Solved: There was a fault P1172 fuel pump insufficient flow. This is related to the fuel transfer pump that transfers fuel from the rear tank to the front tank. The ECU turns on the transfer pump when the fuel level of the rear tank is higher than the front tank. If the transfer pump runs for more than 2 minutes and the ECU doesn't see the front tank's level rise and the rear tank's level lower, it trips this fault and disables the fuel gauge, sending it to empty. In my case, the connector for the transfer pump had disconnected.

I thought the fault was related to the CP3 injection pump because I know this CP3 is on its way out. Once I cleared the check engine light, the gauge worked again, for a couple of minutes. Scanned faults and the only fault that came back was P1172. I figured, how does it know about any CP3 issues when the engine wasn't even running? Looked up the code and found out it's related to the fuel TRANSFER pump, not injection pump.

 

Did this problem get resolved in your truck? i have a 2001 gmc sierra 3500 gasser with the exact same thing going on. i have been looking for someone to ask about the specific workings of the fuel system. Does the pcm take the sending units signal then tell the gauge cluster what to say essentially?

 

Yes. It got resolved. See the 2nd post. Yes. It does take the fuel level from both sending units, combine the readings in the PCM, then the PCM sends the level signal to the gauge on the cluster. If the PCM detects certain faults, it will disable the fuel gauge entirely. The gauge can also stop working because of a faulty sending unit, faulty wiring, etc. Attached is the wiring diagram. Test as I did in my 1st post. You'll be testing the wiring at the PCM connectors. So, you don't need to drop the fuel tanks to begin this test. The hardest part of this will be making sure you're probing the correct contacts on the correct connectors. There are no connector end views for the PCM connectors C1 and C2. I think I had to open up the rear of the connectors and double check the wire colors to be sure I had the correct wires. As shown in the diagram, the resistance should be between 40 ohms to 250 ohms (empty to full) at each sending unit.

 

This is EXACTLY what i was looking for! I ohmed out both senders already, checked the wiring all the way to the fuse box and couldnt find any breaks etc. Cleaned all the grounds, even tried brand new senders plugged into the trucks harness and neither would make the gauge work. Another guy had said he changed the plug on his rear tank and that solved his issue. Tomorrow im going to pin out the rear tank plug a foot from it so that should test the connection there as well.

 

ALso, where is the pcm on your truck? i beleive mine is on the lh side of the radiator shroud down next to the battery. The other odd thing with mine is my scan tool reads 0 percent fuel level monitored and displayed and only one time today did it read 55 percent fuel level, then the gauge dropped back to e again. real strange.

 

The PCM for your gas engine should look like the pic attached. There is another computer in that area that is NOT the PCM. I think it's the TCM. I think the computer on the shroud is the TCM.

 

You don't need to check any wires to/from any fuse box. ONLY AT THE PCM. At PCM connector C1 ("blue"), there is an orange-black wire, I believe it is in position 80. That same wire sends a signal to BOTH sending units. The the signal goes through the level sensors in the sending units and the is returned to PCM connector C2 ("red"). For the primary sending unit (i think this is the front tank), the wire is Purple, in position 54 in connector C2. For the secondary sending unit (rear tank?) the wire is Dark Blue, position 73 in connector C2.

I have found that these wiring colors are USUALLY correct. So again, just find these wire ends. Test them for ohms. If you get no continuity at all, then there's your problem. If you get continuity, fill the tank, check the reading. Siphon it empty. Check the reading. Is it losing signal?

 

So here’s a question. I have a 2003 GMC sierra 2500 duramax. And the truck had a second tank installed but was removed and the wires were cut. There’s a purple for sure. A black? And two greens? And since those got cut my fuel gauge hasn’t worked correctly. It will read super inaccurately. Or just drop to empty and light comes on. Any insight on how to fix this would be great.

 

Another thing... Any problem with the little transfer pump will also disable the fuel gauge. Again, in my case there was a problem with the transfer pump circuit and I had fault P1172. The pump's only job is to take fuel from one tank and move it to the other tank to keep them equal. A problem here trips a fault and disables the gauge. ALSO if the PCM tells the pump to move fuel from one tank to the other and it doesn't detect the fuel level drop in one tank and rise in the other tank after an x number of minutes, it will also disable the gauge. It's a really stupid system. They should have just hooked up a hose between the 2 tanks and skipped the whole stupid transfer pump.

 

The pump's only job is to take fuel from one tank and move it to the other tank to keep them equa

The transfer pump has nothing to do with starting your truck.

 

excellent info. thank you again. ill be working on this again today.

 

How could you bypass the transfer pump? I have an 01 duramax with Allison transmission and the rear tank has been removed. I have to use starting fluid to even get the truck to start. Priming it does not work. You could pump it all day long and as it will prime up as it should the truck still will not start without the starting fluid

 

Did you have any luck bypassing the transfer pump?

 

I also have this issue on my 2003 gmc 2500 duramax. I removed the rear tank cutting the wires going to it not thinking. And now I have no fuel gauge. It will give me super inaccurate readings or no reading at all. Any help is appreciated.

 

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