessive wear. Replace any worn or damaged parts. Examine the relief valve body O-rings for signs of deterioration and replace if necessary (Moss Motors Part # 290-930 &290-925). Check the relief valve spring for signs of collapse or weakening. Its free length should be 3 cm. If the spring is fouled by dirt or weak, the pump will not generate the 400 psi of pressure necessary to operate the overdrive. Reassembly is the reverse of the above order. Make sure the relief valve is installed correctly. I once put that piece in upside down and it took me hours to figure out why the overdrive would refuse to engage.

Now you are ready to check the Solenoid valve. Unscrew the four screws securing the solenoid cover (name plate) and then remove both the cover and its gasket. Remove both the solenoid and the operating valve assembly by carefully pulling on the solenoid lead. Withdraw the solenoid rod and the operating valve assembly from the solenoid housing. Press the solenoid coil and the base cap from the housing. Remove the operating valve plunger and ball by shaking from the solenoid rod. Examine the valve ball and its seat for pitting and scoring. Replace all damaged or suspicious parts. The ball may be reseated by lightly tapping it onto the seat using a wooden dowel rod as a drift punch. Inspect the O-ring seals (Moss Motors Part # 290-935, 290-940, & 290-9450) for signs of deterioration and replace if necessary. Reassembly is the reverse of the above order.

Finally, you are ready to tackle the pump and the non-return valve. Remove the O-Ring (Moss Motors Part # 462-620) and then unscrew the pump retaining plug. It is the one with two holes in its face. Remove the Non-return valve spring and ball. Remove the pump body, the pump plunger spring, and plunger. Taking care not to damage the bore of the pump body, use a suitable drift to separate the non-return valve seat from the pump body. Examine the O-ring seals (Moss Motors Part # 290-915) for signs of deterioration and replace if necessary. Examine the non-return valve ball and its seat for pitting and scoring. Replace all damaged or suspicious parts. The ball may be reseated by lightly tapping it onto the seat with a wooden dowel rod. Carefully reinstall the non-return valve seat into the pump body. Insert the pump into the casing, ensuring that the flat side of the plunger is towards the rear of the Overdrive unit. Reassembly is the reverse of the above order.

If these procedures do not get the overdrive up and functioning, then you will need to tackle the electrical possibilities:

Overdrive transmissions are equipped with an inhibitor switch operated by a pin driven by a pad on the side of the gear selector mechanism in order to prevent the overdrive unit from being activated while in the first/second gear plane or while in the reverse gear plane. The only way to get a fourth-only gearbox to operate in third is to change this mechanism, which will require removal of the gearbox. If you engage reverse gear with the overdrive engaged, then you will likely do damage to the overdrive unit. If after doing so your overdrive still works normally when going forward with the overdrive unit both engaged and disengaged, and is OK in reverse and on the overrun, then you have been very lucky. There is a one-way clutch consisting of rollers in a tapered housing. Run this the wrong way, i.e., when in reverse gear, and the rollers will be wedged into the taper, either jamming them both together permanently or, at the least, distorting them. Disconnect the overdrive by unplugging the yellow/red wire from the gearbox loom from where it joins the main loom by the fuse box until you get the inhibitor switch fixed.

This, of course, brings up the basic issue of how can you test to find out if the inhibitor switch is functioning? The switch is located on the top of the unit and impossible to get at with the gearbox in place in the car. There are two basic approaches, the first mechanical, and the second electrical.

Change from Overdrive third gear into second gear, note the engine speed, and then switch the manual switch off. If the engine speed increases, then the overdrive was engaged in second gear (which should not have happened) and hence would probably also be engaged if you were to shift into reverse gear as this would be prevented by a functioning inhibitor switch.

If you want to do a more rigorous test on the inhibitor switch, disconnect the yellow/red in the gearbox loom from where it joins the yellow in the main loom by the fuse box and connect a test-lamp or voltmeter that is switched to its twelve volts scale from the yellow/red to the purple on the fuse box. By moving the gear lever back and forth across the gate from the third/fourth gear plane to the first/second gear plane and to the reverse gear planes you should see the test lamp glow or the voltmeter register twelve volts in the third/fourth gear plane but not in the first/second gear or reverse gear planes. Wiggle it back and forth and every time that you take it out of the ¾ plane the test lamp must stop glowing or the voltmeter must register 0 Volts. If it tends to keep glowing even a bit or register a few volts, even only one time in 20, the inhibitor switch is sticking and should be replaced in order to protect the overdrive unit.

You can pull the solenoid out without draining the oil. However, it will drip, so place a pan underneath. In order to bench test, just apply twelve volts to it. The plunger should center in the coil. Make sure the little ball is reinstalled where it belongs.

Of course, it is always possible that you will have to completely disassemble the Overdrive unit in order to troubleshoot it. To disassemble the Overdrive Unit, remove the filter screen cover and the filter screen. Remove the relief valve plug and then remove the relief valve. Remove the pump plug and remove the pump, taking care not to lose the ball bearing). Unfasten the bolts securing the ID plate and remove the solenoid, taking care not to lose the ball bearing. Unfasten the nuts that hold the casing together, then separate the halves. Remove the four nuts that hold the piston operating bars, withdraw the sliding member, and then refit the bars and the nuts in order to retain the springs and the spacers. Using needle nosed pliers, pull out the operating pistons. Remove both the speedometer drive bearing and the speedometer pinion gear. Clamp the drive flange and remove its retaining nut. Withdraw the flange and the rear oil seal.

Examine the teeth inside of the annulus for excessive wear. Inspect the one-way clutch for correct operation. Examine the planet gear teeth for excessive wear and check its bearings for smooth operation. Likewise, examine the sun gear teeth for excessive wear. Check the sliding member bearing for smooth operation. Inspect the cone clutch for burning, loose rivets and wear. Inspect the bronze bushing in the front casing (or thrust washer) for wear or missing parts. Inspect both the relief valve and the pump for wear or heavy scoring. Check the ball valve seats for correct seating. Examine the front casing for loose circlips. Place the pump one-way valve spring in its recess in the pump retaining screw and ensure that it projects 1/16” to 1/8” above the surface. If not, gently stretch the spring by running a thin screwdriver up the coils sideways. Finally, ensure there are no sharp edges on the pump retaining screw that might catch on the spring - if necessary, place it in a drill and use a fine file to lightly chamfer it.

In order to reassemble the Overdrive unit, clean all of the parts thoroughly, then lightly oil the bearings and the gear assemblies. Replace the O-rings on the operating pistons, lightly oil them, and refit them into the front casing. Refit the sliding member with the cutouts on its bars facing forward. Fit a new rear oil seal, and then lightly oil the seal surface. Install the drive flange and torque its retaining nut to 55-60 Ft-lbs. Align the marks on the planet gear and insert it into the annulus gear. Apply a thin coating of red Loctite to the mating surfaces and then loosely assemble the front and rear casings. In order to ensure correct alignment, align the internal splines and slide the unit temporarily onto the gearbox mainshaft (third motion shaft). Tighten the nuts that clamp the two halves together and then remove the unit from the mainshaft (third motion shaft), taking care not to rotate drive flange from this point onward. Replace the O-rings on the relief valve assembly and refit it into the casing. Replace the O-rings on the pump assembly and refit it into the casing, noting that the flattened side of the pump-operating shaft faces rearwards against the casing. Install the new filter screen and the filter screen cover. Be aware that the positions of both the Overdrive filter screen and its sump can be reversed. Paying attention to the internal layout should give you the proper guidance as to their proper orientation. Replace the O-rings on the solenoid assembly and refit it into the casing. Replace the O-ring on the speedometer drive bearing, and the refit the speedometer pinion and bearing into the casing.

Place a new gasket on gearbox adapter housing. Refit the circlip, locating ball, and pump drive cam to the gearbox mainshaft (third motion shaft). Rotate the mainshaft (third motion shaft) so that the cam lobe is uppermost, and then carefully slide the Overdrive unit onto the mainshaft (third motion shaft). If it fails to reach the studs, then the internal splines are not aligned. In this case, remove the Overdrive unit and, while using two wrenches to lift the operating bars, use a long screwdriver to rotate the annular clutch until the splines are aligned. If they fail to mate within ¼” to 3/8” of movement, then the pump-operating roller is caught on the cam. Should this prove to be the case, use a long, thin screwdriver to depress the pump plunger. Refit the retaining nuts, noting that some must be fitted with the Overdrive unit slightly raised from the adapter housing. Place a new gasket onto the gearlever extension housing and then refit the housing while locating the plastic bushing into its hole in the selector arm.
Steve S.

With a sudden failure like this I would vote for electrical. The od solenoid takes a few amps and it possible for a contact in the way to go high impedance rather than open, measuring the volts appearing on its terminl when operate it should tell you this.
Stan Best

The best electrical test to do is one of the few occasions when you should check for current rather than voltage. Even if you measure 12v right at the solenoid you don't know if the solenoid winding is good or not. You could measure the resistance but that is at a very low current, the standard current may be making it go open-circuit. Likewise the lockout switch, these can appear to be high-resistance when measured with a testmeter or appear to have continuity when measuring open-circuit voltage, the only real test is with its design voltage and current i.e. with 12v and the solenoid as a load.

Locate the connector in the engine bay where the yellow from the main harness joins the yellow or yellow/red in the gearbox harness. Part this and insert an ammeter, set to 1 amp. Turn on the ignition and manual switch and select an overdrive gear. You should see 800mA drawn, and that indicates there is full circuit continuity, and so the problem is mechanical or hydraulic, see below.

If you see no current drawn, then check you have 12v on the yellow coming from the main harness (if it isn't check both sides of the manual switch on the dashboard) and if that is there check the resistance to ground of the yellow/red or yellow in the gearbox harness. You should see 15 ohms, if open circuit you will need to check both sides of the lock-out switch on the gearbox. This is tricky to get to, you will need to remove the centre console and tunnel carpet, remove the access panel on top of the tunnel, and maybe even undo the bolts fastening the rear cross-member to the chassis rails and lower the tail of the gearbox on a jack.

If you see the correct current, push the connectors back together again testing for 12v both sides and try driving it, as it could just have been a bad connection in that connector which parting it and reconencting it may have cleared.

If not, then even with the correct current the plunger could be sticking, so the first step is to remove the solenoid (watch out for the little ball-bearing on top of the plunger), push the plunger to the bottom of the solenoid, power it, and check the plunger is pulled to the middle of the solenoid. If that is OK it looks like a hydraulic problem.
Paul Hunt

WOW! That's some detailed response.... I'll hire an electrical engineer and physics major to decifer it all, or call a farmer to get it all done.

Thanks.
R.W Anderson

O.K. Here is my update on the OD problem.

I have OD again, and in keeping with one of my earliest lessons in mechanics - "if it worked yesterday and it didn't make a big noise or something fell off since then, it isn't anything big".

Well it turned out to be the wire that comes down along the left side of the OD unit and goes into the OD unit. Its a cloth covered plastic coated wire with a male/female black plastic connector near the top of the OD unit and another similar connector down nearer the point of entry into the OD unit.

The wire must have been down to a strand or two of wire at the lower connector as that is where I found the broken wire.

Making a new wire for the lower connector wasn't a problem, but in the process the male end to the upper
connector came out. Now that took awhile to fix as I needed a neighbor with smaller arms and hands to get up over the transimission to hold that upper connector in place while inserting the end back in.

Why there are two connectors in such a short distance is odd, as one would rarely see if the upper connector is loose or breaking.

But the better news is that my shifting into OD works much better than it used to now that there is a solid wire with solid connections. Up until the wire breaking the few strands of thin wire that were in place were making a connection, but not a good one.

Now when I shift into OD, it almost lunges on shift if is so solid.

I used to push the clutch in when going into OD, but some have implied this isn't necessary.

So thanks for all the assistance, I've printed out Steve's extensive guidance from above for my records, in case next time it is something bigger.

However, now I wonder how many other wires are like this that one cannot see as they are inside connectors. The wire to my points was like this at one time, just a few strands of wire left holding things together.

Thanks again.


R.W Anderson

Hi,

I have just the opposite problem. My MGB (1974 1/2) is always in overdrive now and I can't do anything to get it out of overdrive.

I do not mind driving it that way, but it is annoying because I cannot park the car where I would have to back it up - yes, I am familiar with the requirement to never, never put the transmission into reverse with such a condition.

I would think that the OD switch on the steering wheel column is bad, because I cannot switch it out of OD, but I do not know for sure. Based on the article herein, it seems that the only way to get the OD to turn off is to disconnect the red and yellow wire near the fuse box. Unfortunately, the wiring on my car is in such poor condition (along with a lot of wired splices in the wiring) that I cannot tell which wire is the right one to disconnect, since the colors are so faded and stained. Is the wire one that goes through one of the one-wire disconnects and changes to a different color on the other side of the connector? I found one of those.

Is there easy alternative way to protect my transmission from an errant shift into reverse?

What is required to change the lockout switch? I called several car suppliers about the OD cutout switch on the gear box, British Victoria, etc, but the sales people I talked with had no idea what I was talking about, nor could they find it in the parts book they had. Is this switch still available?

What do you recommend?

Thanks,

Jon
Jon

Jon - it can't just be the manual switch as the lockout switch on the gearbox would still disable OD in reverse and non-OD forward gears. You should be able to tell if the lockout switch is working by changing up from a non-OD gear to an OD gear at low speed, the revs should drop with the new gear, then drop a bit more as OD cuts in. If you have OD in all forward gears and can't turn it off manually either, then either you have two problems i.e. the manual switch *is* faulty, and the lockout switch is too or has been bypassed by a PO. Alternatively maybe OD isn't engaging at all, how would you know?

There should only be three wires in the gearbox harness - green and green/brown for the reversing lights, and yellow or yellow/red for the OD. The green/brown will be connected to the rear harness so that should be easy to find and discount, the other two will go into the main harness. Of these two the green should show 12v when the ignition is on, and the yellow should only show 12v when the ignition *and* overdrive manual switch is on. If both show 12v with the overdrive manual switch off (i.e. because the OD manual switch *is* faulty) then pull the green circuit fuse out and that should remove the 12v from the green wire, making the remaining wire that carries 12v the OD wire.

Don't buy parts until you are sure the existing ones are faulty, it could just be a wiring fault or a PO problem. Test for voltage on both sides of the relevant switches, with the output wire disconnected in each case, and if the output terminal on the switch *still* shows 12v as the switch is operated and release the switch is indeed faulty, but both switches are more likely to fail open-circuit than closed, the lockout switch being more robust than the manual switch.

To get at the lockout switch you will probably need to unbolt the rear crossmember from the chassis rails and lower the gearbox tail on a jack, as well as remove the tunnel cubby, carpet and small access panel.
Paul Hunt

Jon, Several years ago I wrote an article for the AMGBA showing how to bypass the lockout switch using a NAPA cutout relay wired into the reverse lighting circuit. When the backup lights are engaged the relay cuts the power to the OD preventing engagement in reverse. RAY
RAY

Why would you want to? The reverse light switch is more likely to fail open-circuit than the overdrive lockout switch is to fail closed.
Paul Hunt

Oh, and adding more components and wiring is adding more to the chances of something failing. The lockout switch is fail-safe, the reverse switch, relay and associated wiring isn't. Probably why the factory didn't do that in the first place and same themselves a few pence.
Paul Hunt

After 15 years of use I have experienced no problems whatsoever. It allows for OD use in all forward gears and protects the unit from engaging in reverse. If it is assembled and wired with care the system will last as long, if not longer than, other equitment on the car. I have recieved many requests over the years for the particulars involved in the components and layout of this system. It is far from complicated and uses off the shelf parts. RAY
RAY

I dont think its a good thing to have overdrive in first. When I was trying to sort out my overdrive and it was stuck on in all gears, the annular clutch in the overdrive unit would slip if I tried to pull away with any vigour. It would wear out pretty fast if that was happening all the time.
Ross Kelly

Overdrive stuck on is more likey to be a hydraulic or mechanical problem IMHO. Maybe one of the fluid paths is gummed up so the pressure wont release? I would start with the usual suspects (after checking that the voltage to the solenoid is folowing the O/D and inhibitor switches ) , filter and solenoid and if stil no joy maybe the detergent effect of ATF will help but if not then it may need to be pulled and rebuilt.
Stan Best

Ray - you would have to install 200,000 and wait 40 years to be able to claim it is as reliable as the factory arangement ;o)

You can confirm it it is permanently engaged for *electrical* reasons by simply driving along in any gear, lifting off the throttle, and switch the ignition on and off. If it is stuck in and it is electrical then the revs will switch up and down while the speed remains relatively constant. May cause explosions in the exhaust.
Paul Hunt

Paul, The factory designed the overdrive units to be used in conjunction with specific gear ratios and torque loadings. My posting implies that the OD unit can be used in this fashion if the driver uses caution in the lower ranges. With the system that I built, I can hear the relay disengage when shifting into reverse. This will interupt the power going to the OD solonoid if I inadvertantly left the OD switch in the on position. The factory realized that a lot of drivers might abuse the system as I have arranged it and locked out OD in the lower gears as a precaution. RAY
RAY

It's not that they thought that drivers might 'abuse' the system, driving at full throttle in any gear isn't abuse, they knew that the type of OD installed wouldn't take the torque loading at full throttle in 1st and 2nd. The factory lockout switch is their primarily to prevent *destruction* of the OD in reverse, which isn't abuse either.

On the factory V8 they retrospectively removed OD from 3rd gear as in use that was causing problems with the much higher torque of the V8 even though it uses higher hydraulic pressures. Incidentally this is nothing to do with why OD was removed from late model North American cars.

You will never convince me that your additional components and wiring, especially as it is dependant on the wiring skills of individual installers, is better than the factory switch.
Paul Hunt

This thread was discussed between 06/07/2008 and 24/07/2008

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