745 overdrive relay

IPDUSA has the OD relay. It does not hurt to replace the relay. Are you sure that the OD relay is faulty. Here is the wiring diagram for a 1994 940. This is the oldest diagram that I have. It should be the same for the 1992, 740.
3/43 is the shifter button. 3/43 blue wire should have 12 volts, KOEO or running. Press the button sends 12V to 2/8 OD relay, pin 4. 12V goes out from 2/8, pin 8, to 3/42-1, 4th gear inhibitor switch. If the gear shift lever is in 4th gear only, 3/42-1 grounds 2/8 OD relay pin 8 to close or kick OD relay. OD relay closed, sends 12 volts, pin3 to the dash light bulb and to the OD solenoid 8/1 to turn on the OD unit. I have seen the wire(s) break at the bottom of the shift lever, fall off of the OD button, rub thru and short out on the gear box going to the OD solenoid and not blow fuse 12. Check fuse 12. Faulty OD solenoid. Bare wires going to the 4th gear inhibitor. The 4th gear inhibitor switch is #59 in the gearbox pic. #59 might be on the passenger side not the driver side. Don't remember. The OD wiring PDF is a better pic.