Gmc sending unit

Asked by May 17, 2017 at 02:59 AM about the 1988 GMC Sierra 2500 2 Dr C2500 Standard Cab LB

Question type: Maintenance & Repair

Help! Changing fuel sending unit 88 5.7 and the
plug is cut off of truck. I got a black with white
stripe,purple and gray coming off sending unit.
Truck has black with white stripe, tan with white
stripe and pink
What goes where?

18 Answers


If was me with the unit in tanks and tank still dropped I would put the two black with white strip together and then the purple and pink together, then the grey and tan with white stripe wires together and try it out. Now this is a total guess but yet I would think from years of auto work that this would be my first try. But look real close to see if either the purple or grey wires from sending unit has a strip if so put it with the tan with white strip and the other two together.


If those combinations are not correct with the tank down and no pressure on tank it will or can just blow the fuse so you may want to locate the fuse panel and pump fuse and buy a pack of those so if you blow fuses you have replacements, 9 times out of ten the strip wires go together as do the non-strip wires.

I tried black to black- Gray to tan and pink to purple also swaped pink to Gray and tan to purple but still didn't turn on pump

Fuses are good - new fuel pump relay also. Gauge reads complete empty and check gauge light is on


Does the pump have a diagram or symbols on the pump where wire are coming out. One is power, one is fuel level gauge, other ground. Figure out which is fuel level it goes to the float arm and the truck wire goes to the gauge put those together, then you need to see which is power on the truck, same color wire will be the pump fuse or relay wire and if it does not change before it get to the pump thats your power wire, match to the pump + side. the other is ground, may can trace it back to ground on truck frame.


The gauge wire is a ground wire that runs from unit to gauge, it is no the ground for the pump. So basically there is one power wire.


O.k. found a diagram pink is the fuel level guage in your dash.


Black white is ground wire on truck harness and tan white is from ECM, relay, fuel pump signal wire on your harness.


On the diagram it looks like the tan/white wire from your trunk harness goes to the grey wire for right fuel pump or tan wire for left fuel pump since your pump has the grey wire hook tan with white strip to grey pump wire.


Show the pink going from pump and harness to ground, but the black & white wire grounding at the fuel module. Go to this page URL, copy & paste to browser window.



Unless I see it wrong and it is late and I am about to hit the sack but looks like it may should be like I said first post. Relay or ecm may not be getting or giving power or ground is bad may be a fuse elsewhere you need to check.

Ok got it wired and strapped back up and it had power then power went back out . Crank and crank and crank and it does nothing, if I jere the wheel or shake the truck somehow it starts. Was having same issue before I changed sending unit and pump. Idk what it could be, once it starts I can drive anywhere but once it sits for lil it does same thing

My oil pressure gauge is bouncing everywhere! Does that have anything to do with it?



Sounds like break in a wire, likely inside the insulation, check for a wire that bends weird, may can find it with the multimeter, Could be a loose ground wire to frame or just needs wire brushed at that area and put back on. Can use dielectric grease on it after brushing or scraping the frame metal then reattaching the ground wire. Check from block to frame too.


That answer above is for the post that starts out "Ok got it wired and strapped back up".


Could all be related if you look back at the diagram the Oil pressure switch is tied into the wiring from ECU to Relay to ECU fuse, Check the grounds for ECU clean rust free ect and for fuse block and like said before any & all grounds to frame and look & test for wires that may have burns, breaks, frayed, look like they bend because of breaks inside insulation & make sure all electrical connections are good and I would used electrical board contact cleaner and liberal amount of dielectric grease on all electrical connectors and check oil sender, most are located behind distributor near rear of engine block.

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