Need a schematic for OD relay/switch 95 volvo 940

The brake light switch has a separate wired switch in it that puts power to the OD lockup when pedal is up. When pedal is depressed the brake lights come on and the OD goes off. If this switch connected to the brake pedal is faulty or the wires are not connected, OD will not work for it isn't getting a signal to do so.

DTC P0871: OD PRESSURE SWITCH SENSE CIRCUIT

DTC P0871 will set if one of the pressure switches are open or closed at wrong time in a given gear. Transmission system will default to immediate shutdown routine.

The following items may be area of concern:
Related relay DTCs present.
Transmission control relay output circuit open.
Pressure switch inoperative.
Intermittent wiring and connector problems.
OD pressure switch sense circuit open.
OD pressure switch sense circuit shorted to ground.
OD pressure switch sense circuit shorted to voltage.
Faulty TCM.

I've gone through some of the diagnostics on this and unless your an expert at transmission concerns I would get some expert help with this one.

You need to have the transmission serviced at a Transmission shop. There is a problem with one of the Solenoids in the Transmission.

Several suggestions, some cheap, others not. The overdrive relay is the first suspect. It's located on the relay rack behind the fuse panel (usually). Cheap fix. About $10 on Ebay. Just be sure the part numbers match. I always carry a spare. The speed sensor might be another source of problems. It measures RPMs and feeds the info to the computer. Not an easy do-it-yourself fix. Also, the transmission control module. An easy swap with a spare from Ebay. It hides under the dashboard, passenger side. A solenoid on the transmission locks the OD in and out. Not an easy do-it-yourself fix. Volvos are notorious for flaky issues like this, so I would try all the simple fixes before bringing it to your mechanic.

Relay located behind fuse box.

check fuel pump and fuse and relay+change fuel filter.

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....

If the light is on you have power to the system. Remove the access panel to the Fuse / Relay panel by removing the ashtray then feel the O/D relay (usualy white) as you click the O/D button. If no click change the relay. If problem still occurs, change the O/D Solenoid on the left side of the transmission (2 bolts, 1 wire, 20 minutes). Be sure you THOROUGHLY clean the area around the solenoid before beginning work. Also, spray area with carb cleaner or brake spray prior to reassembly.

Overdrive relay usually fixes the problem. With the engine idling quietly, press the OD switch on the shift lever. Do you hear a click? If not, it is probably the relay. Located behind the glove box, passenger side. toward the center console. Pick up a used one on Ebay ( about $10 bucks) and swap it out. Cured my problem right away. Be sure you change the correct relay because Volvo hides several back there. I would buy the spare, then match it with the one now installed. If not the relay, you will need a new solenoid in the transmission. About $200 installed.

You have an overdrive relay issue. The tansmission is really a four-speed box, but the OD selector either locks our fourth gear or allows the car to shift normally into fourth. I use the OD on my volvo all the time, because the revs drop in half when the OD is activated, The relay is located behind the glove box, toward the center console. $10 buck on Ebay and your transmission should shift correctly again. I would try that BEFORE more expensive fixes, like OD solenoid $200 or transmission control module. Just for kicks a picked up a spare ECU on Ebay for $10 because these get costly if and when they go. Easy fix, though, a couple of plugs.