Open Fuel Pump Circuit

TEST CONDITIONS
1. RPM equal to or greater than 24.

FAILURE CONDITIONS
Fuel pump feedback voltage less than or equal to 2 volts for 1 second.

ACTION TAKEN
1. ECM turns on "Engine Control System Fault" telltale.
2. ECM commands BCM to display "Service Now" message on Driver Information Center (DIC).

DESCRIPTION
The fuel pump receives power from CKT 120 through either the fuel pump relay or the oil pressure sensor. The ECM energizes the fuel pump relay for 2 seconds at key on to provide fuel pressure for starting. After the 2 second fuel pump prime, the ECM will not power the fuel pump relay until distributor reference pulses are received indicating that the engine is turning (crank or run). The oil pressure sensor is provided as a "backup" to the fuel pump relay. Normal operating engine oil pressure will close the normally open oil pressure switch contacts, providing a connection between CKT 239 and 120. If a relay fault occurs, the engine will continue to run as the oil pressure switch powers the fuel pump. A vehicle with the relay inoperative will crank until oil pressure builds (oil telltale off) then start and run as the oil pressure switch provides power to the fuel pump. The ECM monitors voltage on the fuel pump power CKT 120 to detect fuel pump voltage supply faults. Code EO20 is set when the ECM sees the fuel pump not energized (0 volts on the feedback) with the engine cranking or running. The code is designed to detect a fuel pump relay fault; relay not powering the fuel pump.

Notes on Fault Tree
1. Checking for engine to start with fuel pump powered through fuel pump relay. If car does not start, go to diagnostic chart.
2. 0 volts with engine running indicates an open from fuel pump power circuit to ECM - fuel pump relay is not at fault.
3. Checking for proper control of the fuel pump relay by the ECM.
4. Checking for open from fuel pump fuse to relay pin "4".
5. Checking for open from relay pin "5" to ground.
6. If light turns on, then all relay circuitry is OK. Fault is the relay.
7. If the relay or relay drive circuits are grounded, the ECM may be damaged by excessive current draw. Repair the fault and check for proper operation.
8. Fault is most likely at ECM connector or ECM. Before ECM is replaced, perform "ECM Replacement Check", Chart C-1Chart C-1 Vehicle Will Not Enter Diagnostics

NOTES OF INTERMITTENTS
If Code EO20 is stored as history, unplug the oil pressure switch, start the engine, and allow to idle. Manipulate affected wiring, check the relay for proper installation into relay at socket. If the fault is induced, the engine will stall, Code EO20 will set. If Code EO20 sets without an engine stall, the cause is a intermittent open CKT 120 between ECM pin E11 and splice to fuel pump relay.