Greetings! I've got a weird issue with my truck charging system. I've gone through a couple of alternator in the last year. Now, even with a new one it won't charge. I have 11.9v on the trigger wire when the truck is running with the batteries at 12.1v. It seems as though the truck is not triggering the alternator. I understand the ecm controls the alternator with that one wire and supplies a voltage that determines how much current the alternator should put out? What do I need to feed that wire to trigger the alternator to charge? I've read both 5v and 12v through a resistor. I ran 12v to the c terminal and that didn't seem to trigger it like I've read elsewhere. Thoughts?

 

When you took the alt to the corner parts store for the free test, how did it perform?

 

They said it was fine. I ran 12.6v (battery voltage) to it when I got home and it charges. Must be the ECM not sending the right voltage? Or something else in that circuit messing the voltage up?

 

OK fine... what were those voltages while they were testing it?

12.6 on the output terminal is LOW. LLLLLLLLLLLLLLOOOOOOOOOOOOOOOWWWWWWWWWWWWWWWWW. Should be more like 14.5 or somewhere around there. If it did 12.6 on the "bench", it's NOT "OK". It's a POS.

​​​​​​​And the voltages were ... ???

 

I'm not sure what it did on the bench. I think it's just a computerized test where it says good or bad. Something easy for the employees.

When I got it back I jumpered it off the battery (12.6v) and it was putting out 14+ volts. The ECM is sending 11.9v to it and that doesn't seem to be enough.

I guess I'm wondering what strategy the ECM is using to control the output. Does the output ramp up as the ECM increases voltage on that wire? Or is it sending a waveform with a duty cycle signal to control it?

 

It's called the "Field Duty Cycle" wire, of that helps any. I don't know, but my guess is that it's pulsed to maintain a certain amperage output.

 

That's pretty much it. The alt is energized only part time, switching rapidly between on and off, according to the electrical system demand (including charging). 11.9V seems reasonable.

I'd take it back.

If possible, get the higher output one that came out in some later year, maybe 04 or 05 or so. It bolts right up, except that it might need the next longer length belt. The option code for it was KG3. It has something like 40 or 50% more capacity. Near 150 amps instead of 105 or thereabouts.

 

It has a 240amp one in there. The one before "died" in one month and this one never worked. Starting to think it's an ECM issue. Ugh!

 

Not bloody likely. Not "impossible" of course; just, not likely. I'd exhaust all other avenues of thought before jumping to that one. Broken wire, unplugged plug, loose connection, burned up connector, that sort of thing. "The simplest explanation that fits all the facts is most likely to be the right one".

 

Occams razor style?! There are a couple other issues is why I'm starting to think ECM. If I jump the alternator it works and also the O2 sensors are doing some weird crap. I'm going to try a 3rd alternator, ground it with a wire, replace the 02 sensors and pull plugs and wires before I go ECM definitively. It worked perfectly for years prior to me putting a turbo right next to the ECM. It's heat shielded and wrapped but who knows. Regardless, I appreciate any and all input. Thank you!