Wire to draw PCM power

[Gordonr1973]

http://67-72chevytrucks.com/vboard/s...d.php?t=461018

Hi,

quick question for you all if you have a minute...

I understand that not all wires have power to them when the key is cranking the motor.

I need to find a wire to draw power off during cranking as well as when the key is in the 'on position' for my PCM so my engine will fire and continue to run.

I don't have power in the wire I chose before when the key is in on position.

Which wire did you use to power my PCM?

I got my PCM & wiring harness from PSI Conversions and their instructions & diagrams are pretty weak.

I think that they expect me to supply the source wire (maybe one that is always on) to the relay in their supplied fuse block and the relay will switch it into two power sources....one always on and one one with ignition??) I'm not 100% positive but I will do some investigating today.

Thanks!



(this below is a msg that Ben sent me to help out and my response if it gives a bit more info to narrow down stuff)



Thanks Ben;

I'll look again for a distributor wire I can use....I rewired my truck with a Painless harness and it's been a battle to get things right; next time I will rewire myself using the existing stock harnesses for the truck & the LS motor.

Do you know which PCM pins these two sources of power are going to the PCM?

I got my PCM & wiring harness from PSI Conversions and their instructions & diagrams are pretty weak.

I think that they expect me to supply the source wire (maybe one that is always on) to the relay in their supplied fuse block and the relay will switch it into two power sources....one always on and one one with ignition??) I'm not 100% positive but I will do some investigaing today.

I can get the motor to crank no problem and there is fuel pressure to the rails but I just can't get it to fire and I'm pretty sure this PCM power issue I'm having is the issue to fix.

Gordon



Gordon

Gordon, The large purple wire should only have power during cranking and is only used to activate the starter. On my swap, I did add a relay to the starter circuit and used the truck's purple wire for + and the neutral safety switch for - to activate the relay. The relay I added is unnecessary for for normal operation, I just did it to take the load off of the ignition switch because I've seen a few melt and to make use of the LS neutral safety switch.
You probably know this, but the PCM needs two sources of power, constant which should always have power as long as the battery is hooked up and Key on power. For "key on power" for I was able to use the wire that originally went to the distributor. The distributor wire triggers a relay which the supplies the PCM and everything else on the engine with power.

Hope this helps, Ben