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Old 11-28-2020, 10:28 AM   #1
jessemthompson
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Weird electrical problem

I have a gen4 5.3 from an 07 suburban in my 72. The other day driving down the road in it and the truck turn off. I hit the key and it started right back up. Further down the road it did it again. This time it would not start again. I could not hea the fuel pump priming, so I assumed I needed to replace my pump. I had a pump at the house, so no big deal. Anyways, I lift the bed to access my tank. I check the power wire with the key switched on and have nothing. So I go to my fuse panel and my fuse is fine. I check my relay and I have constant power. I hit the key and the pcm doesnít seem to be sending power to the activation wire (green/white stripe). So I disconnect the plug that runs from the fuse panel to the pcm to check the connection and when I replugged it in, the fuel pump primes! So I check my wires to make sure no pins are loose and they all seem fine. So with the key on, anytime you disconnect and reconnect the plug, the pump primes. Also with power on when I take out the pcm power fuse and plug it back in the pump primes. But cycle the key to the truck and the pump doesnít prime.
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Old 11-28-2020, 12:26 PM   #2
ls1nova71
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Re: Weird electrical problem

Sounds like your ignition power wire that powers up the ECM isn't getting power. Have you checked that wire? Could also be a bad ignition switch if it isn't putting power on that wire.
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Old 11-28-2020, 11:37 PM   #3
lutronjim
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Re: Weird electrical problem

Kinda/sorta sounds like my problem where when the key is is 'crank position' PCM isn't getting power.
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Old 11-29-2020, 11:57 AM   #4
truckdude239
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Re: Weird electrical problem

check your grounds i had same issuse mine was ground problem on back of the cylinder head
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Old 11-29-2020, 06:41 PM   #5
jessemthompson
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Re: Weird electrical problem

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Originally Posted by ls1nova71 View Post
Sounds like your ignition power wire that powers up the ECM isn't getting power. Have you checked that wire? Could also be a bad ignition switch if it isn't putting power on that wire.
Ignition switch fixed the issue. Back up and rolling.
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Old 11-29-2020, 06:55 PM   #6
jessemthompson
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Re: Weird electrical problem

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Originally Posted by truckdude239 View Post
check your grounds i had same issuse mine was ground problem on back of the cylinder head
When I did the swap in 15, I made sure I grounded everything very well. This was the first issue Iíve really had since the swap.
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Old 12-01-2020, 12:41 PM   #7
truckdude239
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Re: Weird electrical problem

i drove mine abour 300 miles started having random cut offs broke down like 3 miles from a friends shop not restart cosated into had them check it out turns out i neve rbolted the grounds to back of the head still havent figured out how i made it that long
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1971 Chevy c-10 under going a 4.8l LSx swap

Build Thread http://67-72chevytrucks.com/vboard/s...d.php?t=421305

2007 Honda Accord my daily 145kmiles
2002 Honda Accord 4 door With 330k(sisters car)
2005 toyota Avalon 228k( brothers car)
2002 Sububran 5.3 245k
2000 Tahoe 5.3l 378

General manager for Marco's Carwash & lube
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Old 12-20-2020, 10:26 AM   #8
AussieinNC
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Re: Weird electrical problem

When you did the upgrade, did you run the PCM electrical feed thru a relay controlled by the ign switch or did you wire the feed directly thru the switch?

The standard ign switch is not designed to handle the current load of the pcm....

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Old 12-20-2020, 06:35 PM   #9
jessemthompson
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Re: Weird electrical problem

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Originally Posted by AussieinNC View Post
When you did the upgrade, did you run the PCM electrical feed thru a relay controlled by the ign switch or did you wire the feed directly thru the switch?

The standard ign switch is not designed to handle the current load of the pcm....

I used an aftermarket harness by bp automotive. I donít believe there is a relay for the power of the pcm. I know I didnít go directly into the ignition switch. Iíd have to go back and look where I tapped in for power. Itís been 5 years ago.
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