Fuel pump relay coil voltage issue
Thread Starter
Staging Lane
Joined: Dec 2016
Posts: 78
Likes: 20
From: Walnut Creek, CA
Fuel pump relay coil voltage issue For some reason I’m now only getting 2.54 volts with the key on to the coil of my fuel pump relay. The wire to the coil is green/white strip and comes from the K20 X1 connector, terminal 44.
I did recently replace the battery after it was drained down to 1 volt by a horn button that stuck down (no horn). Otherwise no changes.
Any clues?
I did recently replace the battery after it was drained down to 1 volt by a horn button that stuck down (no horn). Otherwise no changes.
Any clues?
Not sure what kinda car you have, or what the setup is. But alot of the time the ecm will ground the relay coil and its powered by a fuse. Do you Have a wiring diagram for you car?
Thread Starter
Staging Lane
Joined: Dec 2016
Posts: 78
Likes: 20
From: Walnut Creek, CA
It’s a 66 Chevelle and it’s a LFX V6. The ECM is similar to an LS.
There is a fuse between the ECM and the relay. The fuse is good. I also swapped the relay for the fan relay. Didn’t help.
Where I hid the ECM is kinda a bitch to get at but I guess I’ll have to pull it so i can check the ECM pin.
Aarrgghhhh…
Do ECMs fail out of the blue?
There is a fuse between the ECM and the relay. The fuse is good. I also swapped the relay for the fan relay. Didn’t help.
Where I hid the ECM is kinda a bitch to get at but I guess I’ll have to pull it so i can check the ECM pin.
Aarrgghhhh…
Do ECMs fail out of the blue?
Thread Starter
Staging Lane
Joined: Dec 2016
Posts: 78
Likes: 20
From: Walnut Creek, CA
Interesting. Which side of a fuse socket would you consider the ground side? The fuse is in line on the 12 volt signal from the ECM.
Did you mean the ground side of the relay?
Did you mean the ground side of the relay?
Thread Starter
Staging Lane
Joined: Dec 2016
Posts: 78
Likes: 20
From: Walnut Creek, CA
I’m also hearing a very vent noise when the key is on. I’m thinking it’s the ECM. Almost like a very faint ringing of my ears. Very subtle. My younger brother noticed it first.
I did check the fuse socket and one side is ground and zero ohms.
What would the repercussions be if I just used a wire from the ignition switch to the coil of the fuel pump relay? Would the ECM set any faults or?? if it wasn’t turning the pump on itself? This was how I did it on my LS swap in my 67. But the guy who did this harness for me set it up the way it is now, ie, the ECM triggering the relay coil.
I did check the fuse socket and one side is ground and zero ohms.
What would the repercussions be if I just used a wire from the ignition switch to the coil of the fuel pump relay? Would the ECM set any faults or?? if it wasn’t turning the pump on itself? This was how I did it on my LS swap in my 67. But the guy who did this harness for me set it up the way it is now, ie, the ECM triggering the relay coil.
On The Tree
Joined: Nov 2015
Posts: 132
Likes: 17
Sorry, was in a hurry; it's just a bit more complicated than that. What you want to know is if the ground is solid on the fuel pump. You may have already tested that in effect, depending on where you connected the neg side of your meter when you took the low voltage reading. I should have said check the neg side of the fuel pump back to a good ground like the neg post on the battery. If that's good you need to follow the positive lead to it's source.
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What would the repercussions be if I just used a wire from the ignition switch to the coil of the fuel pump relay? Would the ECM set any faults or?? if it wasn’t turning the pump on itself? This was how I did it on my LS swap in my 67. But the guy who did this harness for me set it up the way it is now, ie, the ECM triggering the relay coil.
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Verdad Gallardo Looking at a wiring diagram for a lfx camaro, the ecm sends a signal to the fpcm to turn on the pump. Is it supposed to send 12v, 5v, or 2.5v? Idk the wiring diagram doesn't say, and also maybe the ecm can be programmed to just control a regular relay.
edit, I just found your build thread. i would look at where you had to extend the harness.. check the voltage before the splices (ecm side) then in the middle of the splice,, then after the splice. Then do the following:
Like I said before I would jump 12v to the relay coil and see if it all works correctly. If so you could make a more permanent circuit. Is this a direct injection engine? This is an interesting swap, and a quick Google search tells me it's more common than I knew.
edit, I just found your build thread. i would look at where you had to extend the harness.. check the voltage before the splices (ecm side) then in the middle of the splice,, then after the splice. Then do the following:
Like I said before I would jump 12v to the relay coil and see if it all works correctly. If so you could make a more permanent circuit. Is this a direct injection engine? This is an interesting swap, and a quick Google search tells me it's more common than I knew.
Last edited by Lsxford; Oct 30, 2022 at 10:54 AM.
Thread Starter
Staging Lane
Joined: Dec 2016
Posts: 78
Likes: 20
From: Walnut Creek, CA
I did jumper 12v to the output of the relay and the pump ran fine. I have not jumped 12v to the coil on the relay which would test the relay but I have tried multiple relays and they didn’t help.
I also see that the 465 FP control circuit doesn’t specifically say whether it is 12v, 5v or 2.5v but everything I’ve read so far indicates it’s supposed to be 12v. Plus the guy who did the original harness work set it up for the ECM pin 44 to drive the the relay. I can’t imagine he would set it up to use 5v or 2.5v. In my mind this is WAY to low a voltage for the coil.
I also see that the 465 FP control circuit doesn’t specifically say whether it is 12v, 5v or 2.5v but everything I’ve read so far indicates it’s supposed to be 12v. Plus the guy who did the original harness work set it up for the ECM pin 44 to drive the the relay. I can’t imagine he would set it up to use 5v or 2.5v. In my mind this is WAY to low a voltage for the coil.