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MG MGB Technical - MGB Overdrive problem

1973 MGB the overdrive has been hesitant to come out when cold but otherwise has been fine. Was doing 65-70 on interstate with the card fully warmed-up and it jumped out of overdrive. I noticed smoke from the rear and pulled over. The solenoid plate is cracked and the solenoid is partially pushed out. Did something stick on the inside or was there too much pressure? Any help would be greatly appreciated!
T Lawrence

The pressure relief valve should have limited the pressure, if that is blocked then I can imagine that a lot more pressure could develop, which could press down on the solenoid plunger, and effectively try and push it out of the bottom. That could well have cracked the plate, and shorted the solenoid to the body of the OD. That could have caused the smoke as the OD circuit was unfused from the factory, although all the wiring back from the OD through the manual switch to the ignition switch, and from there down to the starter solenoid, may have been damaged. OTOH it may have been oil escaping from the broken solenoid you saw, and smoking from that. And the solenoid plate may have been damaged anyway, finally gave way releasing oil, so with that repaired the OD may well function as it should.

I'd replace the solenoid, remove and clean the relief valve, change the oil and clean the filters, and fuse the OD wiring - inserting an in-line at the manual switch is probably to most effective. After that it really needs a high-pressure gauge on the OD and testing to check that pressure is limited to 400-420psi with OD engaged.
PaulH Solihull

All the parts, that you will need to repair the overdrive, are available except the solenoid plate. This will have to be sourced from a breaker's yard or from someone who rebuilds overdrive units such as Chris Betson at Octarine Motors. RAY
rjm RAY

This thread was discussed between 29/01/2012 and 30/01/2012

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