Alternator, regulator, ECU & check engine light issues

[o2bnmud]

So yesterday I was trying to determine if my alternator or regulator was bad (Large case G1 alternator with electronic regulator), so I unplugged the field wire from the alternator while it was running to see if there was a change in voltage at the battery.

There wasn't, still showing only 12 volts. So that led me to believe it was the alternator and not the regulator. Since it is a large case and I have a 351W, it's a pain to get out so I was hoping it was the regulator. After swapping alternators, as soon I hook up the negative cable the check engine light comes on. I turn the key on, light goes off and it turn over fine but won't start.

I don't hear the fuel pump turn on when I turn the key, so I checked all the fuses, the fuel pump and ECU relays, and the inertia switch. All are good. Hook the battery back up, same thing as before. I try it again with the alternator disconnected, still nothing. I figure it must be something simple because everything else was working before this. I double/triple check my connections and all looks good. I try to manually jump the fuel pump from the test plug, nothing happens. I plug in my code reader (OBDI), and it just shows the 000 display and won't even start the KOEO test.

This morning I plug in another computer, and I get exactly the same results. So now I'm thinking it must be a short in the wiring somewhere? I'm really just starting to troubleshoot more, but I figured any insight from the collective might help save me time.

A little more about my setup: 351W with 87 mustang MAS computer, this has been installed and running for about 16+yrs. Added a custom chip from Gary Vance a few years ago. C4 trans. Large case G1 alternator with electronic regulator.

Thanks,

Tony

 

[Timmy390]

Verify you have power to the ECM is where I would start. Then verify ground....

Tim

 

[Broncobowsher]

Key on, disconnect the field wire, measure for voltage on the field wire. no voltage, alternator is probably fine as it isn't getting a signal to make power.

No voltage (what I suspect) check at the regulator. I can't remember right now, but there should be one terminal that always has 12V (the direct to the battery sense wire) and a switched wire that turns on with the ignition. This one I suspect isn't turning on. As well as the switched power to the ECM. Or this may be backfeeding the computer.

It's really hard to guess how someone else wired something in. Factory wiring is confusing enough. But add in someone else's idea of how things should work and it gets even crazier.

 

[Steve83]

Post LOTS of good pics of the battery, its terminals, the other ends of each battery cable, the alternator, both ends of its charging cable, its voltage regulator & wiring, the body ground, the frame ground, the starter relay, the fuse block (front & back), any splices to the factory harness... The more you show us, the more likely we can offer useful suggestions.

It would also help us help you to see ALL the truck's details in your signature, so we can review them without having to scroll back & look for them.