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I am getting all kinds of check alt messages, abs error and air bag lights. New battery so I am thinking about replacing the alternator but what chance could it be the ECM.
 

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Run the idle about 1,000 RPM and check your voltage at the battery. You should be near 14 volts. If it's closer to 12V your charging system isn't working correctly and then it could be the alternator. The ECM (engine control module) does not play into it.
 

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2006 models have the voltage regulators as a part of the ECM, 2004 models dont, hence yes there could be a problem with the emc on mine. just dont know how common it is to go out. Would suck if it was the ecm
 

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I also stand corrected and also deleted the post that contained the misinformation. I took that quote directly from the GM Service Info site. I guess I would have found out when and if I had an alternator/regulator problem. Something I haven't yet experienced the joy of on my 2005. But rather than the ECM, Fox Mulder says that the field control is done by the PCM.
 

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I'm currently having issues with mine as well. Random ABS fault/ loss of gauges, very sporadic. Come to find out when this all happens my voltage spikes to 17.2 volts, and verified at the battery terminals with a DMM. Alternator issue or crazy computer issue? Over the last two days this happened, the only time I was able to keep it under some sort of control was to run all accessories while driving it home. Also battery appears to be spewing water from overvoltage. Battery is one year old.
 

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Nothing worse than a dead thread of unresolved issues so.....
I took my alternator out and took it into the rebuilder. He told me he couldn't find a replacement part for the alternator for the voltage control. I said I thought it was computer controlled. He says yeah, but there is still some regulator on the alternator itself to receive instruction from the computer. He bench tested it and couldn't find anything wrong. I went ahead and replaced the alternator with a new OEM one from rockauto for 200 bucks. So far, so good. Have not had a single issue yet, been a week. There still may be an underlying cause I'm unaware of yet, but it does seem to have fixed it so far. It was a pain to get the alternator out and back in. Yes, you do have to take off the power steering pump and disconnect the hard line going to the pump. Also the fan on the right side will have to have the bolts taken out and move the fan about as you wrestle with the long bolt coming out of the alternator. Topped off power steering fluid, filled and burped the coolant, and all is well. It actually went pretty smooth, and only took a couple of hours taking my time.
 

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you can just pull the ps return hose and fanagle the alternator out that way, without taking the pump and hard line off. did have to move the fan shroud and wrestle with that around the upper rad hose.
 
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