940 volvo comes out of overdrive without touching button

1. Check the relay.
2. Check the solenoid and wire harness connecting each other. Usually wires become brittle and broke off.

depends where you buy it

check gearbox oil level remove gear lever switch check feed to switch with meter or test lamp no feed check fuse if ok put 2 wires together car in 4th gear ignition on should hear click from overdrive solenoied if ok fit new switch if not check wires from lever to gearbox for breaks think you might find problem here if not check inhibitor switch and then solenoid hope this helps you

The most common cause of intermittent problems is a poor ground connection.Check the engine and the chassis grounds for tightness and clean surfaces.

Relay located behind fuse box.

You can find it behind the fuse box. To access the relays. Remove the plastic cover on the cigarette lighter socket, remove the screw, and remove the ash tray to gain access to the fuses and panel whit the lighter socket. The relay is marked number 123 or 136, found all the way to the right second row.

My car just had the same problem and i bought a over-drive bypass plate from IPD and now it works just fine, just put it on yesterday. My overdrive solenoid was the problem. So, instead of paying $160 for a new one, i paid $40 for the bypass plate. I drive a 91 Volvo 940 Turbo with over 250K miles and now i have my overdrive back. I think if your overdrive relay was bad, the shift light would come on on the dashboard. So it may just be your solenoid. Go to http://www.ipdusa.com/Volvo-940960/Transmission-&-Drive-Train/Automatic-Transmission/Automatic-Over-Drive-Solenoid-Bypass-Kit/p-136-278-676-1031/

could be groung short,,,check gnd from batt neg to chassis grn and ...check alternator gnd ...check alternator belt tension.....tighten if loose check voltage at switch.....switch needs 12 volts from battery and if is not getting correct nvoltage from alternator may be acting faulty....

Check speesensor at transmission.Have somebody scan it.

There is a button on the side of the "shifter" knob that engages and disengages 4th gear.