9th July
written by Todd Harrison

In this video I diagnose and repair a failed HVAC system. My sister’s heating and air conditioning unit failed to be able to start the blower motor (indoor fan). The AC compress seemed to be working fine but without the blower the  unit eventual would turn off without any cooling. The fan motor just didn’t seem to be getting power. I did quickly check that the motor turned free and that the run capacitor tested fine at 7.5uF on the motor. I then checked voltage at the control board and there was no 120 VAC for the fan’s high speed (cool) output connector. When I jumper-ed the relays that control the fan it started right up and the whole AC system was working fine. This means something on the board was not controlling the fan relay correctly. I did short the Red and Green 24 volt control lines just to make sure there was not a control line problem but no luck. I then tested the output from the PIC micro controller and I was not getting the pull to ground from the darlington transistor array IC for the fan relay, however, the PIC seemed fine otherwise. I replaced the darlington transistor array IC because it did have a failed output on the very leg which controlled the fan relay. Unfortunately the PIC output pin for this control must have also been damage because the replaced darlington transistor array IC did not fix the problem. I was able to order a used but known good board off eBay for $75 which did fix the problem so the PIC was for sure damaged. I was into the unit so far I decided to ordered a new blower motor and run capacitor being I know those items where ~20 years old and the motor was acting a bit rough. The extra work wasn’t needed but it was easy enough to do while I was into the unit and ordering parts anyway.

Lennox reply letter video – also see photo of letter at the bottom of this post.

CLICK PHOTO for gallery view and click a SECOND time for hi-resolution image. Click thumbnails on lower right and lower left of gallery to navigate gallery photos.

Lennox reply letter.Still001

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