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04 Envoy, 200k+ miles
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OK, now we are on to something! The first thing you have to realize about this vehicle is that everything is controlled by a computer module of one kind or another. The Engine Control Module (or Powertrain Control Module) (ECMor PCM) covers the engine and partially the transmission. Then there is the Body Control Module (BCM) which controls the interior and exterior lights, the door open chime, the seat belt chime, the radio, the HVAC, etc. (it's a long list). Then there is the liftgate module often referred to as the read BCM.

Now, in order for the TrailBlazer to run properly, it must have good clean voltage in the range of 12.2 to 14.0 VDC (give or take a little). AC ripple out of the alternator must be 20 mV or less otherwise it will mess with the electronics. Electrical connectors must be fully seated and tight and clean. Grounds are super important and they must also be clean and tight. Battery cables need to be free of the green grunge disease, and unless you really want problems, please heed the following:

(I know I put this in earlier, but it needs to be stated over and over)
DO NOT DISCONNECT THE BATTERY - REMOVE THE APPROPRIATE FUSE(S). Why? Because when you reconnect the battery, the HVAC actuators inside of the dash are commanded to run a recalibration procedure which stresses the old brittle plastic gears inside the actuators and the brittle plastic gears break and leaves you unable to control where the air comes out, or control the temperature of the air, etc. Replacing at least one of them literally requires the removal of the entire dash! So, if you ever need to actually disconnect the battery, such as in the case of needing to install a new battery, be sure and use some kind of Keep Memory Alive device to avoid the HVAC actuator recalibration routine.

Now, I am not a pro, but I've been repairing cars since the mid 1970s and about the only thing I am hesitant to mess with is an automatic transmission. I'll rebuild Rochester Quadrajets all day long, but other than doing an AT fluid and filter change, I don't mess with transmissions.

Now a couple of random goodies before I forget. There is an important ground in the passenger compartment and it's located under the carpet immediately to the right of the gas pedal where the foot rests. For some reason, it loosens up and corrodes and when it does, it causes havoc. The alternator is controlled by the ECM. Weird unexplainable electrical gremlins such as the windows going down on their own, the radio changing stations by itself, the clock in the radio resetting randomly, dash lights misbehaving, etc., point to an ignition switch going bad. With the ignition switches, it is not a matter of if it will go bad, it's a matter of when it will go bad.

Well, since rodents have been having a feast on your TrailBlazer, I fear you are going to have to endure some nightmarish things for awhile as you track various things down. Now, if you have a high parasitic current drain, the first thing I would do is to pull the fuse for the OnStar module. I'm sorry but I don't know which fuse that is, but I do know you can download a copy of the owners manual from the GM website.

I do have a copy of the factory service manual in pdf form and it is broken up into sections. If you want one, you can download it from the GMTNation website. Here is the direct link: Need service manuals? Get them here!

Make sure your alternator is putting out adequate voltage and current, get a load test on the battery just to make sure it does not have a bad cell.

Good Luck and please don't be shy!
Chem_Man, Thanks for the awesome links from Mooseman/GMTNation!
 
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