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Thursday, February 29, 2024

Doug's Domain

Doug Vetter, ATP/CFI

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Sunday, December 12, 2010

HVAC Controller Gremlins Return

Way back in 2003 I replaced the HVAC controller after it exhibited progressively more frequent symptoms and then failed entirely. At the time I had the dealer diagnose the problem and the solution was predictable: I spent $325 on a new controller and $80 in labor installing and coding it.

Yesterday morning I decided to do some errands so I hopped into the car and turned the key. At that instant I remembered I'd forgotten something in the house so I went back in to get it. Not a minute later I walked back out to the car and stopped in my tracks when I heard the aux fan spin up. Knowing full well that I'd just started the car and the temperature was in the "balmy" 40's I knew engine temperature alone could not explain why the fan was running.

I got in the car and quickly realized that the HVAC controller display backlighting was not functioning. I could barely make out the blower fan speed and tried to adjust it. Nothing happened. I then tried some of the other buttons and realized that those didn't work either. Since I write software for a living and am all too familiar with the three finger salute I turned the car off, waited a few moments, and then restarted it. Not surprisingly, the controller returned to life.

In any case, I'd seen this movie before. If left alone, the problem would likely worsen over the next few weeks and the controller would eventually stop working entirely. That's an inconvenience in warm weather but a serious problem in winter so I let my fingers do some Googling.

A quick check of my parts supplier revealed that the controllers are now $440 and have a $200 core charge. I know the core charge did not exist in 2003 because the dealer gave me the old unit back and I (foolishly) threw it out, thinking at the time that the unit was unrepairable and I would never be in a position to use it in again anyway. After all, who would have guessed I'd keep this thing thirteen years?

Fortunately, this time around I have an arsenal of knowledge available to me. As it turns out the problem is caused by a failing (or failed) capacitor used in the controller's power supply filtering circuitry. There is risk in attempting the repair of course, but as I see it the worst that will happen is I'll have to spring for a new controller and the related coding charges. If I succeed, however, I'll wind up saving at least $500. That's enough of an incentive to leverage my knowledge of electronics as well as my soldering equipment and technique.

The replacement part is on its way from DigiKey and I hope to complete the repair next weekend. Naturally, I plan to thoroughly document the process for the benefit of all.

Mileage: 201100, Parts: $10