None of what follows is important for solving the problem with the truck in this thread. However, I think it's important to correct a small misunderstanding or two.
There are two things that are not correct.
1. The normally-closed contacts on the fuel pump relay are not part of the active circuit when the pump is driven by the oil pressure switch. The normally-closed contacts do have a useful function, but NOT with the oil pressure switch. More on that later, and
2. 400 RPM has nothing to do with anything in the fuel pump power supply.
Power for the fuel pump comes from the battery or the alternator, via the Junction Block on the cowl, under the lil' plastic cover on the passenger side. A fusible link supplies protection to Circuit 440, orange wire. This circuit splits into two branches. One branch supplies power directly to the oil pressure switch, the other branch supplies power to the Normally Open (NO) contacts of the fuel pump relay. The power to the oil pressure switch does NOT go thorough the Normally Closed (NC) contacts of the relay, in fact you could REMOVE the relay and the oil pressure switch would still turn the pump on when it sensed enough pressure to close it's contacts.
If the relay and the oil pressure switch (and the wire harness connecting this mess together) is properly functioning, the current that drives the fuel pump is split between the relay and the oil pressure switch based on Ohm's Law--the division of current may not be equal depending on which part(s) of the circuit have higher resistance.
If the oil pressure switch fails--or the engine has really terrible oil pressure, the pump will be powered by the ECM which energizes the fuel pump relay. The ECM must sense that the engine is running via signal from the pickup coil and ignition module. No ignition pulses, relay is turned off, pump doesn't run.
Conversely, if the relay fails--or is removed--you only need oil pressure to run the pump, but crank times are extended while the engine builds oil pressure.
I believe that the ECM needs to see 400 RPM to begin computer-controlled spark advance, but NOT to power the fuel pump.
What do the NC contacts in the fuel pump relay do? They're connected to a wire, Circuit 490, which if manually connected to system voltage, will run the fuel pump for testing purposes. The ignition does not even have to be "on".
For any operation of the fuel pump--powered by the ECM-energized relay, powered by the oil pump switch, or powered by the test wire, the fuel pump inline fuse must be good. A popped inline fuse will prevent fuel pump operation.
There is also the "Hot Fuel Handling Module" which is connected to the fuel pump relay and the fuel pump circuit on the protected side of the inline fuse. It's tied into the aux. cooling fan relay, and increases the "priming" time. I've never seen a vehicle that has this module. In '88, only LO5 and L19 got them, and only then in some markets. (High altitude? California?)
Agreed.
No. I'm looking at the wiring diagram for my '88 K1500 (C-K , 1500-3500 is all the same) I believe what follows is "GM Standard" engineering, including vehicles other than GMT400, and model years other than '88.
There are two things that are not correct.
1. The normally-closed contacts on the fuel pump relay are not part of the active circuit when the pump is driven by the oil pressure switch. The normally-closed contacts do have a useful function, but NOT with the oil pressure switch. More on that later, and
2. 400 RPM has nothing to do with anything in the fuel pump power supply.
Power for the fuel pump comes from the battery or the alternator, via the Junction Block on the cowl, under the lil' plastic cover on the passenger side. A fusible link supplies protection to Circuit 440, orange wire. This circuit splits into two branches. One branch supplies power directly to the oil pressure switch, the other branch supplies power to the Normally Open (NO) contacts of the fuel pump relay. The power to the oil pressure switch does NOT go thorough the Normally Closed (NC) contacts of the relay, in fact you could REMOVE the relay and the oil pressure switch would still turn the pump on when it sensed enough pressure to close it's contacts.
If the relay and the oil pressure switch (and the wire harness connecting this mess together) is properly functioning, the current that drives the fuel pump is split between the relay and the oil pressure switch based on Ohm's Law--the division of current may not be equal depending on which part(s) of the circuit have higher resistance.
If the oil pressure switch fails--or the engine has really terrible oil pressure, the pump will be powered by the ECM which energizes the fuel pump relay. The ECM must sense that the engine is running via signal from the pickup coil and ignition module. No ignition pulses, relay is turned off, pump doesn't run.
Conversely, if the relay fails--or is removed--you only need oil pressure to run the pump, but crank times are extended while the engine builds oil pressure.
I believe that the ECM needs to see 400 RPM to begin computer-controlled spark advance, but NOT to power the fuel pump.
What do the NC contacts in the fuel pump relay do? They're connected to a wire, Circuit 490, which if manually connected to system voltage, will run the fuel pump for testing purposes. The ignition does not even have to be "on".
For any operation of the fuel pump--powered by the ECM-energized relay, powered by the oil pump switch, or powered by the test wire, the fuel pump inline fuse must be good. A popped inline fuse will prevent fuel pump operation.
There is also the "Hot Fuel Handling Module" which is connected to the fuel pump relay and the fuel pump circuit on the protected side of the inline fuse. It's tied into the aux. cooling fan relay, and increases the "priming" time. I've never seen a vehicle that has this module. In '88, only LO5 and L19 got them, and only then in some markets. (High altitude? California?)
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