1999 Chevrolet/Geo Tahoe - 4WD Fuel Pump Electrical Circuit Diagnosis Refer to Fuel Controls. Circuit Description When the ignition switch is turned ON, the VCM energizes the fuel pump relay which powers the in-tank fuel pump ON. The in-tank fuel pump remains ON as long as the engine is cranking or running and the VCM is receiving the distributor reference pulses. If there are no reference pulses, the VCM shuts the fuel pump OFF within 2 seconds after the ignition was turned ON or if the engine stops. The VCM will also turn ON the fuel pump for 2 seconds when the ignition is turned to the OFF position. Diagnostic Aids If the fuel pump relay circuit checks out OK, refer to Fuel System Diagnosis. Excessive fuel may also cause long cranking times which is accompanied by the following conditions: A start that is not as fast as normal. A puff of black smoke at the tailpipe. An improperly connected or faulty EVAP component can cause this problem. Refer to Evaporative Emission (EVAP) Control System Diagnosis. One or more leaking poppet nozzles (CSFI injectors) may also extend the cranking time. Perform the Injector Balance Test. Refer to Fuel Injector Balance Test with Tech 2. Test Description Number(s) below refer to step number(s) on the Diagnostic Table. 1. The Powertrain OBD System Check prompts the technician to complete some basic checks and stores the freeze frame and failure records data on the scan tool, if applicable. This creates an electronic copy of the data taken when the malfunction occurred. The information is then stored in the scan tool for later reference. 3. If the no start condition was due to a open fuse, locate and repair any shorts that may have caused the fuse to open before replacement. 4. This step determines if the fuel pump circuit can be controlled by the VCM. The VCM should energize the fuel pump relay and turn the fuel pump ON when commanded by the scan tool. If the engine is not cranking or running, the VCM should de-energize the relay and turn OFF the fuel pump after 2 seconds. 5. Jumpering the fuel pump positive voltage terminal to the fuel pump relay ignition positive voltage terminal verifies that the fuel pump relay ignition positive voltage circuit, the fuel pump positive voltage circuit, the fuel pump and fuel ground are all OK. 8. This step checks for an open ground circuit. 17. Replacement VCMs must be reprogrammed. Refer to the latest Techline information for programming
procedures. 18. This step checks the fuel pump ground circuit on the fuel tank side of the body harness connector only. If the fuel pump ground circuit is OK, then the fuel pump positive voltage circuit or the fuel pump ground circuit in the fuel tank harness or the fuel pump itself is malfunctioning. 21. This test will determine if a short to ground on the fuel pump positive voltage circuit caused the fuse to open. To prevent mis-diagnosis, be sure the fuel pump harness is disconnected before the test. Fuel Pump Circuit Diagnosis Step Action Was the Powertrain On-Board Diagnostic (OBD) System Check performed? 1 Value (s) Yes No Go to Powertrain On Board Diagnostic (OBD) System Go to Step 2 Check 2 Does the fuel pump run continuously with the ignition key OFF? Go to Step 26 Go to Step 3 3 4 5 Inspect the fuel pump relay fuse. Is the fuse OK? 1. Turn the ignition switch ON. 2. Install a scan tool. 3. Using the scan tool, command the fuel pump relay ON. Does the in-tank fuel pump run when commanded ON? 1. Disconnect the fuel pump relay. 2. Using a fused jumper wire, connect the fuel pump positive voltage terminal to the fuel pump relay ignition positive voltage terminal. Go to Step 4 Go to Step 21 Go to Engine Cranks but Does Not Run Go to Step 5 6 7 8 9 Does the fuel pump run? 1. With a test lamp connected to ground, probe the fuel pump relay control terminal. 2. Command the fuel pump ON with the scan tool. With a test lamp connected to ground, probe the ignition positive voltage terminal to the fuel pump relay. With a test lamp connected to B+, probe the ground terminal to the fuel pump relay. Check for an open or short to ground in the fuel pump control circuit and repair as necessary. 1. Reinstall the fuel pump relay. 2. Disconnect the fuel pump electrical harness at the rear body harness connector. 3. Probe the fuel pump positive voltage terminal (body side) Go to Step 6 Go to Step 7 Go to Step 8 Go to Step 9 Go to Step 10 Go to Step 11 Go to Step 12 Go to Step 13 Go to Step 31 Go to Step 14
10 with a test lamp connected to ground. 4. Command the fuel pump ON with the scan tool. Go to Step 15 Go to Step 16 11 Repair the open in the ignition positive voltage circuit for the fuel pump relay. Go to Step 31 12 Replace the fuel pump relay. Refer to Fuel Pump Relay Replacement. Go to Step 31 13 Repair the open ground circuit to the fuel pump relay. Go to Step 31 14 Inspect connections and terminals at the VCM and repair as necessary. Go to Step 31 Go to Step 17 15 With a test lamp connected to B+, probe the fuel pump ground circuit (body harness side). Go to Step 18 Go to Step 19 16 Repair the open or short to ground in the fuel pump positive voltage circuit. Go to Step 31 17 1. Replace the VCM. 2. Refer to VCM Replacement/Programming. 3. Perform the Passlock Reprogramming Procedure. 4. Perform the Crankshaft Variation Learn Procedure. Refer to CKP System Variation Learn Procedure. Go to Step 31 18 Check the fuel pump positive voltage and ground circuits at the body connector (tank side) for an open. Repair the circuit as necessary, refer to Connector Repairs in Wiring Systems. Go to Step 31 Go to Step 20 19 Repair the open fuel pump ground circuit. Go to Step 31 Before replacing the fuel sender, refer to Intermittents and Poor Connections Diagnosis. 20 Replace the fuel pump. Refer to Fuel Sender Assembly Replacement. Go to Step 31 Caution
The fuel pump will operate for 2 seconds after you have turned off the ignition. Make sure that all the fuel pipes and fittings are securely fastened in order to prevent personal injury. 21 1. Turn the ignition switch OFF. 2. Disconnect the fuel pump electrical harness at the rear body harness connector. 3. Disconnect the fuel pump relay. 4. Probe the fuel pump positive voltage terminal at the relay connector with a test lamp connected to B+. Go to Step 22 Go to Step 23 22 Repair the short to ground in the fuel pump positive voltage circuit. Go to Step 31 23 With the test lamp still connected to B+, probe the fuel pump relay ignition positive voltage circuit terminal. Go to Step 24 Go to Step 25 24 Repair the short to ground in the fuel pump relay ignition positive voltage circuit. Go to Step 31 25 Check the fuel pump positive voltage circuit in the fuel pump harness (tank side) for a short to ground and repair as necessary. Go to Step 31 Go to Step 20 26 Disconnect the fuel pump relay. Does the fuel pump continue to run? Go to Step 30 Go to Step 27 27 With a test lamp connected to ground, probe the fuel pump relay control cavity with the ignition key in the OFF position. Go to Step 29 Go to Step 28 28 Replace the fuel pump relay. Refer to Fuel Pump Relay Replacement Go to Step 31 29 Check for a short to voltage on the fuel pump relay control line. Go to Step 31 30 Check for a short to voltage on the fuel pump positive voltage line from the fuel pump relay to the fuel pump. Go to Step 31 31 1. Using the scan tool, clear the DTCs. 2. Attempt to start the engine. Does the engine start and continue to run? Go to Step 32 Go to Step 4
32 1. Allow the engine to idle until normal operation temperature is reached. 2. Check if any DTCs are set. Are any DTCs displayed that have not been diagnosed? Go to applicable DTC table System OK