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MG MGB Technical - Another Overdrive Question

Yet another overdrive question.

The overdrive on my 69 BGT engages fine for the first 5-10 minutes of driving. Once the engine has warmed up, it cuts out and does not re-engage.

I have replaced the o-rings and cleaned the filter. The gearbox is full with clean oil. The pump was recently inspected as well. The solenoid is fine.
I also replaced the small seal where the solenoid is located. I have also replaced the rear gearbox seal.

The have never experienced any problems in reverse.

My feeling is that my overdrive is losing pressure when warm. It sometime engages and disengages before cutting out entirely.

Could this simply be that the unit is worn out?

Is there a possible electrical problem that I may be overlooking?



64 MGB
69 BGT
67 912
Jonathan Seal

I had a similar problem with the overdrive on my 66 Mark-1 B. I removed a fiber washer that was under the inhibitor switch and the overdrive worked fine.
Tony Shoviak

I had similar problem, actually almost identical. I ended up replacing almost everything except the actual overdrive unit. Still no good.

I started with draining and replacing the oil, but when that didn't make any difference I went through all the other things you mentioned. Eventually I gave up because I didn't want to buy a new overdrive unit.

About 6 months later I did another complete oil change and I did the gearbox again (not that I thought it needed it, but I had a new oil pump I wanted to use). Suddenly my overdrive is working again and it hasn't had a problem since. In fact I'm amazed how much easier it kicks into overdrive now. It was never this good.

Beats me how replacing the oil the second time fixed it. Maybe it's just a coincidence. Maybe it has something to do with the order I did things. If you did your oil first, maybe try doing it again and cross your fingers. Miracles can happen. George Bush got re-elected remember. :)

I had a similar problem with a Jaguar 140. I would drive it for 30-40 miles and the OD would slowly fade out. I pulled it and everything was okay. Re-assembled it and it did exactly the same thing. I ended up using 30 W oil in place of 20w-50 .The OD has worked perfect ever since.
conrad sanders


I have to agree with you regarding 'miracles'. I have owned this car for about 8 years and have noticed that the car does sometime cure itself for reasons I cannot explain.

I just changed the transmission oil this week. Maybe I just need to drive around some more. In the worst case scenario, I will just have to live with a four speed for a while.

Other than a worn out overdrive (clutch). I have been thinking that it might have something to do with the inhibitor switch washers or the solenoid ball.

You could write an entire book on O/D problems and fault diagnosis.

If the overdrive is dead, I have been thinking about replacing it with a Nissan 5 speed.

Jonathan Seal

Hi all. I might have something applicable! One time, I filled my MGB OD gearbox with a detergent oil. Of course, it foamed too much and the overdrive did not work. I changed it again (to straight 30wt non-detergent); and it still didn't work. When I checked it, though - it was still foamy inside! I changed oil again, and it "kind-of" worked; and on the third change, it was back to working correctly.

I think that there are nooks and crannies that can retain oil, even during a change - so it's plausible that a several changes might be necessary to flush all the old out.

At least I was able to recycle the barely-used oil . . .

Good luck!
Philip S Jones

Try adding a pint of Dexron 3 ATF to the gearbox oil - it is highly detergent and will clean out any sticky deposits.

Also it is worth taking out the pump retaining plug under the rectangular cover and teasing the one way valve spring a bit longer so that it stands just proud of its hole in the plug - these lose tension over time and the pump loses pressure.
Chris at Octarine Services

Before doing anything else you should always confirm you have electrical continuity and current flow when overdrive is selected but *not* operating, you would feel very silly if you went to all sorets of mechanical trouble only to find it was a loose conenction or worn switch as Tony found. This is one of the very few occasions when an ammeter is useful for diagnostics, part the appropriate connector - depends on year but on a 69 with dash switch on the back of the switch is best, or with a column switch find where the yellow comes out of the main harness by the fusebox and goes into the gearbox harness. You should see about 800mA flowing when the solenoid is powered. If you see that, then by all means investigate the fluid and mechanicals. If not then it is electrical amd should be easier to resolve.
Paul Hunt

must use ONLY 30W mineral oil in the gearbox. Anything else will lead to eventual problems. Cannot use anything with additives that cause foaming or anything synthetic that will not keep the oil viscosity thick enough hot to operate the OD correctly.

"must use ONLY 30W mineral oil in the gearbox"

Too picky. It's supposed to be engine oil in the 4-cylinder (of whatever grade is correct for your local climate), and gear oil in the V8, it's just not that critical. As Cris says you can add ATF to it to act as a cleaning agent. I've read of a number of people using *all* ATF for a while to cure a problem. I've also read of a couple of people saying ATF cured their problem, so they went back to engine oil and the problem came back, so they stick to ATF. In that case there is a problem with the OD that the use of ATF is masking, and they may eventually get a more serious problem.
Paul Hunt

Regarding the inhibitor switch. Have you tried holding the gearstick to the side when in 3rd and 4th gear? If the switch is almost out of adjustment (due to wear on the switch plunger or the shaft that operates it) then you may get overdrive by wiggling the stick. I cannot remember which way you have to hold it, but it is (obviously) hard left or hard right. I made a temporary fix by taking out the stick, turning it through 180 degrees and replacing it. Lasted months like that before I had the switch replaced.

You can also check the switch with a 12V bodge wire straight to the OD. Just make sure you don't put the car into reverse with power on!


On my V8 if I push the lever to the left OD drops out, but that is OD on 4th only so it may be different for OD on 3rd and 4th.
Paul Hunt


I went for a drive this weekend. I moved the shifter side to side when I was in fourth but there was no change.

A few years back, the overdrive was slow to engage and would sometimes disengage on deceleration. Based on information I received on this website, I replaced the gasket behind the solenoid cover. It made a big difference at that time. It was good for a few years and then I got the problem stated above.

Do you think maybe the problem is around the solenoid area? Do you think there might be a relation between the old problem and the new problem. This is why I thought it might have something to do with the solenoid ball.
Jonathan Seal

You need to check the electrical continuity as I described on 31st October before doing anything else. Until you know whether it is electrical or mechanical you won't know how to deal with it.
Paul Hunt

Regarding transmission oil, I've always used Castrol 20-50 in several MGB O/D transmissions, with no problems. When I first got the car, back in 1986, the O/D was slow to engage, so my mechanic recommended changing the oil, adding a pint of Marvel Mystery Oil. It worked. In this case, I had no idea what oil the PO had in the car.

Allen Bachelder

I thought it was REALLY important NOT to use gear oil in the transmission, because its high in sulphates, and dissolves the phosphur-bronze bushes?

D Ainsworth

" REALLY important NOT to use gear oil in the transmission"

Quite correct...gear oil will kill an overdrive quickly...too viscous in addition to the chemical issues. 30W mineral oil with no additives is the way to go.

Like I say, gear oil is the factory recommended lubricant for V8 boxes. Whatever gear oil may or may not do to *some* gearboxes, it doesn't harm the V8 box which has the same basic internals as the 4-cylinder. And in any case 80/90 weight gear oil has much the same viscosity as 30 weight engine oil.

People keep trotting out this 'gear oil kills gearboxes' business but it simply isn't the case.
Paul Hunt

This thread was discussed between 29/10/2008 and 05/11/2008

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