Bizarre overdrive problem

I have a 1990 Volvo 740 Turbo with the B230FT engine and the Aisin Warner
AW71 automatic.
A few months ago, I replaced the overdrive relay because the transmission
would only shift into fouth when I thumped the side to the centre console,
inside which the relay is located.
Now, I am experiencing a different problem.
The car has been taking itself out of overdrive at random and no amount of
console thumping has any effect on it. The car will then shift back into
overdrive, sometimes after a few seconds, sometimes after several minutes.
I have discovered that if I switch off the headlights (at night) or switch
them on then off (during the day) the warning light on the dash will go off
and the car will shift into overdrive!
Yes, I know it sounds strange but the act of switching off the headlights
will make the transmission shift into overdrive! Just what the *&%$#@! is
going on with my car, anyway?

Frustrated Volvo Driver

 

Very simple turning off lights allows sufficient increase in system voltage to
over ride poor connection in OD circuit,somewhere other than in xmission
tunnel?

 

Ahh, now I've got it!
I crawled under the car and discovered the wire running from inside the car
to the overdrive solinoid, which is located on the side of the transmission.
There is a in-line plug which connects the wire coming from the centre
console to the wire running to the solenoid. I cleaned the conact inside
this plug and have had no problems since. Saved an expensive trip to the
dealer's.

Cheers,
A slightly less frustrated Volvo driver