03 GMC Yukon no fuel pressure

Greetings everyone, I have a client that replaced the fuel pump cuz he didn't have fuel pressure but that's not the problem. I I'm not getting any activity, meaning clicking to the fuel pump relay. I have power to it from the fuse but when I inserted power via my Power Probe to the fuel pump I get no fuel pump power. I did lower the tank and the wire is okay through the wire harness, I check the continuity the other two power wires but I did not check the ground though. And I did not verify that the new fuel pump works . Where I'm at now is to check the PCM connection to see if I get any power from that for 2 seconds after turning the key on. The old fuel pump works so I have a problem somewhere else here. I did verify that there is spark as suggested by our leader. Any suggestions is greatly appreciated. Thank you, Doug

Welcome Doug! I'm moving your thread to the repair section so it gets the necessary attention.

Oh thank you Dylan I did that in a hurry I appreciate it!

You can do a lot of checks from the relay base under the hood with your Power Probe.
I marked the load side pins of the relay on this diagram piece, 30 is going to be the constant power feed, and should light the red LED on the power probe.
You should find 2 grounds probing the rest of the relay pins. One is the ground for the control side, and the other that I have marked is 87. That's the path current must flow to run the pump.
If the one marked 87 doesn't light the green LED, then the circuit is broken between the relay base and the pump. From experience, it's usually either a bad pump (open windings) or a bad pump ground.
If all that checks out, the final pin is the PCM control for the relay. There should be power there for the first key on pump prime, and while cranking the engine.

Thank you Noah, that's exactly what I did today before reading your post. Where did you get your diagram from? I got mine off identafix and a suggestion from a previous fix that a person had for testing. There it was backwards from what yours shows, where 87 was the power not 30. Showed 30 went to the pump. At the time i thought it was different. Had to lower the tank again to get to the connector on the pump and the harness and found that the fuel pump was working, just not pumping fuel through. Couldn't hear it activating when the tank was up so had to lower the tank to verify that. Tomorrow I will take the pump back out to find out why it's not pumping gas, probably installed wrong not allowing fuel to pump through from the sump filter. Hope all that made sense. I will post a picture showing the diagram and procedure that I followed to show the difference in what you gave me which is much clearer. Thanks again!

It still could be a poor pump ground even though the pump is running. I've seen that on Chevy trucks before.
The power probe isn't a great tool for finding resistance in circuits.

With a scope and an amp clamp, you could measure the current being pulled by the pump, and observe the initial turn on amperage spike to verify the power and ground for the pump.

Without a scope, you could do a ground to ground voltage drop test.

Set your meter to volts dc, place the black lead on a good ground. I suggest using the ground lead from your power probe since it's connected directly to battery negative.

Backprobe the pump ground wire at the pump (or pierce it close to the pump) and connect your positive meter lead.

With the pump running, there should be no significant voltage on the ground wire. A few hundred milivolts wouldn't be an issue, but if you're reading more than say half a volt or a couple of volts on the ground wire with the pump running, there's a problem with the ground. This is a good video showing what I'm talking about.


I'll be honest with you, if I suspect a bad ground, and the power probe is already out, I'll just ground the damn wire see if the truck starts. That doesn't really teach you anything about finding voltage drop, and only strengthens your reliance on the power probe, but sometimes it's just faster in the real world.

I'm also thinking of a ScannerDanner video where his students wired in a fuel pump reverse polarity. The pump would run, had good power and ground, nice amperage wave form, no fuel pressure. He proved it by calculating fuel pump rpm using the current waveform.

Oh, and I didn't verify if it was 30 or 87 connected to the fuse, I was just going on experience. Whichever one has constant power leads to the fuse regardless of how I labeled it on the diagram snippet.
That was from Alldata. I'm super happy with the new Alldata system.

Noah wrote: I'm super happy with the new Alldata system.

I've used Alldata, a couple different times, on the week-long trial. Their wiring schematics could not compare to Mitchell/ShopKey. The last time a salesman called wanting me to "give it another try", I told them No because they didn't compare. He told me they were going through some BIG changes. However, at that time (about 2 months ago), the changes hadn't been implemented, yet. I told him to call back when they made the change.

It looks like it's time to give them another try.

They've got a great search engine now too.

Yeah I found out it was a bad fuel pump, actually the guy put a new fuel pump in so I checked all the other connections and relay powers and drowned out and come to find out the new fuel pump wasn't working either so I'm putting another one in. Thanks for all your info.

I've been using Identafix and had a real PITA problem with PDF files not opening up on my browser on my Android devices to be able to view electrical diagrams. But I saw that with the new Chrome and Firefox Beta versions now everything works great. This is going on my second month using identafix and I wanted to try AllData at some point in the Future also. Thanks again for all your great input!