[v8volvo]

Those relays are common for failures. That might be the only issue.

One time I was on a road trip from Washington State to California in the western US, and the OD relay failed in my 760 along the way. I had a "good used" one onboard in the car and put that in, then it turned out to be bad also. Had to finish the trip using a pair of makeshift jumper wires sticking out of the dashboard, having my passenger manually connect/disconnect them when we needed overdrive! New relay fixed it when we got home.

The relay does a lot of work over the course of the car's life, changing in and out of OD multiple times every time the car is driven, so they do wear out.

Test the wires and 4th gear switch though. Also make sure the single ground wire that attaches to the main OD solenoid on the bottom of the transmission is connected, tight, and in good condition.

Check the wires at the base of the shifter too, they get moved repeatedly and like to strip/break. Many people give up on having the switch or button in the shift knob, and change the system to a switch on the dashboard where the wires are not subjected to constant motion every time the driver moves the gear lever.