Fuel pump wiring

[CDW6212R]

I'm not really going to dive into this with you guys except to say this: You really have to watch voltmeter readings if you start seeing weird mid-range values like 7-10V on a circuit without a load, like a relay coil socket. It has to do with the semiconductors on the computer outputs. If you put the slightest load on it, it may drop to zero volts. This is where a simple test light is superior to a voltmeter. I have... geez, at least 5 digital voltmeters, but I use a test light just as often. Just saying...

Good thought, weird voltage readings are "interesting."

I had a no start condition in a 302 swap in my old 86 Crown Vic. It had about 6-8 volts on the start circuit, and I had already been through the wiring, and had it nicely wrapped up. I decided to temporarily use a relay to power the circuit, using the existing voltage to trigger a relay I had(which triggered with that low voltage). I drove that car for four more years and sold it with 335k miles on it. I never got back to tracing that one wire, the relay worked fine.

 

[jd4242]

As TDG explained, the PCM monitors the fuel pump relay output (238) for fault diagnostics. When the PCM energizes the relay via "grounding" 926 if it doesn't measure battery voltage at 238 it knows the relay has failed. When the ignition is on the PCM has power.

So what happens when the pcm doesnt register voltage at "238" when running or when 926 is ""energized"" via grounding

 

[2000StreetRod]

My guess is any of the following DTCs:
87 - primary fuel pump circuit failure
95 - fuel pump circuit open, PCM to motor
96 - fuel pump circuit open, battery to PCM
542 - fuel pump circuit open, PCM to motor
543 - fuel pump circuit open, battery to PCM
556 - primary fuel pump circuit failure

 

[jd4242]

My guess is any of the following DTCs:
87 - primary fuel pump circuit failure
95 - fuel pump circuit open, PCM to motor
96 - fuel pump circuit open, battery to PCM
542 - fuel pump circuit open, PCM to motor
543 - fuel pump circuit open, battery to PCM
556 - primary fuel pump circuit failure

Would any of these force it into open loop

 

[2000StreetRod]

I doubt it. A high priority of the PCM is to reduce emissions and increase fuel economy. Have you checked for any DTCs? Some DTCs can be set without causing a check engine light.