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MG MGB Technical - MGB Overdrive

I am a novice with regards to the workings of my MG.

My overdrive has stopped working, are there any checks I can do myself before taking it to a garage and being faced with a huge bill?

Any help would be gratefully appreciated.
Stuart

Stuart,

The first checks to carry out are:
1. Check level of oil in the gearbox.
2. With ignition on and in top gear, have someone operate the overdrive switch while you have your head close to the solenoid. Listen for it to click.
3. Apply 12 Volts directly to the solenoid.
Listen for it to clock.
If the answer to 2 is no, and to 3 is yes, you probably have a faulty overdrive switch, or inhibitor switch.
If the answer to both 2 and 3 is no, the solenoid is faulty.
If the answer to 2 and 3 is yes the fault is internal.

Mick
M F Anderson

What year is your MGB? Is the O/D switch on the gear knob? If so, the wiring from the switch can become chafed right through with the movements made by the lever. Undo the four screws around the gear lever gaiter and lift it up so you can inspect the wiring.

Mike
Mike Howlett

I have a 1979 B with the later OD. The transmission was rebuilt last year. Prior to the rebuild, the OD worked perfectly. The OD unit was not rebuilt, but simply removed and replaced. Since then, the OD has not worked at all.

I'll check the items listed above. Does anyone have any other ideas--could it have been improperly installed back on to the transmission? Any thoughts would be greatly appreciated.

Thanks in advance.

Charlie
Charlie Tregidgo

Charlie,

There is a switch up near the gearshift on top of the trans that looks like the reverse light switch. It makes sure that the OD only actuates in third and fourth. If the wires are disconnected, the switch is broken or not screwed in enough, the OD won't work.

Mick's diagnostics above are a good place to start. If you hear the click, you know that the electrical system is working. If not, try disconnecting the yellow wire near the fuse box. With the car in third or fourth, touch one probe of a continuity tester to the yellow/red wire that runs to the trans and the other probe to ground. If you get a complete circuit, the switch is working. If not, check the switch.

The switch is tough to get at while the trans is in the car. I'd recommend disconnecting the wires to the switch from underneath the car and touching them together while the key is on and the car is in third or fourth. If you get the click from the solenoid, the switch is the problem.

Warning!! Make sure car is secure on jack stands and no one is near the ignition key while you are underneath!!! You don't want someone to accidently engage the starter with the car in gear and you underneath.

You may prefer to come in from the top by removing the console, and the cover plate around the shifter. You can access the wires to the switch with a long hemostat or needle nose pliers.


Good luck!
Steve
Steve Lipofsky

I had my OD go out suddenly last year. It worked great, shifting on and off normally until I made a 10 minute stop then it quit on me. Turns out the push-on connector on one of the transmission-mounted switches fell off. I'd have to look at a diagram to see which one it was, but after tearing everything apart I found I could actually just touch the switch with my fingers from under the car and push the connector back on (after I squashed it together to tighten it up more).

The first two easiest checks is to (as has been rightly said) listen to the solenoid to see if it's working (the transmission should be 4th gear with the ignition ON while moving the switch on/off). Also check the transmission oil level - the OD/transmission share a common oil supply and low levels do cause headaches with the OD. Some transmissions have a side-fill hole and have no dipstick, some have a dipstick located behind the radio console inside the car - if you can't get a grip on the dipstick, feel for it from under the car and push up on it.

The other solutions are right on too - check the other switches and wires. If everything else seems fine check the voltage and operation of the solenoid itself, if it has +12v going to it and doesn't move then you've found your problem.

If the problem is obviously not electrical (IE: the solenoid is operating properly) then the last option you have before tearing the OD off the car because of mechnical problems is to do the following:

* Drain the transmission oil - the OD and transmission are interconnected and use a common supply of oil, most of which will drain with the transmission drain-plug, leaving some in the OD sump.

* Pull the OD sump cover off, drain any additional oil, and check the filters/magnets for debris. Mechanical damage should be pretty obvious here but if you don't have any metal in the filters either clean them or install new filters (since the filters and gasket are one piece it's probably easier to just replace the whole thing)

* If you didn't find any obvious metal in the OD sump pull the solenoid assembly out of the OD being very careful not to pull apart the wires on the solenoid! Check the machined parts in the valve assembly for damage, heavy scoring, obvious damage, etc. If it passes your inspection replace the O-rings on the solenoid plunger AND the "ball" that sits on top of the plunger (a small dab of grease on the ball will hold it in place while you're reinstalling the solenoid).

All these components need to be in good condition to provide a tight positive switching oil-valve for the OD, torn or worn O-rings or badly scored parts can cause pressurized oil to leak around the valve and cause OD slippage or failure.

* If you go through all this be sure to top up with good quality 20w50 when you're done - Castrol GTX 20w50 is commonly used. The later model MG manuals specified 90w gear oil as an option but it seems to be frowned upon and hypoid oil can cause excellerated wear on some of the parts in earlier transmissions.

good luck!
Scott K

The chances are you *won't* hear a click when applying power to the solenoid as when powered it moves into the centre i.e. it does not hit an end stop and when power is disconnected it will stay there, only returning under oil pressure when the overdrive is rotating.

If you have the manual switch on the dash or column then find the bullet connector where the yellow in the main loom joins the yellow/red in the gearbox loom at the back of the right-hand inner wing. With the ignition off and the manual switch off poke a test-lamp in there connected to the brown at the fusebox. By putting the gear lever into an overdrive gear the test-lamp should glow if you have electrical continuity through the gearbox loom, gearbox switch and solenoid. Now connect the test-lamp to ground instead of 12v, and with the manual switch and ignition both on it should glow with the gearbox both in and out of an overdrive gear.

For a car with the manual switch on the gear lever look for a white/brown going into the gearbox loom. Y
you should have 12v at that connector all the time the ignition is on and with the manual switch on and the gearbox in an overdrive gear. With the ignition off pull that connector apart and connect a test-lamp between the gearbox wire and brown at the fusebox. It should glow with both the manuals switch on and the gearbox in an overdrive gear, but not at any other time.

If all that is OK any problems are hydraulic or mechanical.
Paul Hunt

Thank you all for the comments and suggestions. I will check things out and let you know of the results.

Charlie
Charlie Tregidgo

Thanks for all your help on this. It turned out to be the wires under the gear stick, both of which had come loose.

Cheers

Stuart
Stuart

Sounds like the later version with the switch on the gear lever, in which case you could well have narrowly avoided getting a burnt-out loom. I highly recommend making up an in-line fuse (standard 17amp rated, 35amp blow is fine) with short wires with bullet connectors and inserting it where the white/brown in the main loom joins that in the gearbox loom.
Paul Hunt

I had this problem appear last year on my 78 rubbernose and followed all the steps outlined in most of the suggestions here.
It seems my own problem is mechanical/hydraulic as everything electrical has been ruled out including installing a brand new solenoid which was tested on a bench.
Incidentally, the old solenoid also proved to be fine on the bench so I've got myself a spare one now.
Short on funds I haven't had the unit removed for a mechanical inspection or any further repair, so for the last year I've just made do without the overdrive and been a little slower on the freeways.

Just on the suggestion Paul Hunt made, I wish I had checked here when I started diagnosing my own fault. Your steps involving the leads inside the gearbox loom would have been a lot easier than the complicated way I went about it. I might not have bought a new solenoid just to rule out anything I had missed.

Cheers,
David
David

This thread was discussed between 02/06/2003 and 07/06/2003

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