Strange - Injector works WITHOUT alternator connected!

[autowizard]

My wife has an 86 cherokee with the 2.5 tbi. We have been having driveability issues so we replaced cps and set the tps with a scanner and ran a diagnosis useing a snap on scanner.
So when i tested the fuel injector the scanner said to unplug alternator if vehicle runs replace injector, if vehicle dies injector is ok. This runs like a top with alternator unplugged and it loads up and dies with alternator plugged in. I ohmed it and it is within spec.
So the question is, what does the alternator have to do with the injector?

[msmoorenburg]

AC signal coming from the alt causing issues would be my first guess

[Mudderoy]

AC signal coming from the alt causing issues would be my first guess

So a bad regulator or battery could be really low?

[autowizard]

The alternator checks out. No shorted wires i ohmed all the wires from the pcm. I didnt want to replace injector because of the price but the scanner insists that is the problem, just cant understand why unless its shorts out when the alt kicks in?

[msmoorenburg]

The alternator checks out. No shorted wires i ohmed all the wires from the pcm. I didnt want to replace injector because of the price but the scanner insists that is the problem, just cant understand why unless its shorts out when the alt kicks in?

It checks out doesn't mean it isn't kicking out a ac signal (known issue in the 70 with the whine in the radio) Add the whine to a computer and its gonna play games with everything else.

[autowizard]

Ok thanks for the reply guys im gonna try my spare alt and see what that does.

[autowizard]

I tried 3 different alternators all the same plug them in and injector loads up and kills engine. So im guessing bad injector. Im a diesel mechanic by trade and i have never seen this on anything i have worked on in the last 15 years its strange. Must be a renix thing?

[4.3LXJ]

I'm with you , strange

[LizardRunner]

definately strange but now try replacing the injector and see if that fixes the prob. It obviously isn't the alternator.

[autowizard]

Update i replaced injector ran great for twenty minutes then right back to where i started so at this point im gonna try a new pcm as this is the original one that came with the jeep in 86. If that dont work im gonna find a carb and eliminate all the probs lol

[autowizard]

Another update, bought a reman ecm from oreily and its bad no spark from ecm grr now gotta take it back and order another ecm from someone else. A carb setup is looking good at the moment...

[autowizard]

Ok after 4 supposedly rebuilt ecms from both oriely and autozone i finaly got a good one and jeep runs awesome. Thanks for all the replies and if anyone has a similar problem i would recomend buying a new ecm not a rebuilt one lol.

[4.3LXJ]

I have had the same thing happen with "rebuilt" ECMs. I am convinced they just get cores from anywhere and hope they work. The one I got had been hot enough the motherboard was discolored.