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Related or not?

Condition, cause and correction—connecting the dots
Friday, November 6, 2015 - 09:00
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The B2300 epic

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In regards to our focus vehicle, the maintenance man who owns the Mazda pickup in question told me that as he drove, his A/C airflow would gradually decrease to the point that, while he could hear the fan, he had no air at the register. I asked if the air was going north into the defrost vents, and he promised to investigate and report that piece of data the next time he drove. At our next liaison, he reported that there was no airflow anywhere when the registers died.   

Let me pause here to say that he usually mentioned this concern in passing as he was making his rounds in the shop while I was preoccupied with other tasks, and without giving his concern too much thought I initially theorized that he had a problem with loose leaves in his evaporator case. We have that problem in these parts. Perhaps the leaves or trash would settle away from the evaporator when the truck was sitting fallow and then be picked up one or two at the time until the airflow was mostly blocked? That theory was really kind of weak, and I should have been thinking more clearly, but any time airflow goes away and the fan is still spinning, it’s a foregone that there’s something going on with the evaporator.

A week or so later he told me he was having to jump the little truck off in the mornings. He’s an industrial electrician (a good one) and he checked the battery voltage each time he had to jump the truck off; it was always in some state of discharge, usually below ten volts on his meter, and then one day I had him stop the truck long enough to have a look see at it myself. He parked the truck outside one of my service bays, and I dug my personal meter out of a desk drawer, set it on amps, and we connected it in series with one of the battery cables. I explained that we’d need to let it sit there with the meter connected for at least 30 minutes to let all the boxes charge up their capacitors and whatnot, and so we did.

When the appointed time had expired, I walked to the truck and still found 2 + amps flowing. He was passing by on his security patrol on the golf cart and stopped by to compare notes just as I was getting started. I explained that we’d need to start at the under hood fuse panel, and so he reached and took the cover off – and for a second, the amperage deflected to a lower reading, then returned to just over 2 amps.

We started removing the larger fuses while watching the meter, and finally worked our way down to fuse 25, which feeds the secondary side of the A/C clutch relay, and we heard the telltale “click” of the compressor clutch disengaging. I reinstalled the fuse and heard the “click” as A/C clutch snapped back in, and then removed the clutch relay to see and hear the same disengagement. After that it was as simple as moving the washer pump relay to that relay position and verifying that the clutch worked right.

While we were thus engaged, I had one of my guys removing the blower so we could look in the evaporator case for trash, but now we fully understood why his airflow was going away. If the compressor relay coil is de-energized on this one, the compressor is supposed to drop out, but with the relay contacts welded together, the compressor just kept on refrigerating regardless of the PCM’s command until the evaporator became a block of ice.

After the fact, it was abundantly clear that his symptoms were related. We just didn’t know how until the smoke cleared and the ice melted. Conditions: airflow and parasitic drain. Cause: stuck relay contacts.  Correction – obvious.  

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