1980 Fiat 124 fuel pump

If it's like all the newer cars, the pump should not be running with the ignition switch on and the engine not running. To do so would create a serious fire hazard if a fuel line got ruptured in a crash. Typically the pump runs for one or two seconds when you turn the ignition switch on to insure the fuel pressure is up for starting. After that, the Engine Computer turns the fuel pump relay on again when it sees engine rotation, (cranking or running).

Engine Computers didn't exist yet in 1980 that included running a fuel pump relay so it is more likely your pump is powered by a tap on the oil pressure sending unit. To run for only two seconds sounds like it's getting that two-second burst at first, but no power after that. Given the voltages you found, it sounds like you have 12 volts supplied to the pump, then the other wire gets grounded to turn it on, presumably through that tap on the oil pressure sending unit. The first thing I'd check is the oil level. If it's low, it will take too long for the pressure to build up to turn the fuel pump on.

I'm not familiar with your specific car, just the other models that use similar systems. All fuel pumps will have two wires. If one is bolted to the body, frame, or pump assembly, that's the ground wire and the other one will have 12 volts when it's supposed to be running. That's how all late model circuits work. The 12 volts comes from a relay that is turned on by the Engine Computer.

I'm guessing you have two wires on your pump and neither one goes to ground. Based on what you said about finding 12 volts on one wire, it sounds like the other one gets grounded when the pump is supposed to run.

The oil pressure switch can switch the 12 volt supply or the ground side of the circuit. I'd have to see a wiring diagram to see how they're doing it on your car, but it sounds like they're switching the ground side. To be sure, measure the voltages on both wires, and include the colors, when the ignition switch is on and the engine is not running or cranking. Next, if you can catch it, which wire gets 12 volts right when you turn on the ignition switch, then does it stay there or go back to 0 volts. That might be easier to test with a test light. Most digital voltmeters don't respond fast enough to catch those momentary voltages.

Can you see if the voltage goes away before the engine stops running or after? If it's before, the engine should run for a couple of seconds after the voltage goes away, and that suggests there's two different sources, one for that initial pressure buildup and one for when the engine is running. That points to that oil pressure switch.

If the engine stops running first, then the pump's voltage goes to 0 volts a couple of seconds later, that suggests the pump circuit is working properly and there's something else causing the stalling.

The pump may draw too much current for a switch to handle so there may be a relay in the circuit. If there is, swap it with a different one. Sometimes the contacts get arced or pitted, and the movable arm can get fatigued or corroded to the point it will pass current for a few seconds, then it heats up and develops a poor connection.

I don't mean to confuse the issue, but on late model cars, the Engine Computer turns on a relay when it sees engine rotation by the pulses from two sensors. That relay sends current to the fuel pump or pump relay, AND to the ignition (coils), among other things. If the engine stalls due to a failure of one of those sensors, the computer will turn that relay off a couple of seconds later. A lot of people replace the fuel pump because that's the first thing they find missing and they don't look any further. The clue is spark is missing too.

It occurs to me that there were some models that used a tap on the generator to turn things on. That was one way to power electric heaters for the choke. That way those heaters wouldn't run the battery down if the ignition switch was left on with the engine not running. I don't think you can get enough current from those taps to run a pump motor so here again, there is likely a relay included in the circuit.

Right off the bat I see the pump is grounded and the other wire gets 12 volts to turn it on, just like all the other cars I'm familiar with. There is a fuel pump relay. It's part of a "relay set" shown on the bottom of page 9. It appears current for the pump comes from pin 34 of the control module, (top right corner), and through a fuse feeding the relay, but they don't say which one.

After following this around for a while, it looks almost exactly like the automatic shutdown (ASD) relay circuit Chrysler has been using since the mid '80s. What I can't tell, however, is how the circuit works or what each terminal is for on the control module. That relay set has two relays inside it, one for the fuel pump and one for the injectors and other stuff, just like on Chryslers. Both relays are turned on by grounding a common wire, (circuit 85 / pin 28 of the module), just like on Chryslers.

Now I'm going to go out on a limb and make some educated guesses. The control module turns the fuel pump relay on, I think when it gets pulses from the distributor. The module grounds one side of the relay's coil. !2 volts feeds the other side of the coil from an inline fuse.

Now that the relay is turning on, it switches on the high-current 12 volts to power the pump motor. THAT 12 volts comes from the air flow sensor. That is shown as a switch too. My guess is:

1) There has to be air flow, meaning you're cranking the engine, to get the 12 volts up to the relay set.

2) The fuel pump relay needs 12 volts from the inline fuse, and

3) it needs to be grounded by the module when that sees pulses from the distributor.

If the fuel line is ruptured in a crash, the engine can't run without fuel pressure, so it stalls. There's no pulses from the distributor, so the module turns the fuel pump and injector relays off. Even if the fuel pump relay sticks on or is bypassed, there won't be any 12 volts to switch on to the pump when there's no air flowing through the air flow sensor. You have two things turning the pump off.

Check that inline fuse, and check the fresh air tube between the air flow sensor and the throttle body. If you have that removed, the sensor won't see any air flow and the pump will stop running.

You might try running a temporary wire from a 12 volt source to the pump directly. If the engine stays running, we have to figure out which of those power circuits for the pump is dead. If the engine still stalls after a couple of seconds, and the pump is still running, that points to some other problem, possibly ignition-related, and the pump is turning off in response to the stopped engine, like it's supposed to.

Speaking of ignition problems, can you tell if the engine quits exactly when you release the ignition switch or is it a couple of seconds later? If it's right away, hold the ignition switch in the "crank" position. If the engine stays running as long as you do that, suspect a defective ignition resistor. That was real common on Chryslers in the early '70s. The engine would run as long as the ignition switch was in the "crank" position because part of it bypassed that resistor for easier starting. You could stop the starter by shifting out of "park" for an automatic because the neutral safety switch would turn that circuit off, or you could release the clutch pedal to stop the starter.

Dandy. I never thought of that because we usually see the wires already installed in a plug. Happy to hear you solved it.