PCM issue?

[siirial]

I got a 2006 Yukon XL Denali around 87000 miles a couple weeks ago and have been trying to work out a charging system issue.

Initially, it had a bad battery when I bought it, so I picked up an optima red top. On the drive home, the volts dropped to where it was running off the battery. I stopped and tried starting it and the volts were normal, so I continued home.

Over the last week, it's been giving me the same issue, starts up with good voltage, then drops and runs off the battery. This told me the regulator is faulting, so I picked up a new alternator. Worked fine for a day then started the same issue. Went and got another new alternator. Same thing. Good for a day, then bad. Tried 2 different remans, with same results.

So I figured the optima gel style battery just wasn't going to work with my system, so I put in an old battery from my old truck. Worked for a while then same deal... Starts up normal voltage then back to running off the battery.

Since I seem to get some time with it working well after the battery is disconnected to swap alternators, I'm thinking that the PCM is discharging and resetting. Then after some time it starts to malfunction.

Am I in the market for a new pcm? I don't know what else to look at. Battery, alternator, and starter all test out Ok (when first started) and the ground wire from the battery looks Ok with no visible corrosion.

 

[Sasquatch]

Have you put a meter on the battery to check if it`s charging that way? I ask because the gauge stepper motors are famous for dying on our trucks and yours could be giving a false reading. Before you check it that way you may be chasing a problem that doesn`t exist.

 

[siirial]

I checked voltage at the junction block grounded to the alternator and saw around 12 volts between the first and second alternator change.

I have a Bluetooth odb adapter and it's reading 11.9 volts when the gauge drops, which tells me I'm running off the battery...

This thing is really bugging me, especially since I just got it.

 

[siirial]

So the drive home from class, I watched the scanner vault reading. Initial voltage kicks off at 13.3 and rises to 13.5 within a minute. Then a minute later, it starts to lose a tenth roughly every 30 seconds until i pull over to shut it off and start it up again.

That really sounds like the regulator, but why would the regulator keep going out so soon on new and reman alternators?

 

[siirial]

I brought in the alternator to test it out, and it tested fine on the bench test. 13.3 volts. The guy did another warranty exchange for me. Things went great... for the first run at idle for 20 minutes, then the 3 minute drive to pick up my kid from day care, it started to show signs of the voltage dropping as I pulled in. In the 10 minute drive to the grocery store from there, volts across the alternator were down to below 12.5. I am losing my mind.

 

[siirial]

So I've learned about the battery generator control module this morning, and low and behold, driving with tow mode on or my headlights on, voltage stayed over 14v.

Is there any way to check if my module is going bad? I've had MAF related codes thrown twice when the volts were low due to "fuel economy mode" and I understand that codes like that can happen when voltage is too low for the sensor to read correctly.

 

[afpj]

Have you checked the battery connections and ground connections? The SAVRM (batt control module) works be detecting current flow, so if connections are not tight, it won't work right. THat's the simplest thing to check. Do you see that control module on the negative side cable somewhere?. Should look like it's clamped to the negative batt cable.

 

[siirial]

Yeah, I was curious what that was then I came across information on this system. Connections look good and tight.
I've been driving the last couple of days and no problems or engine codes, just the lower reading on the volt gauge than I'm accustomed to. Will take a bit but I think I can get used to it. I may take out the MAF and clean it when I change my air filter since the code it was throwing was related to the MAF and then would get a lean condition.

It would be nice to be able to test that it isn't malfunctioning.

Also, I have a new battery, but when I put it in, it wouldn't start. The engine would turn over and begin to start, but would cut off right away like the theft lock was triggered. I double checked post tightness and they were secure, so I don't know how I triggered the theft lock.

 

[retiredsparky]

You can have a battery cable that has corrosion inside a crimp connection. I have found bad cables several times over the years when batteries and alternators have been replaced with no success (much like your situation).

 

[siirial]

You can have a battery cable that has corrosion inside a crimp connection. I have found bad cables several times over the years when batteries and alternators have been replaced with no success (much like your situation).

Is that something that can be tested with a meter for differences in voltage at different points from the alt and battery?

What is an expected difference in voltage between alternator and battery posts. If I remember correctly, I saw a couple tenths less at the battery.