GMC SYCLONE 1992 Service Manual Supplement page 168

6E3-A-18 4.3L TURBO (VIN Z) DRIVEABILITY AND EMISSIONS
Circuit Description:
When the ignition switch is turned "ON," the Electronic Control Module (ECM) will activate the fuel pump
relay and run the in-tank fuel pump. The fuel pump will operate as long as the engine is cranking or running
and the ECM is receiving ignition reference pulses.
If there are no reference pulses, the ECM will shut "OFF" the fuel pump within 2 seconds after key "ON."
Should the fuel pump relay or the 12V relay drive from the ECM fail, the fuel pump will be run through an
oil pressure switch back-up circuit.
Test Description:
numbers on the diagnostic chart.
1. This test will determine if the oil pressure switch
is closing and supplying voltage to the fuel pump
when the relay is not functioning.
2. This test will determine if the oil pressure switch
is stuck open.
CHART A-5
FUEL PUMP RELAY CIRCUIT
4.3L TURBO (VIN Z) SYCLONE AND
TYPHOON (PORT)
Numbers below refer to circled
Diagnostic Aids:
An inoperative fuel pump relay can result in long
cranking times, particularly if the engine is cold or
engine oil pressure is low. The extended crank period
is caused by the time necessary for oil pressure to
build enough to close the oil pressure switch and turn
"ON" the fuel pump.