Fuel Pump only getting 6.4 volts

[NeverSummer]

So like it says my fuel pump is not working because it is only getting half of battery voltage. I have searched all over the posts here and have not found anyone to have the same problem.

Lets start with the basics; The battery voltage is 12.5 (at chassis Ground), The voltage at fuel pump is 6.4, I hooked up a battery directly to the fuel pump and it runs. I trace the wires back up to the inertia shut off switch (btw i tested the switch and it works) still 6.4 volts. next place up the line is the PCM which the dark green/yellow wire has 6.4 volts. coming out of the PCM. I don't know where to go from there. the black/yellow wire from the power distribution box has 12.5 volts all the way to the relay, which I tested to be working from the FP lead of the VIP test connector, but triggering the relay is supposed to turn on the pump which it does not do.

I just changed the FP about a week ago because it was not working, nor was the sending unit. The FP worked for about a week then this.

Yes there is gas in the tank.

Any ideas? I'm desperate here!

 

[BrooklynBay]

Welcome to this forum! You have 2 relays involved in controlling the fuel pump. The PCM relay turns on the computer which controls the fuel pump relay. The fuel pump relay turns on the fuel pump. Check both relays.

 

[NeverSummer]

Thanks. Checked both, as far as I understand if the PCM was not working the check engine light would not come on, which it does. the fuel pump relay will click on and off when I connect a jumper to the FP terminal at the VIP test connector. Out of curiosity, I know the green relay is for the fuel pump. What are the other Brown and Black relay for?

 

[BrooklynBay]

You should test the voltage inside of the relay sockets.

 

[MrShorty]

Others have reported the same phenomenon. It seems that the PCM< generates a detectable 6ish V "ghost" potential on the fuel pump circuit. If you were to hook up a test light, that voltage would not have enough power behind it to light it. In short, it basically means that the pump isn't getting anything.

To do this systematically, you need to find out where in the fuel pump circuit the voltage drops from 12.5 to 6.4. Your problem will be there. At this point it sounds like you've test battery to the relay, and inertial switch to pump. Any test in between those two points (there really isn't much left there)?

You say you can get the relay to click by grounding the pump self-test connector, but the pump doesn't come on. Have you verified that the relay is actually closing (do you have +12.5 V after the relay when it is closed)? Have you been able to check to see if the relay closes when the PCM commands it closed when you first turn the key on?

What are the other Brown and Black relay for?

Don't remember which is which, but one of those is the "A/C full-throttle cut off relay" (used by the PCM to cut power to the AC compressor at full throttle), and the other is the main PCM/EEC relay.

 

[NeverSummer]

don't know why but the fuel pump is working now. WTF?! The only thing that has changed is the ambient temp, and I changed the PCM earlier today but I tested it and tried to start it and it didn't work!

Yes I have tested both and it is the same at both. Although, I did not even think to check the voltage coming out of the relay when it is closed...will be my next test if it does not start tomorrow morning.

Thank you for the help

 

[BrooklynBay]

It might have been a bad connection or corrosion in a relay socket or the PCM connector.

 

[NeverSummer]

that's the only thing I can think of at this point