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