So I've finally gotten to digging into my 91s fuel pump and I can't figure out what the wires do. In my repair manual they only have 4 wires and I can't seem to find any good information online

Moving left to right with the connector from the truck facing up (like its clipped in) I have
Gray | Gray black stripe | Blue red stripe | Blue yellow stripe | Dark Green black stripe

I measured 12v on blu/r and 8v on blu/y but I'm not sure what they do. I'm pretty sure grn/blk is the main power supply and I read 0v on it...

Should I rewire the green and black to the ASD one or is it supposed to be 0v?

 

Presuming it's wired like the RAM (whose FSM I have - you really need the factory service manual!), the two greys are ground.

Dark green/black should be 12V from the ASD; but if the ASD isn't on, you'll get 0V on it. (Need to bypass the ASD to check it, or supply a signal of some kind.)

Dark Blue/Yellow is the sending unit.

The Blue/Red is most likely the low fuel light, and is connected to ground while the tank is empty enough (it'll be pulled to 12V while the tank is full enough from the LOW FUEL light).

I bet you have the Haynes or Chilton's manual.

RwP

 

So how do I override the ASD? I replaced the ASD relay so I know it's not that. Could it be a bad wire somewhere? I was thinking of rewiring the dark green back to the ASD relay kn the fusebox

 

A 1991 should have a fuel pump relay in addition to the ASD relay; both are wired in parallel for the coils, but they switch separately.

I'd jump the NO and C contacts in the base (with the relay pulled out).

If you only have the ASD relay, then jump there.

To figure out which contacts TO jump, check the pin out of a traditional Bosch relay (which is what's used there!)

Here's a picture with numbers:



Jump 30 and 87. It should help some that it's a fairly distinctive layout; you might not have a 87a on the socket, that's fine.

That should power the pump constantly, so it's easier to check that you're getting full power back there.

Also, be SURE to measure the voltage with the pump CONNECTED. You could easily have a bad ground that blocks the voltage with the current of the pump, but ain't worth bumpkiss to altering the voltage with the pump disconnected and only your multimeter attached.

RwP

 

I remember when I fired it up and the pump was fine it would make a loud high pitch whine when the key was on. Now when I turn it over there is no noise. I dont have anyone to help until the weekend otherwise I'm going to try and turn it over to see if that green and black wire has power... probably the best way to tell if the connector is working

 

Remember, it'll only have power for about 2 seconds unless you're cranking or it's running.

RwP

 

4 years later! Ive got no fuel/no start no whir in the tank. I have Jumped 30 and 87.
Ralph youre saying:
"That should power the pump constantly, so it's easier to check that you're getting full power back there.
Also, be SURE to measure the voltage with the pump CONNECTED. You could easily have a bad ground that blocks the voltage with the current of the pump, but ain't worth bumpkiss to altering the voltage with the pumpdisconnected and only your multimeter attached."

i dont understand your last sentence ... shouldnt i disconnect the connector at the tank . connect Voltmeter to the green(hopefully 12v) and 1 grey (gnd) ?

​​​​​​​how check voltage while still connected?

 

Since no one else replied, Ralph passed away. He was very generous with his knowledge on here. When using his instructions it is probably best to try to be as literal as possible. Otherwise I have no idea how to solve your problem. I guess maybe some meters have sharp electrode probes you can stick into the wire or up the back side of the plug?

 

thank you and what a great memory for all Ralph did to help out.
My issue was solved ( No Start/No fuel) it was a corroded green wire from ASD all the way out to the fuel pump connector at the tank.. ran one new long wire for proper 12V .. after prime by shorting out the ASD.. engine fired up!

 

If memory serves the issue you describe was a TSB. Technical Service Bulletin.