How do I disengage my overdrive, which actually down shifts, on my 1990 Volvo so it doesn't keep going on intermittently? I've already changed the overdrive relay and checked all wiring. It only happens occassionally but is frustrating when it downshifts and stays there on the freeway. El

I suspect you could have a problem with the actual contacts in the fuse box. Make sure the OD relay is well seated and when this happens, give it a wiggle and see if normal function returns. Drive for a couple days with the ashtray and storage bin/ cig lighter removed to get good access to the relay. I bet you find that it's loose/ intermittent contacts there.
Mine did that (88 740GLE).
Rob Bareiss, New London, CT 86 244, 87 244, 88 744

Overdrive can be disengaged by pushing the small button on the side of the shifter. When the overdrive is disengaged, the amber "up arrow" dash indicator should be illuminated.

Have you ensured that the transmission fluid level is correct?

Not sure about the '90, but I believe my tranny "defaults" to non-overdrive when the relay is removed.

I imagine Volvo didn't change too much in the line of wiring up of the relays between 86 and 90, but you never know.

I don't recommend this "fix" without some electrical scematic knowlege, but it seems to me you would be able to "override" the relay effectively by jumping out the OD relay socket board in the correct locations. This will fool the circuitry into thinking the relay is being powered up, making the magnetic field required to throw the contacts of the relay to "make" in the OD direction.

What you want to do is locate the LOAD side of the relay and figure out what terminals are used when the OD is on. You don't care about anything on the coil circuit.

I have removed the plastic cover of my relay before and put the darn thing back in to watch what it does. Once you figure out what way the contacts make the connections in OD, you can remove the relay and trace out the load side to find the stabs of the relay that correspond to the slots in the board.

This should work, if done properly. You know... I really wish I had thought of this when mine was acting up before.

Now whoever buys my car is stuck with a new relay (installed)and 2 spare relays. Go figure