Hello new here and have a question. I have an 03 c4500 topkick that is throwing a p1172 code. The fuel Guage is on when I clear the codes and after some time it cuts out again.

I have tested the rear tank resistance. 249 ohms. The customer went and filled up the other day, rear take took 0 gal and front took 12 gal. I checked power and ground on rear tank. Have 4.8/4.9 v on the connector. The transfer pump is PCM driven I have no voltage on the gray wire and ground on the black wire at the tra sfer pump. I tested the fuel pump relay and it works. It has 83 ohms. For that relay I have four wires black with is ground orange which is power also pin 30. Green with quite trace that sometimes has B+ sometimes no reading the Grey wire which supplies power to the transfer pump when using my power probe has a 100 ohm reading. My question is two part. A does or has anyone ran into this issue before? B. Will I only get voltage on the Grey wire when the pcm tells the transfer pump to come on? If so with the supplied info is that enough to diag a new pcm or would it be something in the fuse box. 

Thanks in advance

A does or has anyone ran into this issue before?

Can't say I have!

B. Will I only get voltage on the Grey wire when the pcm tells the transfer pump to come on?

I believe so? Service info doesn't spell out the exact strategy it uses to run the transfer pump, but it does say have the P1172 test conditions:

 

If so with the supplied info is that enough to diag a new pcm or would it be something in the fuse box. 

Not enough to call a PCM, IMO. Not yet, anyway.

You mentioned finding 100Ω on the gray wire at the fuel pump relay? That's a red flag for a pump circuit issue. This is the pump diagram for reference:

 

I'd suggest this:

Key off, fuel pump relay removed, jump pin 30 and 87. Or, orange to grey. This'll send power directly to the transfer pump.

Do you hear the pump run? If no, go to the pump and recheck for power and ground at the transfer pump, pump connected. If power and ground is present, the pump sucks.

If you do hear the pump run, we can go into testing the relay control circuit. Let us know what you find.