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Dave Nova
Dave Nova, GMC Technician
Category: GM
Satisfied Customers: 18959
Experience:  ASE Master Certification. GM World Class Certification
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Fuel pump went out replaced it now the gas gauge not working

Resolved Question:

fuel pump went out replaced it now the gas gauge not working
Submitted: 3 years ago.
Category: GM
Expert:  Dave Nova replied 3 years ago.
Hello and thank you for using JustAnswer. My name is XXXXX XXXXX I will do my best to help you with your GMC problem.\\

Did new fuel pump module come with a sending unit?
Customer: replied 3 years ago.

yes it was a oem module

Expert:  Dave Nova replied 3 years ago.
And fuel gauge is inoperative with both the old and new sending unit?

Did you have to replace the connector that goes to top of fuel tank module?
Customer: replied 3 years ago.

fuel gauge worked fine the the pcm B fuse blew and the pump stopped working I replace the module with a oem module the the gas gauge was not working so i put the old sending unit on the new module but it did the same thing. no i did not replace the connector that plugs into the module

 

Expert:  Dave Nova replied 3 years ago.
With the tank connector unplugged, turn the key on, fuel gauge should be all the way one direction. Then install a jumper wire across from the thin black wire to the purple wire at the connector and see if gauge goes all the way the other direction. Let me know what you find.
Customer: replied 3 years ago.

so i have to drop the tank?

Expert:  Dave Nova replied 3 years ago.
Not all the way, but just enough to reach the connector. Your only other alternative is if you have an advanced scan tool that shows live data, then you can look directly at the reading from the sending unit.
Customer: replied 3 years ago.
ok i turn the key on the gauge is on E i add the jumper wire the gauge does not move still on E
Expert:  Dave Nova replied 3 years ago.
Is jumper installed in harness side, not fuel tank side?
Customer: replied 3 years ago.
yes the jumper is in the harness that plugs into the module from the small black to the small purple wires
Expert:  Dave Nova replied 3 years ago.
This shows that the problem is not in the tank sending unit, but is in the wiring or the gauge itself. The stepper motors that move the gauge needles are very prone to failure on these trucks.
Customer: replied 3 years ago.

i have replaced all the stepper motors. could the fuel pump have burrned up something when it blew?

 

Expert:  Dave Nova replied 3 years ago.
It is not likely. The sending unit is on a separate circuit. Try the jumper wire from the thick black to the purple wire, just to make sure the ground circuit is good. . The signal goes from the sending unit to the PCM first, then the PCM sends a serial data signal to the gauge cluster. If you turn up nothing then the next step will be testing with a scan tool.
Customer: replied 3 years ago.
it will not move off E with the other ground. I have a new pcm is it worth a try ? any other things it could be?
Expert:  Dave Nova replied 3 years ago.
The problem is either in the PCM or the wiring. The PCM has to be programmed to the vehicle, so if your new PCM has already been programmed, then give it a try. I would also inspect the harness to the tank, make sure the wires have not gotten pinched or cut while installing the tank.
Customer: replied 3 years ago.

i have a new programmed pcm the harness to the tank looks good. can a fuel pump burn up the pcm?

 

Expert:  Dave Nova replied 3 years ago.
Not normally, the fuel pump circuit is separated from the sending unit circuit. The PCM also has fail safe circuitry built in so that a short will normally not damage the PCM. As with all other electronic components, it is still possible for the PCM to fail, but not likely.
Customer: replied 3 years ago.
put the new pcm in same thing ..so now what any other things to look for ...start to trac the wires back off the pump module?
Expert:  Dave Nova replied 3 years ago.
Yes I would check wiring. If you have an ohmmeter then take a reading at the tank unit. Same terminals that thin black and purple wires connect to. Then you can measure resistance again at PCM connector with tank unit plugged in to see if you get the same reading in both places.
Dave Nova, GMC Technician
Category: GM
Satisfied Customers: 18959
Experience: ASE Master Certification. GM World Class Certification
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Customer: replied 3 years ago.
i will try that. one thing sometimes when i start the truck the gauge will go up to 1/2 or 1/4 tank for about 30 sec. then drop to E and stay on E. the truck only has about 5 gal of gas in it so I have no idea what the 1/2 tank reading is about
Expert:  Dave Nova replied 3 years ago.
That is odd. See if you get an ohmmeter reading at the tank, then at the PCM connector. If you need to know which terminals to test at the PCM, let me know.
Customer: replied 3 years ago.

so almost everytime i start the truck it will go to 1/8 to 1/2 tank of gas for 20 sec then drop to E and the low fuel light will turn on .....I restart the truck same thing .. ok so set the ohm meter to like 200k and check the reading across the purple and black wire? what are the wires at the pcm?

Expert:  Dave Nova replied 3 years ago.
Yes, read at the tank module, then at the PCM with tank connector plugged in.

The black wire is at PCM connector C1, pin 23
The purple wire is at PCM connector C1, pin 54

PCM connector C1 (blue)
graphic

PCM connector C2
graphic
Customer: replied 3 years ago.
i get nothing across the wires at the module what setting do i need the ohm meter to be on(NNN) NNN-NNNN20k 200k 2000k ? I an reading across the black and purple wires right?
Expert:  Dave Nova replied 3 years ago.
Yes, reading across the black and puple. If you are reading at the wires, make sure the connector is plugged in. 200 ohm setting would be best. Tank sending unit ranges from 0 - 90 ohms.
Customer: replied 3 years ago.
i get 136.6 at the module but i can not get a reading at the pcm
Expert:  Dave Nova replied 3 years ago.
You have an open wire then. Try connecting the ohmmeter from purple wire at tank harness to purple wire at PCM, you may need a piece of wire to extend meter leads. The do the same for the black wire to see which one is open. It may be easier to run a new wire instead of finding the break in the existing wiring.
Customer: replied 3 years ago.
the purple wire is orange the pcm does it change coler?
Expert:  Dave Nova replied 3 years ago.
Should be same color. Sorry for the misinformation earlier. The purple wire is at connector C2, pin 54
Customer: replied 3 years ago.
ok c2 let me try that
Expert:  Dave Nova replied 3 years ago.
Sounds good, sorry about the typo.
Customer: replied 3 years ago.
the wires look ok i jumped them from the plug to the pcm
Expert:  Dave Nova replied 3 years ago.
Do you get a good reading at the PCM, same as at the tank?

Take your ohmmeter and read the other sensor you have that is not in the tank. Take a reading at full and empty and see what you get.
Customer: replied 3 years ago.
at the tank i get 136.6 at the pcm i get 153.3....... the othe sender is 41.0 empty and 192.2 full
Expert:  Dave Nova replied 3 years ago.
Well, the readings are definitely within the range of the sensor. The wiring to the PCM is okay and the PCM you have already tried to swap out. The signal from the PCM to the gauge cluster is a single wire serial data signal, since the other gauges all work, the serial data signal is okay, this just leaves the fuel gauge within the cluster as being the problem..
Customer: replied 3 years ago.

ok let me pull the cluster and swap the stepper moter

 

Expert:  Dave Nova replied 3 years ago.
Make sure the solder connections are good as well.
Customer: replied 3 years ago.
ok i swaped the tach and the fuel stepper and it is doing the same thing gas gauge will go to 1/4 tank for 15 sec then drop to E
Expert:  Dave Nova replied 3 years ago.
Could be an error in the circuitry in the gauge cluster.
Customer: replied 3 years ago.
anyway to test this? other than put a new cluster in it ? it is just odd that this started the same time as the fuel pump blew
Expert:  Dave Nova replied 3 years ago.
An advanced scan tool will let you read data from the PCM and from the instrument cluster that way you would know for sure where the problem lies. Your choices are limited, you cna use the dealers Tech 2 or the Auto Enginuity scan tool.
Customer: replied 3 years ago.
i put a gauge cluster in from a diffrent truck and the gas gauge did the same thing. must be something with the sending unit/wireing any other ideas? rewire it?
Expert:  Dave Nova replied 3 years ago.
You could try running new wires from the tank connector to the PCM. Just connect a couple of wires temporarily to see if this fixes the problem. If it does, then take the time to run the wires permanently.
Customer: replied 3 years ago.
thanks for all the help I will give it a try /drop you a tip thanks
Expert:  Dave Nova replied 3 years ago.
Let me know how things go. I sure want to see this one get fixed.
Customer: replied 3 years ago.

ok I got it. the plug on the new oem module was bad. I put the old plug in the new module and everything works. It was making a connection (hence the good ohm reading) but not good enough to support the gas gauge.

thanks for all your help

my son must have hit accept on one of your replys so do i accept the amswer again or can i just go the the website and give you a tip?

thanks again

Expert:  Dave Nova replied 3 years ago.
Thanks for the update. If you click accept again they will charge your account the full amount of the question. it is probably better to give a tip.

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