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MG MGB Technical - `74 B Overdrive goes on by itself

When starting, overdrive engages and disengages in normal manor. After driving 5 minutes overdrive engages with out using switch and does not disengage until car is parked and cooled down. Also, in reverse, will not back up, feels like clutch is slipping. After extended driving, car feels like it goes into overdrive in 1st gear. I gather this malfunction has to do with pressure build up in the transmission, if so how do I solve the problem?
w d miers

Worrying! Reversing like that could well destroy the one-way clutch inside the OD. If it definitely isn't electrical (the only way to prove that is to disconnect the OD solenoid from the gearbox switch) then it sounds like the low pressure relief valve is blocked, allowing sufficient pressure to develop, or the solenoid and ball doesn't have enough clearance to the ball seat. Drain the oil, remove the sump, clean the filter (not it may tear on removal and need replacing), remove the hex plug on the bottom above which are all the low and high pressure valve components, remove the solenoid being careful to catch the ball, and thoroughly clean everything. Check the solenoid plunger operates smoothly. You could also run ATF in it for a while, which others have said acts as a cleaning agent. Personally I would then put the proper 20W/50 oil back, but some have said theirs only works with ATF. Obviously faulty then, but it beats removal and overhaul.
Paul Hunt 2010

This thread was discussed on 06/07/2010

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