Chart A-5 Fuel System Electrical Test

Test Description: The following numbered steps correspond with step numbers on diagnostic chart.

1. If the fuse is blown, a short to ground in CKTs 120, 639 or the fuel pump itself is the cause.

2. Determines if the fuel pump circuit is being controlled by the ECM. The ECM should energize the fuel pump relay. The engine is not cranking or running so the ECM should de-energize the relay within 2 seconds after ignition is turned "ON".

3. Turns on the fuel pump if CKT 120 wiring is OK. If the pump runs, it is a basic fuel delivery problem.

4. This test will determine if a short to ground on CKT 120 caused the fuse to blow. To prevent a mis-diagnosis, be sure the fuel pump is disconnected before the test.

5. Checks for a short to ground in the fuel pump relay harness CKT 639.

Chart A-5 - Fuel System Electrical Test (Part 2 of 2):

Test Description (Cont'd): The following numbered steps correspond with the step numbers on diagnostic chart.

6. Checks for open in relay ground, CKT 450 (Terminal "5").

7. Determines if the ECM is in control of the fuel pump relay through circuit 465 (Terminal "2").

8. The fuel pump control circuit includes an engine oil pressure switch with a separate set of normally open contacts. The switch closes at about (4 lbs) 28 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. This step checks the oil pressure switch to be sure it provides battery feed to the fuel pump should the pump relay fail.

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 But Will Not Run".

9. If the fuel pump relay control coil resistance is less than 20 ohms (shorted), repeat ECM failures may result.