Hello, all,

New to this vehicle, as my son recently bought it. Now it won't start. I hear no fuel pump action, so felt that it is not running. I do not have a complete wiring diagram, which makes things a bit difficult. I have found some for the fuel system, and have a pretty good grasp of how it all ties together. He checked the voltage at the inertia switch with the key on and it was just a volt or two. Shouldn't this come basically straight from the front junction box with 12v? I switched the fuel pump relay with the adjacent one, and the voltage is still only 2v. Any other reasons 12v wouldn't be going to the inertia switch? Does anybody have a wiring diagram for this beast? I know of the module failing inside of the pump being a problem, but that would seem moot if there is only 2v getting to the inertia switch.

 

The fuel pump relay is only energized for 2-3 seconds when the key is first turned to RUN from OFF. Are you checking it only during that interval? After that interval and until the engine starts, the voltage would read some undefined value, usually between 2-8 volts due to the hi_Z input status port on the PCM.

 

See if this diagram helps for it to make better sense to you.

 

Thanks, both that have responded. The diagram makes perfect sense, but unfortunately does not solve the problem. My thought was that the FPDM (in the fuel pump on this vintage truck) received constant power, and was modulated by the PCM. If I am understanding, though, the PCM could still interrupt the Fuel Pump relay, causing no current to the Inertia switch (and onto the pump). Puzzling, though, is that there is LOW current, as opposed to NO current, which tells me the FP relay is activating, correct? I switched 2 identical relays on the junction box to eliminate the possibility of burned contacts, and that did nothing. I also checked the plug in diodes for the FP and PCM, and they both worked correctly, thought I do not know what resistance they should show.Any other thoughts on what could cause LOW voltage to the Inertia switch?

Thanks for your help!

 

That is incorrect. Follow what I said earlier EXACTLY.

 

Thanks, projectsho! Unfortunately, when first turning the key on, or cycling it, the voltage at the inertia switch is (I was wrong before) less than a volt. Meter checked at the battery is 12.8. Not sure where the voltage drop is.

Thanks for your input.

 

Okay, I had to review how the newer FPDM circuit works. The FP relay is supposed to be energized and pass power thru the inertia switch whenever the ignition is in RUN or START (not just during initial priming cycle). I was thinking of the older system in my previous posting.

Check and verify that fuse F9 under the dash is supplying power through it and that the FP relay is energizing. A failure in that part of the circuit would prevent power from getting to the inertia switch on the DG-YE wire.

 

You could also remove the FP relay and see if the power goes away, if it does not then that power reading you're getting comes strictly from the PCM. Next with the FP relay out, check for power on terminal 5 of the relay socket just to make sure 12v is reaching the relay..