I have found a 240 1986 for sale with the four gears set up plus overdive which is electrically engaged with a switch on top of the shift lever. The original switch is missing. I tested it on the road with a temporary switch triggered by a poassenger. The overdive will engage when the car is in fourth gear and curent made to go momentarily through the switch, however if the switch is again opened temporarily to let the curent pass the overdrive does not disengage. If the gear box is put in neutral and again put in the forth gear position,the overdrive remains engaged; if the gearbox is put in third gear and then in fourth gear the overdrive is disengaged.This make for a cumbersome downshifting procedure.

I would like to know if this problem is likely to be cured by replacing or fosterizing the overdrive relay (such would be good news) but if there is a problem with the overdrive unit proper , how hard and expensive would it be to fix it or to find and install a replacement unit for a do it yourself amateur

Thanks in advance for your advice