D2 A8/S8 (1997 - 2003)
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Each side has its own blend doors, but there is also the heater core valve, or just the HVAC head
Posted by: hyperlogos (20841) on 2018-02-12 19:49:05
In Reply to: Blend door issues? No driver's side A/C! posted by AustinEngy on 2018-02-12 18:18:57

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If you hear weird ticking noises coming from your dash (try low fan and a variety of temp settings) you probably have a bad climate servo. They are a PITA to replace, but it can be done without removing the dash so there's that. Otherwise there is a good chance that one of the solenoid valves in the after-run pump/heater core valve module has gone bad. This is only sold as a unit ($$$) but it's common for just one solenoid to go bad. The HVAC control unit itself is also a common point of failure, usually after a blend door has stuck itself closed because VAG loves to use blend door seals that turn into glue in old age. It's common across all VAG products from what I can tell, but it's not unusual in general. The climate servos are dumb -- they are just a motor and a potentiometer, no electronics, and all the logic is inside of the control head.

The only mode doors in the central unit affect both sides.

The first step is to read codes from the HVAC control unit, which doesn't give any external indication when there is a fault, but it does store codes. Unfortunately, if it fails, it may also fail to set codes. After I changed the control unit in my 1997, the new unit was able to set codes that told me which servos I had to replace.
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