An excerpt from OBD1 Fuel Supply System Checks
Lift Pump Electrical Circuit
1. This procedure applies direct voltage to run the lift pump. If the pump runs, it may be a fuel pump relay circuit problem which step 2 will locate. - Apply fused 12vt to lift pump test terminal. -Listen for lift pump. Does lift pump run? NO: Open in CKT 120 or CKT 450 or CKT 490 to the fuel pump or faulty lift pump. YES: Go to step 2.
2. This step checks voltage from the battery and the ground circuit to the relay. - Disconnect Lift Pump Relay. - Ingnition “ON”, engine “OFF.” - Probe relay harness connector terminal “B!” with a test light to ground. Is test light “ON”? NO: Repair open CKT 440. YES: Go to step 3.
3. This step determines if there is voltage from the battery, terminal “A” to terminal “D” on the relay connector. - Connect test light between harness connector terminal “B1” and “A1”. Is test light “ON”? NO: Repair open ground CKT 450. YES: -Connect test light between terminal “B3” and ground. -Crank engine. Is test light “ON”? NO: Open in CKT 806. YES: Go to step 4.
4. This completes the fuel pump relay circuit, but the oil pressure switch should also be diagnosed. - Faulty connection at relay, open in CKT 450 or faulty Relay. Continue testing oil pressure switch. - Engine rinning. -Engine at normal operating temperature. - Oil pressure normal. - Disconnect fuel pump relay. - Using a volt meter connected to ground, probe fuel pump relay harness connector terminal “A3”. Is there B+ voltage? NO: Open in CKT 440 or 120 to the oil pressure switch or faulty oil pressure switch. YES: - Reconnect fuel pump relay. - Ignition “OFF”. - Probe fuel pump relay terminal “A3” with a test light to ground. Is test light “ON”? No: No trouble found. YES: Faulty oil pressure switch.
Note: When all diagnosis and repairs are complete, CLEAR DTC(s) and verify operation.