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MG MGB Technical - Overdrive Troubleshooting

Guys (and Girls) - Help!

Having trouble pinning down the problem with the overdrive on my 1981 MGB GT LE. I took the gaiter and gasket of the gear lever to check the connections and they all look fine. With the ignition on and the car in 4th, the circuit tests live (white striped wire). However, the overdrive still doesn't kick in. What are the connections to the gear knob shift? I have red and black leads into the switch - does red go to the left or right? I think it would just mean that off would be on and vice versa if I had the connections reversed, so it looks as though the switch is faulty. Anything else I can check?

Thanks,
Bob

P.S. Also I need to replace one of the alloys and the local parts dealer has a Triumph Stag alloy in stock. However it states 5.5 on the side as opposed to 5J which is stamped on my wheels. Will it make any difference? It is still a 14 inch rim and an identical design?

R S Price

Bob,

Doesn't matter which switch wire goes where.

If there is 12V at the solenoid with the gearlever in 3rd or 4th and the switch on then look elsewhere - low oil level will stop the OD working
Chris at Octarine Services

Bob,
The overdrive has a switch that screws into the gearbox that verifies you are in 3rd or 4th
If this switch is not working then the O/D will not engage.
Ron
R Garcia

Last time my O/D failed it was an open circuit solenoid , you could measure the volts on it but it did not move. You confirm this by disconnecting the volts and measuring resistance to ground , should be a few ohms .
S Best

Bob
The Stag wheel has a different cast centre.
Dvae
Dave Wellings

If you have never replaced the O-rings in your OD, they could be part of your problelm. Between the solenoid, pressure relief valve assembly, and pump assembly, there are six to eight O-rings that can be replaced with realitive ease with the transmission still in the car.

Look in one of the parts suppiers catalogs or the Bentley manual for an exploded diagram that will show you where they are.

Good luck.
glq Greg

Thanks for all the help! Hopefully I will get to the bottom of it.

Bob
R S Price

To check continuity of the OD circuit disconnect the yellow/red in the gearbox loom from the yellow (up to 77 or so) or white/brown (77 on) in the main loom by the fusebox and insert a test lamp between them. With the ignition on, the manual switch on, and the gearbox in an OD gear the lamp should light. It should be out with any other combination. 12v on the white/brown is only the output of the ignition relay, there is still the manual switch and the gearbox switch and several connectors to get through before it gets as far as the solenoid.
Paul Hunt

This thread was discussed between 24/04/2004 and 05/05/2004

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