Hello fellow DIY'ers,

I have a 1990 745 turbo which has been acting funny lately.

For no reason while in motion over 30 mph the over drive engages.

Now sometimes I can push the button and turn it off.

Most times it won't shut off until it wants to on it's own.

What should I look for to properly diagnose this issue?

Should we assume that the solenoid is bad?

Much thanks,
Greg

 

The default position for the OD is off until energized; then it engages. So, for some reason, the OD is getting energized. I would not first suspect the solenoid. Something is energizing the circuit. I'd look at the shifter button and the relay area first. Do you have an auto or manual?

 

Automatic, I wish it was standard.
So do I just dismantle the button on the selector?
Also where is the relay, under the glove box like on a 240?

Thanks swift. G-

 

You're in luck paisano! Beginning with the 7 series, Volvo centralized most all of the relays. They are behind the fuse tray which is, itself, behind the ashtray. Kind of difficult to see in their but this is the arrangement...

I wouldn't go ***** nilly disecting the o.d. button unless it's a hobby or something. Those auto shifter ***** sometimes don't go back together as simply as they disassemble. I'd try to test the wiring first. Honestly, the o.d. relay failing is your best bet. That's the most obvious place where it could happen; you have power to energize the circuit and a circuit that normally controls the electricity.

 

It's been happening infrequently now for a while.
But just this last week it came on and would not shut off.
Cool, I'll get a relay and see if that does it.

 

The relay may be OK. I have a similar problem with my 94 940. The overdrive relay is fairly large and heavy and I find that it has worn a bit loose in the connector, sometimes making contact, other times not. Before replaceing it, try wiggling this relay and push it down to see if this helps out.