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MG MGB Technical - overdrive problem

71 roadster, Had intermittent problems with the overdrive in the past, its been working fine for quite a while. its starting to play up again. Now its continually, when switched on rapidly jumping in and out of overdrive a couple of times a second. I fear it could do damage to other parts of the transmission. Thanks
Trevor Harvey

Sounds like an electrical problem, either a marginal gearbox switch (quite common) or a poor connection elsewhere. However going in and out a couple of times a second is quite a bit faster than I would expect for either an electrical or a hydraulic problem.

First check the oil level.

Second, if you have an ammeter - ideally an analogue one - connect that at the manual switch on the dash i.e. take one of the wires off the switch, connect to one of the meter leads, and connect the other lead back to the switch. Watch the meter when it starts jumping in an out. You should normally see a steady 1 amp or so when running, so if that starts jumping about in time with the OD then you do have an electrical problem.

If that is steady, then unfortunately it's problem inside the OD itself. The next thing would be to connect a high-pressure gauge to the OD and watch what happens to the pressure, although that really needs the car to be raised and run at a standstill with OD engaged as it's not really feasible to drive with the gauge connected. Consequently that needs the problem to occur while you are watching.
paulh4

Much easier first option is just to pull the gear lever towards yo when it does that - if it stops jumping then the inhibitor switch needs adjusting.
Chris at Octarine Services

Thanks Paul and Chris. Whats the best way to adjust the inhibitor switch. I thought this was giving problems in the past but making better conections to the overdrive switch on the dash seemed to improve it.
Trevor Harvey

From my forum at octarine-services.co.uk/forum

Put a jack under the removable gearbox crossmember and remove the 4 bolts holding it, lower the crossmember. Take off the radio console / armrest and the tunnel carpet, undo the screws holding the top cover of the tunnel round the gear lever (4 synchro) or the big tunnel cover (3 synchro). You will see the switch on the left hand side at the front of the gear lever turret. Pull off the 2 spade connectors and using a long stout screwdriver (4 synchro), tap the side of the switch to loosen it (anti clockwise) - it has a very coarse thread and will unscrew easily. Normally there will be 2 washers under the switch - test the switch for correct operation and if OK refit with just one washer. Refit the crossmember and test operation before refitting the tunnel cover.
Chris at Octarine Services

Agree with Chris. The switch has a tendency to move and just a little movement could put it on the edge of going in and out. It is a PITA to get to the switch, but if all your electrical connections on the engage/disengage switch and the connection to the solenoid are tight, then bite the bullet and check the transmission switch.
Bruce Cunha

Yes, if pulling the gear lever towards you stops it happening then go for the switch. But if it doesn't then do the electrical test before anything else (other than oil level).

Another test is when it is NOT jumping in and out, see if you can reproduce the "rapidly jumping in and out of overdrive a couple of times a second" by using the manual switch. If you can't i.e. it is taking longer to manually switch in and out then when the problem occurs, then I doubt the problem is electrical. But if you can, then again do the current check. It's possible bad connections are only just activating the solenoid, which could possibly be accounting for the rapid engagement/disengagement.
paulh4

Hydraulic pressure gauge can be used while driving, it just needs a long enough hose to bring it up to by the door mirror. Much more informative than when on the hoist as the system is under load and the oil can get riht up to temperature. For example, I recently had a problem which didn't occur until about 10 miles had been done, then a gradual residual pressure started building up, and just kept increasing.
Paul Walbran

Did you use a right-angle fitting? I was thinking that a straight fitting as on the gauge would reduce ground clearance, perhaps dangerously so.
paulh4

Yes, a right angle fitting and a hose of snug length to the 1/4 light. It is a little clumsy getting in and out of the car, but worth it.
Paul Walbran

This thread was discussed between 15/11/2017 and 21/11/2017

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