Questions and Answers
I was looking at a wiring diagram on an 1988 Chevy, for the fuel pump, It seems there are two hot wires going to the fuel pump. One goes through the oil pressure switch, the other through the fuel pump relay, and it doesn't appear that they are connected. If the oil pressure switch can't shut off the juice from the relay, what good is it? I guess I could be mis-reading the wiring diagram. What do you think? Also does Chrysler and the imports also use safety switches?
Emergency Fuel Shutoff
Q. It's my understanding, that when electric fuel pumps became the norm, car makers needed to put some kind of safety switch on the fuel pump to shut it down in case of an accident. Ford uses the inertia switch. GM I guess uses the oil pressure switch. If the oil pressure drops below 3 psi, it shuts off the juice to the fuel pump. Is that right?
A. All fuel injected cars have some type of fuel shutoff in case of an accident. In the case you cited, the oil pressure switch does not, in itself, shut off the fuel pump. It sends a signal to the computer and if certain conditions are met, the computer will shut off the fuel pump. The primary way the computer knows that the engine is running is from a reference signal from the distributor. The ECU will not allow any fuel supply until it receives distributor reference pulses. As a backup system to the fuel pump relay the fuel pump also can be energized by the fuel pump switch and engine oil pressure sensor. The sensor has two internal circuits. One operates the oil pressure indicator or gage on the instrument cluster, and the other is a normally open switch which closes when oil pressure reaches about28 kPa (4 psi). If the fuel pump relay fails, the fuel pump switch and engine oil pressure sensor runs the fuel pump. An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. The fuel pump switch and engine oil pressure sensor energizes the fuel pump as soon as oil pressure reaches about28 kPa (4 psi).
In general most imported cars use the Air Flow Meter to determine if the car is running. If the engine is running, air is being drawn in. If the air flow stops, it's assumed the engine is stopped and the fuel pump is shut down. In older vane type AFMs, there were a set of fuel pump contacts in the AFM itself. Now the ECU shuts off the pump. Some use a signal from the distributor or Crank Angle Sensor.
Chrysler uses an Automatic Shut Down relay and the fuel pump relay to shut off fuel. If the ECU does not get a signal from both the crankshaft and camshaft sensors within one second of engine cranking, the ECU deactivates the fuel pump and ASD relays, not only shutting down power to the fuel pump, but shutting down the injectors as well.
