View Full Version : Fuel Pump won't work.. but car runs fine with relay bridged?

February 8th, 2015, 02:06 PM
ok so we have swapped an engine & pcm from one vehicle to another... both were LS1B's and pretty much simple straight through swap.

I have re-linked the VATS & updated the VIN number. We can't seem to get the fuel pump relay to engage with key-on like it should.

The car runs & drives fine if we bridge the fuel pump relay coil.

Have i missed something with the PCM change process? Have done pcm swaps before without any issues.

February 8th, 2015, 02:42 PM
just for curiosity, why swap PCM?
The fuel pump relay should have 4 wires, one should have constant 12v, ground, 12v from pcm that activates the fuel pump relay, and the other one goes to the positive side of the fuel pump.. check those with a multimeter, your constant 12v and 12v output to the fuelpump should be ok.. check for one ground, and check for 12v when you open the switch,,,, one of those is your problem,

February 8th, 2015, 02:44 PM
Pin #9 dk green/ white on the red/green connector is the one that activates the fuel pump relay, check continuity from this pin to one of the 4 wires thatt goes to the fuel pump relay

February 8th, 2015, 03:17 PM
Thanks 400ss. We swapped the pcm's out as the pcm is the recipient car was locked out with HP and the new motor had been upgraded and would require re-tuning.

February 8th, 2015, 03:20 PM
i see, nice.... try it out, you can check the pin #9 on the PCM, it should have positive voltage when you cycle the key to ignition, if this wire doesnt output this voltage maybe the pcm is bad, but if it does have the signal, check if it goes to the relay. if it does foes to the relay check the ground wire

February 8th, 2015, 03:47 PM
I had 2 x spare pcm's to check and both behaved the same, its like the PCM permission is not happening therefore no power on the relay coil.

February 8th, 2015, 03:50 PM
Fuel pump activation wire will have voltage. It doesnt matter if the vats is on. Vats will turn off injectors pulse. Ithink

February 8th, 2015, 03:51 PM
When you first turn key to ignition this wire should have voltage for 2-3 seconds. To prime the pump. Sorry ifidont explain myself

February 8th, 2015, 04:58 PM
Turn the vats off & see what happens.

February 16th, 2015, 12:03 PM
Yeah i sent him the PCM with VATS disabled originally and when it wouldn't start that's when I went to the owner and tried a spare pcm & also completed VATS re-linking process.
I'll touch base with him and see if he has checked the wiring as above....