Chart A-5 Fuel System Electrical Test
Chart A-5 - Fuel System Electrical Test (Part 1 of 2):
Wiring Diagram for Fuel System Electrical Test.:
CHART A-5 (Part 1 of 2) FUEL SYSTEM ELECTRICAL TEST
Circuit Description:
When the ignition switch is turned "ON", the Electronic Control Module (ECM) will energize the fuel pump relay which completes the circuit to the in-tank fuel pump. It will remain "ON" as long as the engine is cranking or running, and the ECM is receiving Computer Controlled Coil Ignition (C3-I) reference pulses. If there are no reference pulses, the ECM will de-energize the fuel pump relay within 2 seconds after key "ON", or the engine is stopped. The fuel pump will deliver fuel to the fuel rail and injectors, then to the pressure regulator, where the system pressure is controlled. Excess fuel pressure is bypassed back to the fuel tank. The fuel pump test terminal is located in the engine compartment. When the engine is stopped, the pump can be turned "ON" by applying battery voltage to the test terminal. Improper fuel system pressure may contribute to one or all of the following symptoms:
Cranks but won't run.
Code 44 or 45.
Cuts out, may feel like an ignition problem.
Hesitation, Loss of power or poor fuel economy.
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
1. If the fuse is blown, a short to ground in circuits 120, 839 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 misdiagnosis, be sure the fuel pump is disconnected before the test.
5. Checks for a short to ground in the fuel pump relay harness CKT 839.
Chart A-5 - Fuel System Electrical Test (Part 2 of 2):
CHART A-5 (Part 2 of 2) FUEL SYSTEM ELECTRICAL TEST
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
6. Checks for open in relay ground, CKT 450 (terminal "C").
7. Determines if the ECM is in control of the fuel pump relay through CKT 465 (terminal "A").
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 Won't Run".
9. If the fuel pump relay control coil resistance is less than 20 ohms (shorted), repeat ECM failures may result.