HVAC control unit GTS style

Model 1978-1982

This is what the HVAC (Heating Ventilation Air Conditioning)  controller is like:


HVAC now working
Living in Florida you really do need the HVAC to work. My 928 was converted to R-134a before I bought it and the a/c seemed to be working, the compressor was engaged when the a/c button was pressed but all I got inside the car was hot air so I decided to investigate, I did a search and read all the posts on a/c I could find and with Dwayne’s write up, Wally’s paper and John’s site, I set to.

I found that all the vacuum actuators held vacuum which was a relief, the sensor chain was working correctly but the setting motor arm wasn’t moving, testing the vacuum solenoids revealed two not holding vacuum, the white line going to the heater valve and the blue line going to the fresh air/inside air flap.

I bought some used solenoids and a setting motor from Gio and replaced the leaking solenoids but the setting motor was like mine, broken, I had seen a post awhile back about replacing the transistors in a setting motor so I removed the circuit board and replaced the transistors with some new ones, when refitted and the temperature slider move from one end to the other I could now hear it moving the flaps.

One other job was to check the front of the evaporator for clogged fins and debris, I removed the rubber bellows between the blower and heater box, this is just enough to get an eyeball of the front of the evaporator, mine was clear. If it was dirty you would need to remove the blower to have enough room to get a brush and vac in there to clean it.

On testing on the road I now have cold A/C and controllable, the temperature at the center vent went down to 30f, outside temperature was 88f. When the system works it works well.
After all this I understand the 928’s a little better but without the help of those mentioned above and all of the other posts I’d still be in the dark or should that be sweating profusely.
Picture of transistors I replaced (2 x BC 327, 2 x BC 337)

Ivan Gleaves.




HVAC controller 1991 model