New HVAC Woe- heater valve only shuts off with A/C
07-17-2014, 10:40 AM
#1
Burning Brakes
Thread Starter
New HVAC Woe- heater valve only shuts off with A/C
Hi everyone,
Another one for the brain-trust:
Symptom: hot air blowing through the vents unless the A/C is ON.
All of the HVAC diaphragms hold vacuum. The vacuum check valve was recently replaced, and works perfectly.
The heater valve allows heat into the heater core always, unless the A/C is on, at which point, it dutifully shuts off.
Is this the famous HVAC control unit relay? Have any of you encountered this problem?
Another one for the brain-trust:
Symptom: hot air blowing through the vents unless the A/C is ON.
All of the HVAC diaphragms hold vacuum. The vacuum check valve was recently replaced, and works perfectly.
The heater valve allows heat into the heater core always, unless the A/C is on, at which point, it dutifully shuts off.
Is this the famous HVAC control unit relay? Have any of you encountered this problem?
07-17-2014, 12:00 PM
#2
Electron Wrangler
Lifetime Rennlist
Member
Lifetime Rennlist
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This has nothing to do with the usual relay failure which disables the compressor (opposite issue).
This could be that your mixing flaps are stuck wide open (setting motor failure) or the temperature sensor loop is open circuit (this causes full heating mode - except when overridden by Max AC (temp slider full left).
Are you sure from observing the valve that it only closes with AC or are you supposing this from the temperature at the vents? Do other valves change mode when this happens? e.g. if you are in vent only mode do the defroster vents open when this happens?
The key questions here are: Is this a temperature regulation issue due to a sensor loop failure OR a loss of vacuum in the system at this time.
The vacuum loss (if it happends) would likely be due to the comb flap solenoid activating (and the actuator/lines leaking massively). The comb flap activates when AC is on in Max AC mode (temp slider full left).
A possible test: with the car off and your ear near the HVAC head unit - move the temp slider to the right of full left - just until you hear the microswitch click.
Retry testing.
Alan
This could be that your mixing flaps are stuck wide open (setting motor failure) or the temperature sensor loop is open circuit (this causes full heating mode - except when overridden by Max AC (temp slider full left).
Are you sure from observing the valve that it only closes with AC or are you supposing this from the temperature at the vents? Do other valves change mode when this happens? e.g. if you are in vent only mode do the defroster vents open when this happens?
The key questions here are: Is this a temperature regulation issue due to a sensor loop failure OR a loss of vacuum in the system at this time.
The vacuum loss (if it happends) would likely be due to the comb flap solenoid activating (and the actuator/lines leaking massively). The comb flap activates when AC is on in Max AC mode (temp slider full left).
A possible test: with the car off and your ear near the HVAC head unit - move the temp slider to the right of full left - just until you hear the microswitch click.
Retry testing.
Alan
07-18-2014, 09:15 PM
#3
Burning Brakes
Thread Starter
Alan,
Thank you for your customary detailed response.
It is the heater valve that I am observing. It does not close unless the AC is on. I will check out the sensor loop- that is a logical place to start. The sensor in the driver's side alternator cooler duct is a likely culprit, I think.
Thank you for your customary detailed response.
It is the heater valve that I am observing. It does not close unless the AC is on. I will check out the sensor loop- that is a logical place to start. The sensor in the driver's side alternator cooler duct is a likely culprit, I think.
07-19-2014, 11:01 AM
#4
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If the outside sensor fails or the connector is corroded the system defaults to telling the HVAC it is cold so the heater valve will be open.
__________________
Does it have the "Do It Yourself" manual transmission, or the superior "Fully Equipped by Porsche" Automatic Transmission?
George Layton March 2014
928 Owners are ".....a secret sect of quietly assured Porsche pragmatists who in near anonymity appreciate the prodigious, easy going prowess of the 928."
Does it have the "Do It Yourself" manual transmission, or the superior "Fully Equipped by Porsche" Automatic Transmission?
George Layton March 2014928 Owners are ".....a secret sect of quietly assured Porsche pragmatists who in near anonymity appreciate the prodigious, easy going prowess of the 928."
07-19-2014, 11:41 AM
#5
Rennlist Member
The HVAC works only at full cold or hot correctly, if the outside sensor fails. The end positions have little microswitches and override the logic.
The heater valve is only controlled by the movement of the temperature mixing flap motor. The motor sits close to your right knee when you drive the car.
Below a picture of the part that controls the heater valve, pretty simple.
The heater valve is only controlled by the movement of the temperature mixing flap motor. The motor sits close to your right knee when you drive the car.
Below a picture of the part that controls the heater valve, pretty simple.