After a long process of repairing my wiring harness due to rodent damage, I cranked the car the other day. Unfortunately the fuel pump is not running, so no start.

I followed C5Diag's video on testing the underhood fuel pump relay and PCM control function, and it appears to be working correctly. I have not yet cleaned the ground at the rear wheel well, but I will do this.

I have 3 questions at this point. First, if I apply battery voltage directly to the socket where terminal 30 of the FP relay goes, should the fuel pump run? Second, is there a way to test the voltage back closer to the pump, short of dropping the tank? Third, assuming I have to replace the pump module, is there a viable factory upgraded part that would work in my 2004 model, such as a C6Z or C6 ZR1?

 

Relay terminal 30 is 12v sent to the fuel pump.
You need a good circuit to the pump, good ground and a functioning fuel pump to run.

 

You can take a jumper wire from pin 30 and 87 and the pump will run…with the fuel pump up top it may be a little more difficult to check power and grounds to the pump…just checking for 12 volts is not the way to check the power…the power wire and ground has to be able to carry the current to allow the pump to run correctly…video below at watch from 55:40…let me see if I can find a wiring diagram to see where you may be able to check at a connector that is accessible.

 

There is a connector C402 (circled) that if you can access it can be used to check for power…don’t have an exact location though.

 

zr1 pump is 3 phase and requires its own module to run

 

in this video, the guy shows how to wire the fuse box to run the fuel pump.

 

Ok, I jumped 30 to 87 and got nothing. I haven't yet jacked up the rear end, so I haven't yet looked for the connector back near the tank. It's cold and rainy here this week, so it may be a few more days. In thinking back, the fuel pressure dropped 10 psi almost immediately when I shut off the car before. I see no evidence of any leaky injectors, so that problem may be the check valve in the pump module. Ergo, I might be killing 2 birds with one stone by replacing the pump module anyway.

 

I wouldn't condemn the pump till you find the connector and check it's power and ground and because of that rodent damage...and as far as the video you can substitute a headlight bulb as the load because the pump is inoperative and make sure the circuit is supplying the proper current with a voltage drop check...these pumps draw around 5-6 amps...maybe someone with the 2004 FSM can supply you with the location of the connector...what evidence do you have that the injectors aren't leaking ?? ...if you have a Mityvac pump you can get a GM female fuel line adapter and after removing the fuel rail inlet line you can pump some air into the fuel rails...if the pressure holds the injectors aren't leaking...an easy and accurate check.

 

Exhaust every remote electrical option before moving on to any in-tank headaches.

I'd also be sure to also refresh the ground in the driver rear wheel well to the rear of the wheel on the frame.

If you get into a separate project of pulling the tank to replace the fuel pump - which do avoid unless it is necessary on the 03-04 setup vs earlier - I'd just go with a Walbro unit. You can keep stock flow or go for greater gph just check to see which ones may be noisy vs not. Many today are NOT. But my point being invest in a bigger pump while you're in there so if you ever decide to add a basic supercharger kit you won't need to worry about a BAP or ever going back into the tanks again.

 

It's interesting that you should mention this as I have a first generation Vortec SC kit for the C5 sitting in boxes in my shop 😀.

 

If you can get everything flowing, then it's easier to slap a BAP on the pump and run with it. Factory will flow enough unless you really start to pulley down. If you decide heck with it? 430 or 450 will get it done. No need for return style system unless you really decided to exceed the needs of 700 or so.

 

So would I just buy the Walbro pump and install it in the existing in-tank module? Is the check valve available as a separate part (or is it even a separate part?)

 

I have gone so far down the rabbit hole that I'm not sure what works with the factory sending unit or not. These Walbros are not dimensionality massive so they look like they'd fit or be close. Best to search on here for the firm facts which I can't give you.

 

I think I may have found an issue. Hopefully THE issue... This is the harness side of the C412 connector, as suggested by C5Diag above. It's really easy to access, as it's clipped into the outside of the frame rail just in front of the left rear wheel, after you take out the wheel well liner. Now I just have to crawl under there and see if I can repair the harness without having to drop the rear-end or tanks. Also included is a picture of the long term solution to the rodent problem.

 

Seeing that damage, I think you ought to buy a snake or two as well.

 

Repaired the wiring harness, and the fuel pump still doesn't run. I will try to load test the circuit, but it is looking more like the tank will have to be dropped. After actually looking at it, I'm not as scared of it.

 

Ok, I found an old sealed beam headlight in the shop, and I hooked it up to pins A and D on the repaired connection C412. It lights up nice and bright for 2 seconds when I turn the key on. Since this is the correct outcome for the load test, is it safe to say that my next step is pulling the tank? The good thing about doing the load test is that it allowed me to check my work before I tape up the harness with Tesa!

 

Did you try supplying 12 volts and a ground directly to the pump ??

 

yes I did that earlier, before I even repaired the harness, to no avail. I set my hot battery down by the back wheel house and hooked it directly to terminals A and D on the pump side of C412.

 

I began the process of dropping the tank today. I got the jack positioned and removed the five 13mm bolts first, not allowing the tank to drop yet. I unhooked the fuel filler and vent tube, and I managed to get the locking ring loose on the big cross tube. When I did lower the tank a bit, apparently that freed the pump end of the pump extension harness, which came out then with no effort, looking like this:

Guess that explains further the failure of the pump to respond to power input. This finding upset me a bit, since earlier this very morning I had decided that if I was going to drop the tank, then I was going to replace the pump regardless. Having thus decided, I went ahead and ordered the Delco pump module from Rock Auto for $231.99 + tax and shipping. Then I went out and started dropping the tank... Anyhoo, I can't reach the top of the pump yet, so I can't unplug the harness to get it out and repair it, so the tank still has to come out. I won't post the pictures of my arms that I took after sliding out from under the car, but let's just say it looked like a murder scene. I highly recommend wearing a thick long sleeve shirt when doing this job.

Now, I have the lock ring loose on the big metal accordion tube between the tanks, but I haven't yet figured out how to draw it back far enough to access the inner lines. Anybody got any secret methods to share?