Chart A-7A & A-7B Fuel System Diagnosis

1. If the fuse is blown, this test will confirm a short to ground on circuit 120. To prevent misdiagnosis, be sure fuel pump is disconnected before test.
2. Determines if the pump circuit is ECM controlled. The ECM will turn ON" the pump relay. Engine is not cranking or running so the ECM will turn OFF" the relay within 2 seconds after ignition is turned "ON".
3. Turns ON" the fuel pump if circuit 120 wiring is OK. If the pump runs, it is a basic fuel delivery problem which the following steps will locate.
4. Checks for battery voltage at the pump relay.
5. Checks relay ground CKT 450.
6. Checks for ECM control of relay through CKT 465.
7. The fuel pump voltage control circuit includes an engine oil pressure switch with a separate set of normally open contacts. The switch closes at about (4 lbs) 27 kPa of oil pressure and provides a second battery feed path to the fuel pump. If the relay fails, the pump will continue to run using the battery feed supplied by the closed oil pressure switch.
A failed pump relay will result in extended engine crank time, because of the time required to build enough oil pressure to close the oil pressure switch and turn "ON" the fuel pump.
There may be instances when the relay has failed but the engine will not crank fast enough to build enough oil pressure to close the switch. This or a faulty oil pressure switch can result in engine Cranks Hut Will Not Run".
8. Checks the oil pressure switch to be sure it provides battery feed to the fuel pump should the pump relay fail.
9. Checks for open oil pressure switch with ignition "OFF". Should the switch stick closed, the fuel pump will continue to run and discharge the battery.