HVAC experts needed
Thread Starter
Instructor
Joined: Sep 2002
Posts: 131
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From: Sardis B.C. Canada
HVAC experts needed
Working on a 85 5 speed with out rear a/c. I am getting no heat adjustment, it is full hot or full cold.
This is what I have confirmed or know
- vacuum at the solenoids is good
- I have replaced the diaphragms at the orange and yellow lines, all others checked and hold vacuum
- I have a variable resistance at the fenderwell temp sensor and dash sensor
- heater valve operates and is good
The system cycles through all its settings and flaps move as they should. Temp out of any vents is warm unless the control is set all the way to the left. Then it is cool. There is no in between.
What I suspect is the servo motor that is on the side of the heater is responsible for adjusting the air temp by allowing cool air to mix with hot. When the temp control is set full left the servo moves to its full travel one direction. When the temp control is moved even a little to the right, the servo travels fully to the opposite direction. There is no in between.
Is this how the air temp is regulated?
All I have left to try is the heater control assy itself but this seems unlikely.
I have searched all the archives and WSSM have not been able to find an expanation of how the air temp is regulated.
Any thoughts would be appreciated.
Thanks in advance,
Darrin
This is what I have confirmed or know
- vacuum at the solenoids is good
- I have replaced the diaphragms at the orange and yellow lines, all others checked and hold vacuum
- I have a variable resistance at the fenderwell temp sensor and dash sensor
- heater valve operates and is good
The system cycles through all its settings and flaps move as they should. Temp out of any vents is warm unless the control is set all the way to the left. Then it is cool. There is no in between.
What I suspect is the servo motor that is on the side of the heater is responsible for adjusting the air temp by allowing cool air to mix with hot. When the temp control is set full left the servo moves to its full travel one direction. When the temp control is moved even a little to the right, the servo travels fully to the opposite direction. There is no in between.
Is this how the air temp is regulated?
All I have left to try is the heater control assy itself but this seems unlikely.
I have searched all the archives and WSSM have not been able to find an expanation of how the air temp is regulated.
Any thoughts would be appreciated.
Thanks in advance,
Darrin
Under the Lift
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Lifetime Rennlist
Member
Joined: Mar 2002
Posts: 18,648
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From: Buckeye, AZ
The setting motor gets signals from the temps sensors. The most likely cause for this ON-OFF/HOT-COLD behavior, in general, is one or both temp sensors, BUT you appear to have verified that they are providing variable resistance signals. Did you verify this at the setting motor plug (resistance between pins 4 and 12)?
Wally's comprehensive 928 AC guide covers this among many other things.
http://www.928gt.com/t-wallyhvac.aspx
Wally's comprehensive 928 AC guide covers this among many other things.
http://www.928gt.com/t-wallyhvac.aspx
Rennlist Member
Joined: Mar 2004
Posts: 1,602
Likes: 9
From: Southport, NC
Working on a 85 5 speed with out rear a/c. I am getting no heat adjustment, it is full hot or full cold.
This is what I have confirmed or know
- vacuum at the solenoids is good
- I have replaced the diaphragms at the orange and yellow lines, all others checked and hold vacuum
- I have a variable resistance at the fenderwell temp sensor and dash sensor
- heater valve operates and is good
The system cycles through all its settings and flaps move as they should. Temp out of any vents is warm unless the control is set all the way to the left. Then it is cool. There is no in between.
What I suspect is the servo motor that is on the side of the heater is responsible for adjusting the air temp by allowing cool air to mix with hot. When the temp control is set full left the servo moves to its full travel one direction. When the temp control is moved even a little to the right, the servo travels fully to the opposite direction. There is no in between.
Is this how the air temp is regulated?
All I have left to try is the heater control assy itself but this seems unlikely.
I have searched all the archives and WSSM have not been able to find an expanation of how the air temp is regulated.
Any thoughts would be appreciated.
Thanks in advance,
Darrin
This is what I have confirmed or know
- vacuum at the solenoids is good
- I have replaced the diaphragms at the orange and yellow lines, all others checked and hold vacuum
- I have a variable resistance at the fenderwell temp sensor and dash sensor
- heater valve operates and is good
The system cycles through all its settings and flaps move as they should. Temp out of any vents is warm unless the control is set all the way to the left. Then it is cool. There is no in between.
What I suspect is the servo motor that is on the side of the heater is responsible for adjusting the air temp by allowing cool air to mix with hot. When the temp control is set full left the servo moves to its full travel one direction. When the temp control is moved even a little to the right, the servo travels fully to the opposite direction. There is no in between.
Is this how the air temp is regulated?
All I have left to try is the heater control assy itself but this seems unlikely.
I have searched all the archives and WSSM have not been able to find an expanation of how the air temp is regulated.
Any thoughts would be appreciated.
Thanks in advance,
Darrin
Drifting
Joined: Sep 2006
Posts: 3,379
Likes: 128
From: Stratford-upon-Avon
You are correct - the mixing motor controls flap varying proportion of ambient air and heated air from heater core.
You really need to disconnect one plug to this motor and see if the whole sensor resistance chain is in spec and varying by about 1k ohm frommone extreme of temp lever to the other.
Good sign the int and ext sensors are varying - is this testing at plugs where they join loom to mixer motor to rule out wiring problems? Failure of one of these sensors gives rise to classic all hot or all cold condition you have. New sensor fixed it (fan motor drawing air over that sensor was gunged up too and barelybturning so I rplaced that at same time as I had centre console out) and all perfect after that. The system is actually simpler to troubleshoot than appears at first sight.
I had same fault. The cause was the internal sensor. Its resistance varied but it was way out of spec. High enough that the sensor chain always presented resistance to mixer motor which fooled it into thinking it was very cold in cabin! I don't recall spec resistance but it's in wsm or you should find it in thread here.
If it checks out ok then may be problem in the (simple) electronics in the setting motor itself where a logic board reads string and other inputs (like the override you can trigger at full cold) and runs mixer flap to appropriate position.
At least your motor is running ok between extremes.
You really need to disconnect one plug to this motor and see if the whole sensor resistance chain is in spec and varying by about 1k ohm frommone extreme of temp lever to the other.
Good sign the int and ext sensors are varying - is this testing at plugs where they join loom to mixer motor to rule out wiring problems? Failure of one of these sensors gives rise to classic all hot or all cold condition you have. New sensor fixed it (fan motor drawing air over that sensor was gunged up too and barelybturning so I rplaced that at same time as I had centre console out) and all perfect after that. The system is actually simpler to troubleshoot than appears at first sight.
I had same fault. The cause was the internal sensor. Its resistance varied but it was way out of spec. High enough that the sensor chain always presented resistance to mixer motor which fooled it into thinking it was very cold in cabin! I don't recall spec resistance but it's in wsm or you should find it in thread here.
If it checks out ok then may be problem in the (simple) electronics in the setting motor itself where a logic board reads string and other inputs (like the override you can trigger at full cold) and runs mixer flap to appropriate position.
At least your motor is running ok between extremes.
Thread Starter
Instructor
Joined: Sep 2002
Posts: 131
Likes: 0
From: Sardis B.C. Canada
Excellent! this is the info I am looking for. I could not find the info about the pin-out of the control plug and or the resistance values of the varistors. I will check it out and get back with results.
Darrin.
Picture is just for entertainment CCGP at Mission 2010
Darrin.
Picture is just for entertainment CCGP at Mission 2010
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Nordschleife Master
Joined: Apr 2003
Posts: 9,601
Likes: 39
From: Abbotsford B.C.
Darrin,
That was a fun event, too bad the silver mazda beside you ended up in the wall!
And the black car almost off the picture is the one I was mainly there to help support. It needs some work imo
That was a fun event, too bad the silver mazda beside you ended up in the wall!
And the black car almost off the picture is the one I was mainly there to help support. It needs some work imo
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Burning Brakes
Joined: Oct 2006
Posts: 869
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From: Bakersfield, Ca.
Darrin, do you have the '928 HVAC pub 1985' from Porsche? It covers the complete system. If not, PM me and i'll send you a pdf copy.
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Joined: Mar 2004
Posts: 1,602
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From: Southport, NC
Thread Starter
Instructor
Joined: Sep 2002
Posts: 131
Likes: 0
From: Sardis B.C. Canada
Update on the all or nothing heat. The interior temp sensor was out of spec. It did change resistance with temp change, this fooled me. Thanks to the great info and exact resistance values in the HVAC guide that Neil H sent me, I was able to confirm that the resistance was out by about 3K ohms. Thanks for sending that Neil, Rennlist comes through again.
Darrin.
Darrin.
