Fuel Pump 1983-91 Waterboxer

From VanagonWiki

Whenever the engine cranks but won't start, the fuel pump and its support systems have to be considered as a likely cause.

The fuel pump is at the heart of the fuel injection system, providing a constant supply of high pressure fuel to be managed and delivered by other parts of the FI system. It is supposed to run 100% of the time that the engine is running. It is a dumb part that just spins and spins as long as it has 2 things, power and ground. The ground is provided on a brown wire that bolts to metal in the engine compartment. The power to run the pump is provided by a fuel pump relay inside the engine compartment relay box, turned on whenever the ECU tells it to turn on.

Yes, the ECU controls the fuel pump relay and the pump can thus only run when the ECU thinks it should and tells it to. The ECU must be present, powered up, and in the right mood to supply ground to the fuel pump relay coil to complete the circuit that turn on the fuel pump relay and thus run the fuel pump. While 12 volts is supplied to the FP relay coil whenever the key is on, it also takes the ground provided by the ECU for the relay coil to turn the FP relay on.

A couple of things can put the ECU into the mood to run the fuel pump:

1. The key switch has just been turned on. Each time the key switch is turned on, the ECU powers up and then turns on the FP relay for 1-2 seconds to pressurize the fuel system.
2. The engine is turning. Whenever the key is on and the engine is turning, timing pulses from the hall effect unit inside the distributor are sent to the ECU and the ECU knows this means that fuel is needed so it supplies ground to the FP relay coil which turns on the FP relay and runs the fuel pump.
   

HOW TO tell if the fuel pump is working?

1. Listen to it. It makes noise when running. Each time the key is turned on it should make this noise for 1-2 seconds. No need to turn the key all the way to start position, just on.
2. Feel it. The pump buzzes or vibrates when it runs. Put your fingers on it while someone turns the key on. It should buzz for 1-2 seconds each time the key is turned on.

WHAT IF it ISN'T working? I can't hear or feel it buzzing what now? Well, a whole list of other things have to work first before the fuel pump will be able to work. Here is that list:

1. Battery must have a charge and a wire from it to the fuse box and dash wiring must carry 12 volt battery power up to the ignition key switch.
2. Ignition key switch must work when turned on.
3. Black wire out from key switch must carry 12 volt battery power back to the engine compartment and to one side of the ignition coil on the left side of the engine compartment.
4. Another smaller black wire must carry this same power from the coil terminal over to the relay box above the coil, and turn on the ECU relay. The ECU relay should click on each time the key is turned on, feel it or listen to it.
5. The same small black wire must also provide this power to one side of the fuel pump relay coil.
6. The red wire from the alternator to the threaded post inside the engine compartment wiring box must have 12 volt battery power on it at all times that the battery is connected, key off or on. (the alternator red wires are connected to the battery via the main starter wire for charging purposes but these also carry battery power the other way to the fuel injection system during starting and the engine relay box must have this power from the battery to work. once the engine is running the alternator then provides the power to the relay box as well forward to the battery).
7. A wire from the threaded post to the relay box must provide battery power to the high current side of both the ECU and fuel pump relays. When the key switch is on, the ECU relay must turn on and pass the high current 12 volts on to the ECU itself, powering it up.
8. The ECU must receive the 12 volts from the ECU relay and also have a good ground.
9. The ECU must turn on and work. needs to be a good ECU and get power and ground.
10. The working ECU must then send a ground signal to the fuel pump relay coil completing the circuit through the relay coil to the 12 volt battery power that was provided by the black wires from the key switch.
11. The output power from the fuel pump relay must now travel out and along a wire to the fuel pump, passing through a 2 wire connector, and reach the + terminal of the fuel pump
12. The fuel pump - terminal must have a ground supplied to it, normally from a brown wire that runs next to its power wire, back through the 2 pin connector, and back to the engine compartment where it is screwed down to metal.
    

If all of the above has been fulfilled, the pump should run. If it doesn't it is likely bad or plugged with debris. The quick test is to measure the voltage at both leads of the pump while someone turns the key on and off a few times.