Can You Log What the ECM is telling the Fuel Pump to do?

[Mingus]

I'm curious if you can log the fuel pump trigger wire??

I'm having an issue that seems like the pump is randomly turning off, killing the engine. I need to find out if the ECM is commanding it to shut down or is the pump or relay acting up.

[blindsquirrel]

There is no condition I'm aware of where the PCM intentionally turns the pump relay signal off unless the PCM itself is powered down, or stops receiving CKP signal (i.e. it thinks the engine has already stopped). Even in a VATS or other 'fuel cut' situation it stops the engine by disabling the injectors, not the pump.

Using a DVOM to monitor the signal at the control side of the relay, and then at the output side of the relay would be something actually useful to diagnose a pump problem or relay problem.

[TheMechanic]

You could use the min max feature on most DVOM's to check that.

Just not fast enough on the post button to get in before you Squirrel. Hahaha

[Mingus]

Thanks, I'll hook up on the relay input and see if I can catch it when it quits.
The problem is that sometimes the car will drive without an issue, and then all of the sudden.... engine quits. It will start right back up immediately without an issue. I was watching the fuel pressure the other day and noticed the pressure dropped to zero right before it quit. It's not running out of fuel. It's got to be something electrical. I'm hoping it's the fuel pump relay starting to go bad.

[blindsquirrel]

If it happens frequently enough you can get an easy yes/no by bypassing the relay. Some safety concerns with that though if it only happens rarely and needs lots of driving - if you crash, the pump keeps running and you could burn to death, so bear that in mind.

If it dies/zero pressure even with the relay bypassed then you know you need to check wiring between the relay and the tank - or the pump itself/something inside the tank on the other side of the connector.

[Ben Charles]

Mingus does it only happen at idle by chance???

Could always put a fuel pressure gauge on a shrader valve and when it happens see if rail pressure has dropped) if not fuel related my guess would be crank sensor

[Mingus]

I found the issue last night.
I started by monitoring the voltage from the ECM that triggers the fuel pump relay. When the engine died, this voltage remained constant (prongs 85/86).
Next, I monitored the voltage going to the relay from the battery (prong 30). This voltage dropped out, then the engine quit. As I looked over the wire to the battery I came across and old toggle switch I had installed Pre-LS to kill the pump while the key is on (trouble shooting purposes). This toggle had started to rub against the metal battery box causing it to intermittently draw more current (not quite a direct short). This eventually overheated the internals of the toggle, disabling it.
I removed the toggle and replaced the wire to the relay.
I'm driving the car to and from work every day this week to make sure I fixed the issue.

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