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MG MGB Technical - overdrive problem

Hi I have a 1974 MGBGT the problem that I have is when I selected overdrive it is very intermittent when cold it works every time then fails to work.The gearbox oil has been changed and is on the full mark I used mineral 20-50 oil this problem was there before oil change.Help!
M Scull

As always check electrical continuity first. An ammeter in series with the manual switch will both show you what is happening, and allow the overdrive to work normally (as far as it is working at the moment anyway). A 1 amp range should be sufficient, the solenoid takes about 800mA at 12v i.e. ignition on engine stopped, closer to an amp when running.

If you don't have an ammeter a 12v bulb in series will glow when you have continuity, but may not pass enough current to allow the solenoid to operate. If the fault is anywhere but the solenoid itself this may show the problem occurring, but if it is the solenoid that is failing as it heats up it may not allow the solenoid to get hot enough to fail, and so lead you to think the problem is internal i.e. hydraulic or mechanical.

Did you clean the filters when you changed the oil?
PaulH Solihull

This thread was discussed on 17/03/2013

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