Hello,
I've been lurking for over a year now looking up various solutions to all the problems I've found in my 1999 "trail"Blazer. While I've managed to replace the heater core, drivers side door (pins were too oval and ate 3 sets of replacement bushings), weld up the completely sheared door striker, replace the door panel with one with an intact door handle AND replace the window regulator and motor with help from this forum, I haven't found a solution to the following problem. Not long after changing the heater core, I installed an aftermarket CD player, and it does not shut off when I open the Driverside door. It shuts off when I open the other 3 though. Even after replacing the entire door, it won't shut off until it times out. Same with the interior lights until the MOST pressing problem started. Maybe 3 months after the Heater core, all hell broke loose around the same time. I lost 1st and 4th gear, shortly after that, automatic headlight (always on) and driving lights (won't turn on) controls stopped working along with the rear hatch release button on the dash along with a slow drain on the battery. It's dead if I don't drive it for 4-5 days. As far as the automatic 4WD/2HI/4HI/4LO, well they were iffy when I bought it. It had well over 200K on everything when I bought it and needed a lot of little things like full tune up, short interval oil changes (I thought the HG's were blown but it was just that bad from previous owner), new tires, the door and striker, radio, and super scrub down inside and out BEFORE the heater core. My question is: I have the solenoids now but should I be looking for a pinched wire or bad ground to the PCM before dropping the pan? I'm 90% sure the truck wasn't on a good service interval and it's going to be sketchy even if I save the tranny fluid and add a qt of lucas trans fix. Is there a chance I lost 1st and 4th due to a bad wiring connection dating back to my Heater Core change? The body is straight and rust free and aside from the dashboard cracks and carpet, I've cleaned the interior up to a pretty nice point. Certainly nice enough to haul the dog back and forth at least at sub freeway speeds.

 

Wow, over 4 years to find out the solution was: The purple BCM plug got trapped in the dash when I changed the heater core and pulled the entire dash out for all the electrical issues. The transmission is still working fine after saving the 'old' t-fluid and replacing the shift solenoids and a new filter (which had IIRC an 04 mfg date on it), recommend using a 'mud pan' typically used for mixing concrete/mortar that you can find at a big box hardware store to catch the fluid though. Why am I updating a 4 year old post? Because hopefully it helps someone else having electrical issues like mine after a heater core change or pulling the dash will know to look at/for the purple BCM plug for answers instead of throwing parts at the problem or 'just dealing with it' like I did for 4 years! Big thanks once again to Christine for pointing me in the right direction by posting a factory service manual troubleshooting list for the ambient light sensor (one of multiple things controlled by that plug) that led to me finding out that I had 'lost' a BCM plug in my haste to just be done with the heater core swap (outside during winter but still a lame mistake on my part). While I hope you don't need it, if you do, I hope this helps.

 

Thanks for updating your post and sharing what you found to be the solution! I had to change a heater core on mine, and I'll agree that it would be super easy to accidentally pinch a harness like that. I'm really glad you were able to find the problem and resolve it.

 

Thanks for the update. I really appreciate when anyone takes the time to come back and post the solution to whatever issue they were having. Very considerate. Thanks

 

First time I saw this post is today. I was going to suggest pulling the dash off to make sure no wires were pinched or plugs left unplugged. I've offered that advice to a few folks over the years and the last thing they wanted to do was to pull the dash off again. I'm glad you posted up your fix to the problems you had and truly appreciate it. Hopefully, this will help others in the future.

 

That's what I'm hoping at least. It proved to me at least that no matter 'how good' a mechanic I am, I can still forget things when I try to hurry things up. The internet (and this forum) are the modern day equivalent of the group of guys with their head under the hood of a car every day when you go past that you'd ask when something went wrong with your car. I could try and save my ego and say nothing or spill the goods and save someone else the embarrassment. Truth is, I've done WAY more embarrassing things than this and survived! 😁 This is just a small payment back to the community for all the times they steered me clear of bigger embarrassments.