spode134

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

Bill Ball

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

neilh

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.

StratfordShark

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.

Bill Ball

The 85 guide Neil mentions will help you nail this down. It covers the temp sensor testing on pages 11-15 and adds some testing details to Wally's guide.

spode134

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

neilh

Darrin, just emailed you the HVAC document.

Lizard928

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

spode134

Great, got to go out, will check it out later.
DG

Emickelsen

Does one of those exist for the '83? That sounds awesome!!

ponenc

Neil: Just PM'd you asking for he same. Thanks, Randy

neilh

There are 3 versions i'm aware of -
1980 , 1985 and 1990, plus Wally's excellent right up.
PM me with an email address and i'll send it to you.

linderpat

Neil - I'm guessing it is the same for an 86. Please send a copy to linderpat at gmail dot com! Thanks in advance!

spode134

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.

mx22

Sorry to dig this old thread up, but any chance I can get a copy of that HVAC manual pdf as well? I would probably need 1990 one, but would take 85 as well...

Thank You!