Need help on 94 ranger either short or bad alternator or something else

[franklin2]

So it runs with the belt off, but alternator connected?

At this point I'm going to suggest removing the PCM, taking the cover off and getting a good look at the electrolytic capacitors on the circuit board. Look for swelling or leaking of the barrel-shaped caps, if they leak there will be a white stain on the circuit board.

If this checks ok, then you will be unplugging and checking *all* of the PCM's outputs. First things first though.

If he has a PCM problem, why would it run ok on just the battery?

 

[SenorNoob]

The only way this makes sense is if the alternator is overcharging or has some AC on the output that is causing the PCM to shut itself off with some kind of protection circuit.

 

[dean94]

I don't really see anything wrong with ecm board

 

[pentode]

What's going on with that card edge connector in the second picture? Looks like it's covered in butterscotch fudge. Is that connector used in the truck or is that just for factory diagnostics or something?

I'm just looking for anything/everything abnormal.

 

[alwaysFlOoReD]

I believe that's a port used for diagnostics or a daughter board in earlier iterations.

 

[SenorNoob]

Isn't that the J3 port? Same one used for programmable tune memory modules?

Looks like very old dielectric grease.

But I fail to see how having the alternator disconnected makes anything work unless the alternator is outputting something it shouldn't be.

 

[pentode]

Isn't that the J3 port? Same one used for programmable tune memory modules?

Looks like very old dielectric grease.

But I fail to see how having the alternator disconnected makes anything work unless the alternator is outputting something it shouldn't be.

Yes, I don’t know how they test alternators at Autozone but if it were me I’d really like to get a scope on the alternator output and see if it’s got a big AC component or something, which I believe you mentioned earlier. I’d also like to nail down whether the truck actually does die with 13.8V applied to the B+.

One of these things will surely lead to the problem.

 

[dean94]

What would be best way to test voltage to see if truck shuts off I've been using jumper cables from another running vehicle that was putting out 13.6 at running cars battery. And I don't have a way to check ac voltage from alternator other than a digital multimeter. I can see if the place that rebuilt alternator can check for ac voltage when they open back up

 

[pentode]

What would be best way to test voltage to see if truck shuts off I've been using jumper cables from another running vehicle that was putting out 13.6 at running cars battery. And I don't have a way to check ac voltage from alternator other than a digital multimeter. I can see if the place that rebuilt alternator can check for ac voltage when they open back up

I would say jumper cables from a working, running vehicle to your battery should clear that question up just fine. No need to worry about exact voltages right now, I’m just curious what the behavior is.

 

[dean94]

hooked jumper cables up to it while it was running it just died. While jumper cables was hooked up tried to restart it it just backfires every so often it may decide to fire up for a split second and just die same way with alternator hooked up mainly backfire but once in blue moon it may fire up for split second and just die and back to backfiring