Blower Control Module Question

[SoCal]

My 95 ATX heater/air blower suddenly became stuck on full, regardless of EATC setting, including OFF. Searched the threads here and concluded I needed to replace my Blower Control Module.

I then purchased and installed a used control module. This one works fine, then intermittently jumps to full fan, then back to working fine again. This happens on any EATC setting, including OFF.
My guess is the used blower control module is bad too (can they fail intermittently?) Or is there something I’m missing?

Any help is appreciated.

 

[hawkeye18]

The BCM (which I love, because in the naval aviation world BCM stands for Beyond Capability of Maintenance - i.e. itsa broke) is basically a device that provides a varying level of resistance on the ground side of the circuit. Off = open, and full = short. There is a large transistor whose purpose is to provide the necessary current handling capability to support the motor on full blast. This is the failure point for most BCMs.

Damned transistors. The P-N junction has been the greatest advance in electronics in the last 1,000 years but it has also given us electronics troubleshooters fits, as they are prone to failure for no good damn reason.

I work in the miniature electronics repair field (circuit card level and smaller) and it is widely known that if a circuit card comes in bad, there is nearly a 75% chance that either an IC chip, transistor or (less commonly) a diode is bad. usually it's an IC; complex electronics and catapult shots don't mix.

Anyway, there is quite a good chance that the replacement BCM also failed. It is happening a lot more recently (past year or two). Perhaps the specific model of transistor used is reaching the end of its lifespan? I have tried to identify the part using FEDLOG but met with no success. The only hope for these units in the long run is to find an electrically identical replacement part. Kinda makes me wish I weren't stuck in Iraq!

 

[llamaking122]

i dont mean to thread jack but does anyone have a pic of where it is located in the car. i couldnt find it. but then again i really didnt know what i was looking for XD

 

[hawkeye18]

i dont mean to thread jack but does anyone have a pic of where it is located in the car. i couldnt find it. but then again i really didnt know what i was looking for XD

Stick your head under the passenger side footwell and look straight up (your back will hate you for this, you might want to move the pass. seat all the way back). You will see two little rectangular tabs on posts that seem to be holding a plastic shield up (they are). Use a pair of pliers - or, if you're brave, your fingers (they're sharp!) - and get those tabs off. The plastic piece should now lower down. And pretty much that's it. It's right there. Removing it is a weee bit more complicated.

The plug for that little guy is behind the glove box; you need to lower it all the way down (squeeze the sides of it where the rubber stopper posts are so they come past the frame and it swings down) and find the two large connectors on the left hand side of the gaping maw. If you look up and towards the front of the car, you can see the white bracket that holds the ECM in place - this is where you put a tweecer or chip. In any case, one of those two plugs on the left goes to the BCM. wiggle the wiring harness to figure out which plug it is. Remove that plug and route it down. The plug can be feisty; just work at it.

Then you have to remove the hose from the module. This should be pretty simple. No, there is no hose on the other plastic tube. Don't worry about it.

Now, use an 8mm socket to get the two retaining bolts out, and... voila! As you pull it (straight) out, you will see the electronics, which are inside the plenum and in the path of cool air. When mine failed, I ascertained that it failed because it seemed to have gotten covered in wet sweater fuzz, and there was a VERY strong "burnt electronics" smell. It was also popping fuses and the smell was coming through the vents. Yeah, the transistor failed in spectacular fashion - it had arced and blown up! Sweet.

Anyway, installation is pretty much reverse of removal. The hose only clips onto one of those tubes so don't worry about it.