Volvo Repair: 1987 volvo 760 turbo-overdrive, volvo 700, volvo 760

Question
When I Purchased the car(1987 volvo 760 turbo) from its previous owner he told me not to use overdrive, which i felt was something i could live with considering the deal i got for a really well maintained 1987 volvo.($700.00)  The idle would dip once in a while sometimes causing the car to stall. A mechanic changed the spark plugs and said he adjusted the timing and it now runs a little rough but never stalls- he did say he really didnt know the true problem and trial and error would be expensive. Sometimes when i start the car now the OD light comes on and when pressing the button to disengage OD it doesnt disable, the problem is in OD it doesnt seem to go into 3rd gear i think? the RPM's run at about 3k (50mph). anyway these are the  questions i have.  Can OD be disconnected? and if so is it a big job/costly? Will the car still run ? can a novice like myself do it?  Thank you any help you offer is appreciated.

Answer
I would first try replacing the overdrive relay this is the most common problem.


the overdrive relay is located on the fuse panel behind the ashtray.remove the ashtray and then remove trim around lighter socket then you will see 2 screws remove the screws and the storage area will come out the relay is white and about 1x1x3 inch.it will be towards the passenger side of the panel.