Justin McMickens

Any help would be greatly appreciated I have a 968 and when driving it loses power to the solenoid that controls vacuum to the heater control valve causing the heat to come on for a second or two then the power comes back to the solenoid allowing it to pull vacuum on the heater control valve blocking the HCV off and the air is cold again. I'm not sure what could be causing the loss of power. Tried swapping relays, control unit, cab air sensor please help still the same symptoms. Power loss to solenoid/hot air tends to happen when coming to a stop.

PaulD_944S2

Check the connectors on the back of the HVAC module.

If they are OK, then most likely an intermittent in the HVAC module. You can try touch-up soldering the circuit board large pads but no guarantees!

Justin McMickens

Thanks for your reply I'm out of ideas... everything looks to be connected fine at the back of the unit, and I've swapped the unit with a known good unit and it still has the same symptoms.

thomasmryan

see if the one way vacuum valve is working on the firewall


edit ...and the vacuum reservoir next to the battery

Justin McMickens

Thanks for the quick responses and guidance. I have checked the 1 way valve it's doing its job and the vacuum reservoir is also holding vacuum very well. I have gone over the vacuum system pretty well and found zero issues with a vacuum leak. Checking for leaks is how I found the solenoid was losing power opening the HCV.