1999 Chevy Suburban C1500 with 5.7L engine. I have 131,000 Miles. I had problems with the fuel pump running 3 weeks ago. The pump stopped one night and came on in the morning and then stopped for good. I took it out and it would run at 12 volts but I saw somewhere that it runs on 5 or 6 volts and I check the plug in the back and it has 5.42 volts. So I changed the relay and checked the fuse and they are ok. So I ordered a new pump and it worked for 2 weeks and then it stopped on day after a long trip. I changed the relay and checked the fuse again and they are good. It was working on and off and as soon as I thought it was fixed it stopped working a few minutes later. I then replaced the fuel pump wiring harness because I know thats usually a problem and it still will not work. I checked the grounds to the pump and they seem to be good. I also checked the pump on 12 volts and it does kick it on but I still think that the initial prime is around 6 volts. I even changed the oil sending unit because I hear the fuel pump is tied into that. I still have around 5 to 6 volts to the back at the plug. I recently checked it and when I plug it in it drops to 0.42 volts and so does the fuel sending unit but that is working. I am running out of what it may be. I am wondering if the fuel pump has a problem, the computer has a problem or there is a bad ground somewhere. Please help. My next step is to get another fuel pump unit?
AAAHHH! I'm having flash backs!
You are correct about the 6 volts at the pump but it's supposed to be there. Your fuel level sending unit runs on 6 volts. Your fuel pump runs on 12 volts. If you changed the harness connector then you had to splice the new connector into the 4 wires. One wire is a black wire with a white stripe on it. This is your pump ground, and it attatches to the frame at the cross member just before or just ahead the fuel tank. Follow this wire to where it connects to the frame ( it's not far ). Make sure the connection is good. Next --the gray wire is the 12 volts that operates the fuel pump. Pull out the fuel pump relay and observe the numbers on the side of it and how they relate to the terminals they plug into. Jump out 85 to 86. This directly jumps out the fuel pump, and it should have 12 volts to it, even without the key being in the ignition. Test to see.
November, 29, 2007 AT 11:00 AM
The gray wire is only putting out 6 volts along with the purple/white. I believe the relay changes it to 6 voltsor the computer does. I purcahsed a fuel pump online but I think it was the one being recalled, so I went out yesterday and bought one from advanced auto parts and it had the new wire harness that was flat. I believe the old pump has an arch in the actual pump itslef because it would drop to.43 volts when it was connected. So we will see how well this works. If anyone else has any ideas what it may be, please let me know because this may ruin the new pump. I am returning the one I bought because I think it was the pumps that were being recalled.
November, 29, 2007 AT 10:46 PM
Jump out the 85 to 86 terminal again, dissconnect the pump, test the gray wire to ground. If you still get 6 volts then you have a bad ground or a bad connection on the gray wire or a bad connection on the ground. These pumps are 12 volts. The 6 you are recalling is the amperage. The proper current draw on a good GM pump is 5 or 6 amps.
Forget about the purple/white- wire. It's suppose to have 6 volts, it's the fuel level sensor wire. The relay does not change the voltage. A relay is just a fancy word for a switch. They call them relays because they have complex mechanisms inside of them which must work together in order for the voltage to be " relayed" to the proper spot. Think of it this way--you want a light on--your brain tells your hands to operate a 120v switch inorder for the 120v light to come on. Your brain does not use 120v to do this. Your brain is a relay for the light. See
The person who designed these connectors on these pumps should be I'm sure you can finish this sentence. Let me know.
November, 30, 2007 AT 12:04 PM
Thanks for the Info. It is still reading 6 volts at the connector on the firewall. So somewhere near the computer and relay area I have a bad connection. Can the computer send the wrong volts? If I cant find the problem with the grey wire is there a way to bypass and create a way to run the pump? Does the computer control the pump? If I bypassed it would it mess up the computer?
December, 1, 2007 AT 8:15 AM
Sorry, there is no way to bypass this wire. You can replace it but it must connect between the terminal 85 or 86 and the fuel pump. Go back to terminal 85 and 86. Test for voltage on both of these terminals. One should be dead and one should have voltage. The one that is dead is the gray wire that goes directly to your pump. Test this wire here for continuity to ground with the fuel pump unpluged; there should not be continuity. If there is then you have a short somewhere in this wire. The one with voltage on it is an orange wire that is hot all the time. This orange wire is a feed from a 20 amp mini fuse that should be marked ECM-8. This fuse is located at the under the hood relay center ( the one where terminal 85 and 86 is located). If there is not 12 volts on this orange wire then the problem is probably located internaly inside of the computer. This orange wire also feeds the oil pressure sensor.