No ECM Signal to fuel pump relay

Tiny
CLINTON78
  • MEMBER
  • 2002 CHEVROLET SILVERADO
  • 5.3L
  • V8
  • 4WD
  • AUTOMATIC
  • 230,000 MILES
For a couple years, my truck has had an intermittent no start issue characterized by the fuel pump not priming. Most of the time when this issue is occurring, the fuel pump doesn't prime and the fuel gauge is buried on E. Sometimes, the fuel gauge does work when the fuel pump isn't; however, after a couple key turn-ons, it usually fails too. Most of the time, if I leave the key in the On position, after several seconds to a few minutes, it will all of the sudden prime and the gauge comes alive. Then it will work for a few days until the next time it randomly doesn't. I have ruled out the Passkey issue. The fuel pump relay works as it should. I always have power to Pin 30. I believe the ECM tells the relay to work via the green/black wire to Pin 85. This should happen for 2 seconds when the key is turned on. I have tested this when the issue is occurring and have confirmed that no signal is being sent from the ECM. I traced the Pin 85 bl/gr wire to Pin 9 on the Red ECM plug and confirmed that there is continuity, which is no surprise since it works half the time. I can make the pump run by using a jumper at the relay and by utilizing the Prime test slot; however, even when the pump is running continuously via the jumper, the truck will not start - just cranks. I've tested every fuse I have inside the cab and under the hood. I've checked grounds. I've created a temporary ground (via jumper cables), and nothing seems to work. I feel like it must be something to do with the ignition not signaling the ECM or the ECM is bad although the CEL does come on when the key is turned on. It's not on when the truck is running, and there are no engine codes or security codes. I can't rationalize why it works to leave the key On for a couple minutes and everything just magically comes alive (most of the time) without me as much as bumping the truck. Is the ECM just screwy?
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Wednesday, July 24th, 2019 AT 7:23 PM

9 Replies

Tiny
SCGRANTURISMO
  • EXPERT
Hello,

Can you check the ground from pin #85 and make sure that it is clean tight, corrosion free, and making good metal to metal contact? This is the ground from the electromagnetic coil inside the relay that receives electricity from a switched power source to pin #86. The electromagnetic coil energizes and pulls the metal switch closed via magnetism and completes the circuit from a fused power source feeding pin #30 and pin #87, which supplies power to the load in the circuit, in this case the fuel pump. The Ground is Ground #105, on the right front of the engine block. I have included the wiring diagram for your vehicle's fuel pump circuit in the diagrams down below, for reference. Please get back to us with what you are able to find out.

Thanks,
Alex
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Wednesday, July 24th, 2019 AT 9:35 PM
Tiny
CLINTON78
  • MEMBER
Thanks for the quick response, Alex. And thanks for correcting me on the Pin #'s. Indeed, 86 is connected to the ECM. I just turned the key on for the first time today. The fuel gauge worked, but it did not prime so I shut it off immediately to do some probing. I had enough ground at Pin 85 to light my test light; however, there was a voltage difference. From the positive battery terminal to Pin 85 measured 10.9 V. The battery from terminal to terminal measured 12.2 V. Everywhere else I've compared voltage has been the same as the battery. That seems a little suspicious. I turned the key to On again and after about 3 or 4 seconds, the pump spontaneously primed. After that, it would prime immediately every time I turned it on just like it has done before. I measured the Pin 85 voltage again to see if it was different - still 10.9 V. I could not locate G105. Could you tell where it is in relation to some of the components?

Also, I wanted to mention that while I have not confirmed it, I suspect that when this issue is occurring that the fuel injectors are not being signaled as well, which I think may explain why using a jumper at the relay operates the pump but does not work to get the truck started.
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Thursday, July 25th, 2019 AT 4:02 PM
Tiny
SCGRANTURISMO
  • EXPERT
Hello again,

I can't find a picture or diagram in either of our reference guides for Ground #105. All I can say is the description tells me that it is located on the lower right front of the engine. Grounds are usually almost ring connectors. A ring connector is a round connector that a fastener goes through and holds the ring connector securely this way. You might want to get some electronic connector cleaner at your local auto parts store. It comes in a spray can and is made by CRC. I have also included a description of how your vehicle's fuel system works in the diagrams down below. Please go through these guides and get back to us with how everything is progressing.

Thanks,
Alex
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Thursday, July 25th, 2019 AT 5:03 PM
Tiny
CLINTON78
  • MEMBER
Okay, so it would appear that I was right on my initial post about which Fuel Pump Relay pin gets the signal from the ECM. I realize this contradicts several wiring diagrams that are available online, but nevertheless, Pin 85 gets the voltage from the green/black wire from the ECM. The other thing I have noticed is that many diagrams available online show and describe that G105 is on the right-front of the engine; however, in the diagrams, it appears that the engine setup is different with the most obvious difference being that the alternator is on the opposite side. Mine is on the driver's side (left) of the engine. The only diagrams I can find that show G105 on the right-front also show the alternator on the passenger side (right). Anyway, so when I test Pin 86, which on my truck appears to be the ground wire for the fuel pump relay, it reads the same as if I were to read the battery directly. The curious thing to me is that when I test Pin 85, which on my truck is connected to Pin 9 on the ECM, it reads full voltage for the 2 second prime time (when the truck is working properly), and it also reads 10.9 V when I test it as a ground. This occurs when the truck is working and when it is not so perhaps it's some sort of backfeeding situation that is normal? I did locate where the large negative battery cable is grounded, and there is a smaller wire grounded in the same location. Everything tested 12+ V there. I found online that the ECM is grounded to G103 which is on the back of the engine. I took that bolt out (no easy task) and inspected/tested the wire. Everything was 12+ V. I should also mention that I have tested the ground pins on both ECM plugins to find that they are good. I would be interested to know which ECM pin gets the signal from the ignition that would tell the fuel system to work.
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Friday, July 26th, 2019 AT 4:17 PM
Tiny
CLINTON78
  • MEMBER
I should also mention that if you were to Google search for anything remotely related to this issue for a Silverado on my computer, you would find that no link or site hasn't already been clicked. Lol. There are so many people that have had this issue, and it seems like there is never any resolution or closure. I.E. No one ever reports back saying, "yep, this fixed it." I hope we can resolve this one here. It would be a gift to humanity. Or at least Silverado owners.
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Friday, July 26th, 2019 AT 4:24 PM
Tiny
SCGRANTURISMO
  • EXPERT
Hello again,

Okay, the Ground wire for the fuel pump relay should never, I repeat never, have more that 0.1V running through it. The ground wire or the ground is bad. In the diagrams down below I am giving you directions on how to test a ground and how to do a voltage drop test to confirm the presence of unwanted resistance in the ground(by ground I mean from Pin #86 on the fuel pump relay to Ground #105. Please go through these guides, test the ground and ground wire for the fuel pump relay, and get back to us with what you are able to find out.

Thanks,
Alex
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Friday, July 26th, 2019 AT 8:13 PM
Tiny
CLINTON78
  • MEMBER
I should probably clarify that when I read 12V on Pin 86, I had the other probe on the positive battery terminal. So that's where the 12V was coming from. When you say 0.1 V, is that the voltage that is supposed to operate the relay via the ECM signal? If so, I should also say that when I take the relay out and put a probe on Pin 85 (ECM Signal), when the truck is working and for 2 seconds of priming, that reads 11.7 V when connected from the negative battery terminal. Am I making a mistake by measuring voltage by connecting the other probe to either battery terminal?
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Saturday, July 27th, 2019 AT 7:00 AM
Tiny
CLINTON78
  • MEMBER
Alex! I figured it out. Completely by dumb luck. The truck got to the point tonight that my trick of waiting for it to 'magically' prime wasn't working. I took the bottom of the fuse block off to see if the connectors were corroded. They were brand new looking so I put everything back together. When I did this, I took several relays out including the two big ignition and starter relays. I just put them back in without paying attention which one went where since they're the same relay. I turned the key on, and the fuel pump primed. I then went to start cranking and nothing. I thought, well that's new. Then I thought, hmm, I wonder if it's because I swapped those two relays. I swapped them back. No fuel pump priming. I then swapped it with the headlights relay. Fuel pump primed, truck started and headlights were very dim, almost out. I did this a few times to be sure, but yes, it was the big ignition relay the whole time. Probably a $10.00 relay. Nowhere on the internet have I found that someone discovered that to be the problem although many people suggested swapping out the ignition harness. Weird thing is that I bench tested that ignition relay a while ago, and it seemed to work fine. I must have caught it on a good day. Such relief. I hope others who have this problem find their way to this thread. Thank you very much.
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Saturday, July 27th, 2019 AT 4:53 PM
Tiny
SCGRANTURISMO
  • EXPERT
Hello again,

Good Job! These are the messages we love to see. If you have any other vehicle related questions, please, feel free to ask them.

Thanks,
Alex
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Saturday, July 27th, 2019 AT 8:27 PM

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