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Truck running rich when alternator plugged in


speed_demon

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My truck runs rich when the alternator is plugged in. When plugged in I get 14.5v at the battery and the truck runs rich. 10% CO and HC ppm to be exact. It runs good but not as crisp as it should. When I unplug the alternator field fuse and/or unplug the alternator connections it runs much better and you can't see even a hint of exhaust.

Any ideas other than "you have a bad alternator"?
 


MAKG

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I doubt this is a bad alternator. It's output is a little high, but not ridiculous. Check power and ground to the computer, and run self-tests.
 

speed_demon

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Welp the truck passed emissions today. It was consistantely 9.9% (1% IM limit) with the alternator plugged in and then .04% (idle) and .52% (2500) CO with the alternator field fuse unplugged. The hc ppm dropped from a consistant 680 (750 IM limit) to 114 (idle) ans 46 (2500). I ran the self tests both running and not. With the alternator field fuse plugged in I got a 42 rich code, and without fuse got no codes. The IM tech said he had never seen anything like it. This seems to be on par with the rest of the problems. Odd and hard to diagnose. I'll never buy a 208k vehicle again. Oh yeah and after I bought it I found out it has a reconstructed title. I should have junked it but oh well its sorted out now.

The alternator passed a non-load test but I still think it's the problem. When cold the voltage bounces around. When warm the voltage is stable at 14.5 but the truck continues to run rich. It could be a short in the alternator wiring circuit but I think more likely an internal short in the alternator. When I pulled the alternator apart it the brushes and slip ring were hanging on by a thread and I think this may be the original alternator (208k miles).

I would however like to get a wiring diagram for that circuit if anyone has it just in case the alternator doesn't fix it. If anyone has one that would be great!
 

AllanD

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Bad rectifier diode.

It is causing a serious AC "leakage" and the inverse voltage spikes are "confusing" the computer.

replace the alternator.
 

MAKG

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I shied away from that answer because the smoothed output was reading high, rather than the usual low one gets from one fried diode in the rectifier (diodes almost always fail to dead shorts, so the output is zero while that particular diode is energized). You've fixed a lot more than I have, so I'd be interested in the logic here.

A single fried rectifier diode will often barely charge the battery. This one appears to be charging just fine.

I was thinking that one of the charging circuit lines was bouncing around with intermittent contact. The whole charging circuit is disabled when the field coils are disconnected.

FYI, speed_demon, "bouncing around" is often hidden by long integration times for digital voltmeters. It's more visible on an analog meter as a vibrating needle. Best is a DSO, but those are very pricey.
 

AllanD

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The inverse spikecan easily do weird (often counter-intuitive) things to the alternator because the regulator will try to compensate....

I'd take a digital mltimeter and set it on AC-Volts range and measure the alternator output voltage at the output terminal stud.

any voltage greater than 0.1VAC indicates a problem
at around 0.3VAC weird shit starts happening.

I wouldn't be a bit suprised in this guy's case to see 2VAC or more
tnd that's enough to start generating false signals in the engine
management system, false PIP, false EGO readings (as the voltage
spike travels on the ground side of the HEGO, etc)



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