No voltage to fuel pump - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Tilt · 10-02-2015, 02:10 PM

Finally got the truck running. Drove it to get the exhaust on it yesterday and back home. Started it up this morning and got half way up the driveway and it dies. Tried to restart and it just turns over. I noticed that the fuel pump wasn't coming on when the keys was first turned on. I checked the fuse and it's good, I have 12V going to the relay, but only about 0.1-0.2V going to the pump while cranking. I swapped the relay with the A/C relay since it was the same and get the same results. Could the computer have crapped out on me? Is there anything else I can check to see if the ecm is bad?

Heavymetl · 10-02-2015, 02:39 PM

I've yet to see a bad GM PCM. I know there out there but that's the last place I'd look.

Just check with a meter that the PCM is commanding the fuel pump on for prime at the initial "key on" for engine start. As long as the computer is putting out that 12v signal to the relay, the problem then lies somewhere in the other wiring you have for the fuel pump.

I typically use large wires for the fuel pump too, 10 guage. Fuel pumps hate low voltage. The relay is supplied the power in my truck and LS swapped Camaro directly from the battery. Make sure you relay has a nice connection and good wiring.

Good luck and let us know what you find.

Oh yeah, make sure the fuel pump is grounded well too!

solidaxel · 10-02-2015, 03:28 PM

Are you able to "jump" past the relay with 12VDC to see if the pump will run?

Tilt · 10-02-2015, 05:07 PM

The fuel pump will run if I jump it to the battery. I only have the 12V main going to the relay. On the priming cycle and while cranking, I'm only getting 0.1-0.2V. This is on a new BP Automotive harness and fuse/relay box.

Heavymetl · 10-02-2015, 05:48 PM

.1-.2v where? At the relay out going to the pump or the +12v signal from the computer?

If you know the pcm signal is good and the pump runs when jumped to the battery, your issue is somewhere in between, which means old fashioned electrical troubleshooting.

Tilt · 10-02-2015, 07:13 PM

Quote:

Originally Posted by Heavymetl

.1-.2v where? At the relay out going to the pump or the +12v signal from the computer?

If you know the pcm signal is good and the pump runs when jumped to the battery, your issue is somewhere in between, which means old fashioned electrical troubleshooting.

12V to one leg, and 0.1-0.2V to the other three while cranking and prime cycle.

I'll Try the ground to the PCM and see if anything changes.

Tilt · 10-02-2015, 07:40 PM

Grounding the PCM changed nothing.

I pulled a plug wire and tried my spark tester and no spark. Tried another and same thing, no spark.

BR3W CITY · 10-02-2015, 06:31 PM

Put an external ground on the ECM as well just in case. Usually an ECM dies in one of two ways; COMPLETELY and drastically, or slowly and miserably. There are very few conditions in which an ECM can be damaged but still function normally otherwise. Grounding the case fixes the most common non-critical failure which is the internal ground dying. The next is the injector driver internally dying.

The less safety concious among us have just run the fuel pump trigger wire and relay off of the ignition switch instead of the ECM. Its best to have a rollover valve if you do that, but it does work.

BR3W CITY · 10-10-2015, 01:14 AM

Thats maybe only the 3rd ECM I've seen be the root cause, so you really got luck of the draw. 99% of the time when people think the ECM is dead, another problem is to blame...but it seems like you actually hit the 1%

10-02-2015, 02:10 PM	#1
Tilt Registered User Join Date: Jul 2014 Location: Dayton, TX Posts: 335	No voltage to fuel pump Finally got the truck running. Drove it to get the exhaust on it yesterday and back home. Started it up this morning and got half way up the driveway and it dies. Tried to restart and it just turns over. I noticed that the fuel pump wasn't coming on when the keys was first turned on. I checked the fuse and it's good, I have 12V going to the relay, but only about 0.1-0.2V going to the pump while cranking. I swapped the relay with the A/C relay since it was the same and get the same results. Could the computer have crapped out on me? Is there anything else I can check to see if the ecm is bad? __________________ '83 Silverado Step Side '85 GMC short bed. '86 Chevy Step Side Custom Deluxe LQ9/4L80 swap (SOLD) Last edited by Tilt; 10-02-2015 at 05:11 PM.

10-02-2015, 02:39 PM	#2
Heavymetl Registered User Join Date: Feb 2012 Location: Southern Ohio Posts: 292	Re: No voltage to fuel pump I've yet to see a bad GM PCM. I know there out there but that's the last place I'd look. Just check with a meter that the PCM is commanding the fuel pump on for prime at the initial "key on" for engine start. As long as the computer is putting out that 12v signal to the relay, the problem then lies somewhere in the other wiring you have for the fuel pump. I typically use large wires for the fuel pump too, 10 guage. Fuel pumps hate low voltage. The relay is supplied the power in my truck and LS swapped Camaro directly from the battery. Make sure you relay has a nice connection and good wiring. Good luck and let us know what you find. Oh yeah, make sure the fuel pump is grounded well too! __________________ 1970 C10 Turbo LM7 5.3/LS6 cam PT7675/4L80E/Holley Dominator EFI/E85 Moser 9" 3.25/4 wheel discs 11.16@121.07

10-02-2015, 03:28 PM	#3
solidaxel Registered User Join Date: Dec 2002 Location: Cactus Patch So. Az Posts: 4,749	Re: No voltage to fuel pump Are you able to "jump" past the relay with 12VDC to see if the pump will run? __________________ 53 TuTone Extended Cab 350 4-Spd 3:08 (SOLD) 53 Chevy Moldy pearl green ZZ-4 4L60E 9" 3:25 55 GMC 1st Black Mll (ZZ4) ZZ6 TKO 600 5 sp 3:73 62 Solidaxle Corvette Roman Red (327 340hp 4spd 3:36) C4 & C5 suspension tube chassis LS 3 4L70E 65 Corvette Coupe 327 350hp 4spd 4:11 78 Black Silverado SWB (350/350) 5.3 & 4L60E 3:42 2000 S-Type 3.0 (wife cruiser) 2003 GMC SCSB 5.3 4L60E 3:42

10-02-2015, 05:07 PM	#4
Tilt Registered User Join Date: Jul 2014 Location: Dayton, TX Posts: 335	Re: No voltage to fuel pump The fuel pump will run if I jump it to the battery. I only have the 12V main going to the relay. On the priming cycle and while cranking, I'm only getting 0.1-0.2V. This is on a new BP Automotive harness and fuse/relay box. __________________ '83 Silverado Step Side '85 GMC short bed. '86 Chevy Step Side Custom Deluxe LQ9/4L80 swap (SOLD) Last edited by Tilt; 10-02-2015 at 05:13 PM.

10-02-2015, 05:48 PM	#5
Heavymetl Registered User Join Date: Feb 2012 Location: Southern Ohio Posts: 292	Re: No voltage to fuel pump .1-.2v where? At the relay out going to the pump or the +12v signal from the computer? If you know the pcm signal is good and the pump runs when jumped to the battery, your issue is somewhere in between, which means old fashioned electrical troubleshooting. __________________ 1970 C10 Turbo LM7 5.3/LS6 cam PT7675/4L80E/Holley Dominator EFI/E85 Moser 9" 3.25/4 wheel discs 11.16@121.07

10-02-2015, 07:40 PM	#7
Tilt Registered User Join Date: Jul 2014 Location: Dayton, TX Posts: 335	Re: No voltage to fuel pump Grounding the PCM changed nothing. I pulled a plug wire and tried my spark tester and no spark. Tried another and same thing, no spark. __________________ '83 Silverado Step Side '85 GMC short bed. '86 Chevy Step Side Custom Deluxe LQ9/4L80 swap (SOLD)

10-02-2015, 06:31 PM	#8
BR3W CITY meowMEOWmeowMEOW Join Date: Sep 2009 Location: MKE WI Posts: 7,128	Re: No voltage to fuel pump Put an external ground on the ECM as well just in case. Usually an ECM dies in one of two ways; COMPLETELY and drastically, or slowly and miserably. There are very few conditions in which an ECM can be damaged but still function normally otherwise. Grounding the case fixes the most common non-critical failure which is the internal ground dying. The next is the injector driver internally dying. The less safety concious among us have just run the fuel pump trigger wire and relay off of the ignition switch instead of the ECM. Its best to have a rollover valve if you do that, but it does work. __________________ *'66 Short Step / SD Tuned / Big Cam LQ4 / Backhalfed /Built 4l80e / #REBUILDEVERYTHING* MY BUILD THE H8RDCPTR //\\ MY YOUTUBE CHANNEL REV J HD

10-10-2015, 01:14 AM	#9
BR3W CITY meowMEOWmeowMEOW Join Date: Sep 2009 Location: MKE WI Posts: 7,128	Re: No voltage to fuel pump Thats maybe only the 3rd ECM I've seen be the root cause, so you really got luck of the draw. 99% of the time when people think the ECM is dead, another problem is to blame...but it seems like you actually hit the 1% __________________ *'66 Short Step / SD Tuned / Big Cam LQ4 / Backhalfed /Built 4l80e / #REBUILDEVERYTHING* MY BUILD THE H8RDCPTR //\\ MY YOUTUBE CHANNEL REV J HD

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