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MG MGB Technical - Overdrive clicks continuously

The car is a '67 GT with 3 synchro gearbox and Type D overdrive. The overdrive has recently stopped working. When overdrive is selected the solenoid can be heard to click rapidly both when driving and when standing in the garage with 4th gear engaged without the engine running. Any ideas?
Graham Gilmore

The D-type has a 2-stage solenoid - a low-resistance pull-in winding (0.7 ohms, draws 17 amps), then when the plunger has moved far enough a contact opens to disconnect that and a higher resistance winding (6 ohms, draws 2 amps) is left connected to hold the plunger operated and engage OD. Sounds like this secondary winding has gone open-circuit.
Paul Hunt

Thanks Paul, I guess that means a new solenoid is needed.
Graham Gilmore

I had the same problem as you about three years ago and Paul also educated me as he has done you - thanks Paul.
I took the solenoid out, carefully opened it up and found the higher resistance wire broken at its soldered joint to the points. You could not see the break, but by gently pushing the wire sideways, it moved away from the joint. I simply soldered a bridging wire across the two.
I believe that with time, resonance from the points causes fatigue in the wire and it breaks at the joint.

Richard.
RH Davidson

Have now fitted a new solenoid and the overdrive works OK now. Thanks for all the advice.
Graham Gilmore

This thread was discussed between 13/07/2009 and 30/07/2009

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