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Tuesday, April 23, 2024

Doug's Domain

Doug Vetter, ATP/CFI

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Saturday, December 18, 2010

HVAC Controller Fixed

When we last saw our hero, he was facing a grave threat -- freezing his nads off driving down the highway at 80MPH without any heat because of a faulty HVAC controller. We now continue with the story...

The first indication of a problem with the HVAC controller appeared last weekend so I ordered a new capacitor from my electronics supplier in an attempt to do a component level repair on the unit as opposed to throwing the better part of $500 at BMW for a new unit. I knew I could expect intermittent operation of the climate control system and thus intermittent heat until it was fixed, but little did I realize how bad it would get as quickly as it did.

By the drive home on Monday the controller went from working perhaps 95% of the time to working little more than 40% of the time. Whenever the controller would stop working, the "Auto" button LED would continue to be illuminated and I could faintly make out the segments of the fan speed indication on the display, but no control on the unit worked, the HVAC blower motor did not run, and the servo motors that automatically control the inner and outer doors remained silent. No degree of menacing looks or threats of physical violence with a clenched fist helped matters. Even a few well-placed bitch slaps on the nearby dashboard structure did not convince the unit to behave. It turned on and off with no apparent rhyme or reason.

(Image: Closeup of capacitor changed to fix hvac controller) While dealing with this headache I discovered a couple things. First of all, once the car is car is warmed up, even if the HVAC blower is not operational some heat does flow through the vents due to ram air pressure, but trust me -- it's not much at all. Second, I found I could take advantage of the unit's non-volatile memory that stores the last temperature and fan speed settings. I adjusted the temperature on both sides to 80 degrees and set the fan speed to about 3/4 of maximum so that when the controller was operating it would generate enough heat to warm the interior sufficiently to compensate for the periods in which it did not work. This had the unfortunate effect of blowing lots of cold air shortly after start up but that was a small price to pay for warmth throughout the drive.

While this little "trick" made the car drivable, by the drive home Tuesday I had had enough.The weather forecast was for cold but sunny weather the remainder of the week so I decided to bring the E46 out of its lair. I hadn't driven the car much since the rear suspension overhaul so knew this would be good for the car as well as good for me. Of course, nothing is ever easy. While browsing my favorite weather site during lunch on Thursday I realized to my horror that the forecast had changed and some light but accumulating snow was predicted for later that afternoon...just in time for the commute home.

As I learned many years ago when I foolishly tried to run summer (not all-season) tires on the E36 in winter, a half in inch of snow might as well be four feet: summer tires make the car downright dangerous to drive in those conditions. I wasn't about to lose my pride and joy just to keep my employer happy so I ditched work a couple hours early to grab the E36. It didn't take much to rationalize the move: better cold than spun out in a ditch, right? And good thing too...I had just managed to put the E46 to bed and close the garage door when I saw the first snowflakes appear.

By the time I got back to my neighborhood 15 minutes later we had about 3/4" of snow on the ground and the ABS was getting a workout. I had to stop at the food store on the way so I took the opportunity to grin a bit, turn off ASC, watch out for the Federales (and, for that matter, the light poles), and play around with the effects of reduced friction in the largely empty parking lot. Mercifully, the HVAC controller decided to play nice and I had heat nearly the entire trip home. What can I say? The car doesn't always love me, but I think it respects me...and I can live with that.

Fast forward to today. The parts came in, I pulled the climate control unit, drove home with it in the E46 and proceeded to swap the defective capacitor with a new tantalum part. I ran into a small snag when I realized I'd forgotten my micro screwdriver set back at the garage, but since I knew full well it didn't have the micro (T7) torx driver best suited for this job I just ran over to Eppy's and picked up a nice 41 piece precision bit set by Sunex Tools for a mere $28. While there, incidentally, I learned that Ideal tools bought SK Tools out of bankruptcy. The CEO of the company told Eppy's management that they expect to be shipping again in March and that they were planning to keep the manufacturing facilities in the US. Hearing that I confirmed my commitment to SK Tools, and good thing too....I have a lot of them.

Back at the workbench I took the opportunity to do a little preventative maintenance and replaced the HVAC temperature sensor fan. While not exactly cheap at $125 I felt it was best to do it while I had the unit out of the car. Interestingly, I discovered that the fan lacks a tachometer output, so I know the HVAC controller hardware or firmware is not monitoring the fan's speed. This in combination with its near silent operation would make it damn near impossible to detect a failure. And since stationary DC fans present a load equivalent to a short, I felt it better to replace the fan now rather than risk possible future damage to the controller I was working so hard to preserve.

On the way back over to the garage with the newly repaired controller I found myself a bit on edge because I could not bench test the unit and so I could not be 100% sure the repair was completed successfully until I reinstalled it in the vehicle. Once I arrived at the garage it took me less than two minutes to pop the OBC and HVAC controller back into the stack, reconnect the battery, and turn the key -- to find the controller working as expected. The acid test, of course, was the 20 minute drive home from the garage...the controller never skipped a beat. My feet were nice and toasty, and I took time to bask in the warmth of the knowledge that I'd just managed to save myself $470 in parts and $130 in labor, or $600 total in little more than two hours of work. I'll say it just in case it isn't obvious: that is one hell of a DIY dividend.

Naturally, I took a bunch of pictures of the process and observed at least one thing that wasn't covered in the DIY I followed. Therefore I hope to publish a DIY of my own as soon as possible.

Mileage: 201320, Tools: $28, Parts: $125, Parts Saved: $440, Labor Saved: $130