A week ago, I heard a short very quiet noise, and the overdrive OFF light came on. I was on my home on a 17 mi drive, including highway and residential speeds. When I got home, I checked the fuses, and found #11 blown. This fuse carries rear defroster and "4th gear" on automatics. It's summer in Texas, so the rear defroster was not on, and hasn't been for some months. I replaced the fuse, and the light went off, and overdrive works again. I assume the little half-second noise was the sound of the fuse blowing.
Last night, the fuse blew again. No unusual driving.
Hi Pat: I suspect your problem is with the wire going to the "4th gear" (over drive?) in the transmission, look for break on the insulation of the wire, or the wire rubbing against the chassis. There is a product sold in the autoparts that is used to dress the wires, if you find a small nick in the wire you can put some electrical tape and then the plastic tubing over it, or, if it is a larger problem, you can consider replacing that section of the harness entirely.
Look under the car at the wire to the transmission, often the insulation will chafe on something and it'll short against the tranny or the body of the car and blow the fuse.
Before seeing your replies, I opened the shifter cage, but saw nothing amiss. Looked at the basic layout in my Bentley, and looked at Brickboard, before jacking the car and throwing my aging back underneath. The OD solenoid has a hard rubber/plastic-like fixture on top, holding the wiring lead securely into the solenoid. However, mine was badly cracked and separating, exposing the wiring and the internal electronics. Off to my favorite pick-n-pull for a cheap used replacement, since these items don't really wear out. During removal of the old solenoid to install the replacement, I got a better inspection of the wiring coming down from the shifter cage, and draping over the top of the transmission and leading to the wire coupling on the solenoid. This section had 3 inches of bare wire, some strands broken. I know Volvo was using "environment friendly" wiring with chalky insulation during the 80's, but read that it was discontinued by '88. Apparently not. I bet a buck that this was the problem, not the exposed top of the solenoid. By the way, most of the 240/740 transmissions at the junk yard had at least a little of the same problem on the rubber top of the solenoid, but none as bad as mine.
Repair completed, with electrical tape and 2 layers of the flexible plastic tubing that wraps around wiring that is exposed to heat, grime, or constant rubbing.
The wiring was much improved later on, but under the car is a very harsh environment, traces of transmission fluid will rot out insulation in no time, combined with the heat, weather, vibration and road debris I really can't figure out why they didn't protect it better.
I had the same thing happen. It was a bare wire just before the connector to the solenoid below the shifter. I repaired the bare wire but the overdrive is still off and now the relay in the center console is buzzing. the only way i could get the over drive to turn on is by running a jumper from the cigerette lighter to the white wire from the overdrive solenoid which I cut.
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