westija

Folks,

I need some help with an issue related to A/C hot anytime I move the lever off the FULL COLD position at the Head Unit. I have been tracing many things and am stumped. So, because I've been on this for a while the post is a bit long. Sorry about that and thanks in advance for help.

I have done multiple searches and found what can be similar issue to one on a thread named "About Cabin temperature Sensor".

Here is my case:
- 89 manual S4 150K+ miles. Very little use (driven less than 1,000 miles per year).
- Bought the car in February 2012. A/C was not working and had been converted from Freon a long time ago. It was not very cold and got hot when moved the lever off the FULL COLD position.
- In April 2012 I have followed Dwayne's A/C Vacuum repair and ended up replacing the following parts that were bad: a couple of bladders / diaphragms (footwell and center comb), the 4-way (by the Brake Booster), the Conector (rubber manifold that connects all solenoids) and 1 solenoid.
- In May 2012, I replaced the Heater Valve and Short Hose and took the car to a local A/C Shop and had the system vacuumed, Drier replaced and system recharged. A/C was very cold but still would not stay cold when move the lever off the FULL COLD.
- In September 2012 tested the Vacuum again and found that Connector (Rubber Manifold for Solenoids) was still letting some air leak and one Solenoid had some marginal leakage. Replaced the Connector and Solenoid. Tested system and was working perfectly.
- Last month I went on for a ride and the A/C was again getting hot when moved the lever off full cold. This time Vacuum is tight and good.

Performed the following tests today as per RL and WSM:
- Setting Motor lever moves when moving the Head Unit lever. It goes from full cold to full hot with no obstruction.
- Resistance between pin 4 and 12 of the Terminal / Plug at the Setting Motor (to test the sensor string): spec 3.7 kOhm lever at Cold (actual 3.25) and 4.7 kOhm at Hot (actual 4.21). Local temperature here is 84F (sorry for those who were hit by the major snow storm this weekend), so these seem fine.
- On the same Terminal / Plug , tested voltage between Pin 3 and Pin 11: spec Battery Voltage (actual battery voltage): OK
- Then on the other Terminal / Plug of the Setting Motor, tested voltage between Pin 3 and ground: spec Battery Voltage (actual: OK); between pin 8 and ground: spec Battery voltage (actual OK); between pin 2 and ground: spec Battery voltage (actual: OK).
- Then the WSM says to at this same location (pin2 and ground) to start moving the lever to full hot and the Voltage should fall to Zero at about 20% of the way. Actual: did not fall to Zero until the lever was at 80F (almost full hot)
- Then tested between pin 9 and ground should find Battery voltage and should drop to Zero at about 10% of the way. Again I found Voltage OK with the lever at Cold but only dropped to Zero when the lever was almost at 80F again.

What do these two results indicate?

Then I moved on to the next two tests: pin 10 and ground and pin 1 and ground to test the center vent swith (I believe).
The fist one (10 and ground) spec: Zero volts with vent opened (actual: ~1V below battery voltage) and then battery voltage after closing the vent (actual: OK). So it fails the first part .
The second one (1 and ground) spec is 0V with vent open (actual: ~ 1V below battery voltage) and battery voltage with vent closed (actual: OK). So again it failed the first part.

But, I think this might be unrelated to the temperature issue (or maybe not?)

Then I went on to review the Head Unit as in one of the postings at the 15-pin plug at the Head unit
- Pins 13 and 14: apec: ?? Actual: 6.4 kOhms (is this supposed to be the same as the sensor string spec? If so, I might be off here)
- Pin 14 at the Head Unit 15-pin connector and Pin 4 of the Setting Motor: spec: ??? Actual: 2.4 kOhm. (maybe off again?)

Finally I went on to the Individual Temperature Sensors (although the first sensor string test indicated they seemed fine).
- Outside Temperature Sensor resistance measured at the sensor connector by the wheel well. Spec ?? Actual: 0.40 kOhm
- Inside Temperature Sensor: measured on Terminal 31. Spec : ?? Actual: 0.46 kOhm.
- Also measure (from the back of the pins with all connected): between pin 1 terminal 31 and pin 2 terminal 29. Spec: ~ 2kOhm (?) and found 1.6 kOhm.

Then when I turn the ignition and observe the Setting Motor arm move, it stays on the cold position only when full cold. It goes all the way to the hot side when I move the lever poff fuill cold. But I have not moved the car for a few days, so maybe it is reading it as "too cold" and going to heat?

Help!!!!!!!!!!!!!!

Thanks in advance

risaac928

My 84 does/did the EXACT-same thing. I noticed when I moved the selector switch all the way to the left and the temp selector to its coldest setting the AC would blow cold...but if I moved it to the right to its next setting it would start to blow warm again.
Finally today it started to work properly today. I know this does not solve your problem but play with the different settings you might stumble on a work around.

NC928S4

I eagerly await responses to this thread as I will embark on the same journey in a few weeks. Good luck getting this sorted out.

Mrmerlin

the usual culprit here is the temperature sensor in the LF fender ,
its part of the duct for the alternator.

https://rennlist.com/forums/928-foru...ix+HVAC+system

joejoe

+1

Alan

You don't need to do all this evaluation - The basic symptoms are classic of an open circuit sensor loop. You just need to test the loop components and connections. Do a resistance evaluation of the loop and the head unit. Plenty of other threads on this with details of how...

Alan

westija

Stan / Alan,

Thanks for the tips, I read your other thread (the one you indicated above) and will give it a try at the car side of the outside sensor connector once I get back home. I have not checked that connector at the car side, but the sensor resistance measured at the sensor side was OK. Still it could be that the sensor resistance was good but the connection to it was not.

Alan,

From your sensor loop testing recommendation, isn't that what I described on mine above that it "passed" ?
"Resistance between pin 4 and 12 of the Terminal / Plug at the Setting Motor (to test the sensor string): spec 3.7 kOhm lever at Cold (actual 3.25) and 4.7 kOhm at Hot (actual 4.21). Local temperature here is 84F"

Isn't it this what you are recommending on this thread indicated by Stan? I am sorry, I am just super confused with all this now.
"check the sensor loop between Plug to HVAC Motor Console LHS (HVAC) Plug Pin 4 (YE)* and Plug to HVAC Motor Console LHS (HVAC) Plug Pin 12 (YE)* should see a 1000 ohms variation between 18C - 30C HVAC setting (65F - 86F)"


Thanks

Alan

I don't know where you got those numbers:

To test the entire sensor loop (from Plug to HVAC Setting Motor Plug Pin 4 (YE)* to Plug Pin 12 (YE)*) you should see an ~1000 ohms variation between 18C - 30C HVAC slider settings within a total range of 3100 ohms - 5300 ohms (depending on ambient temp).

Stan's quoted text there was an incomplete version of a post I made a long time ago (as above)

Your problem is still here - there is a problem somewhere in it - maybe in final connections or intermittent connections...

Alan

westija

Alan,

With the slider at the coldest position I got 3.25 kOhms and with the slider at the hotest position I got 4.21 kOhms. These were measured between pins 4 and 12 of the HVAC motor plug (motor under the steering wheel).
Wouldn't that indicate a good sensor loop?

Thanks

StratfordShark

Your sensor string does sound ok given the resistance readings given.

Testing at plug is good as accounts for wiring too between sensors and the mixing motor.

I suspect the comparator circuit in the mixing motor itself. There are a couple of transistors on the board in the setting motor.

I would remove setting motor and open her up, then desolder any transistors from board (lots of room!) and test each transistor (check for good diodes between base/collector, base/emitter and high resistance emitter/collector so meter on high resistance range).

If any transistor is bad there are easily available equivalents.

Alan

I agree the readings aren't out of range - but I don't trust them based on behavior...

So it is possible its the setting motor - but it would have to be still functional but out of spec for it to work the way you describe - So I'd still first suspect that you have something funky on the loop that you aren't seeing when testing it.. maybe to do with the final connections...

If you can borrow a known good setting motor (or even just connect it across to another vehicle - you can use long wires no problem) and see what happens that would help. Or alternatively simulate the different conditions with resistors.

Alan

westija

Alan, I am thinking you got it right again.

I measured these sensors resistance again and the Outside Sensor resistance measured at T31 was 470 Ohms, then when I measure the Internal Sensor at T29 I found over 2,000 Ohms.

Somehow this was tricking me.I pulled the console out (argh!) and tested it outside to be sure

So, can you guys confirm what I was supposed to read in terms of resistance of the internal sensor before I order another one?
Tonight's temperature inside my garage is a good 80F if not higher.

Seems a new one costs over $120. Ouch! If anyone has an used one that works, pls let me know

Thanks again

StratfordShark

John Speake posted resistance readings for the cabin sensor and he stated 1900 ohm at 86F. Post 19 in this thread:

https://rennlist.com/forums/928-foru...re-sensor.html

It's an NTC thermistor (resistance goes up as temp goes down) so your sensor seems ok?

When my cabin sensor went out of spec (same symptoms as yours - full hot when I moved lever off cold override) it drifted very high and from memory was about 4Kohm (so the setting motor always thought it was very cold in cabin!)

Alan

That may still be OK? - Here are the specs:

2570 ohms at 20C (68F)
2220 ohms at 25C (77F)
1900 ohms at 30C (86F)

Alan

westija

Update - still stumped!

Took my time and retested tonight:
1. Sensor String - between Setting motor pins 4 and 12: 6,240 and 5,320 Ohms. Delta ~ 1,000 Ohms - OK
2. Continuity of the various parts of sensors circuit: all OK (between Setting Motor pin 12 and Head pin 13: 0 Ohm; Head pin 14 and T31 pin 1: 0 Ohm; T31 pin 2 and T29 pin 4: 0.03 Ohm; Setting Motor pin 4 T29 pin 3: 0.04 Ohm)
3. Outside sensor: between pins 3 and 4 of T29: 508 Ohms OK
4. Sensor String including the Setting Motor - between T29 between pins 3 and 4 car side: 6,990 and 6,020 Ohms, delta ~ 1,000 Ohms. The delta is OK, not sure about the individual numbers.
5. Sensor String: between Head pin 14 (car side) and Setting Motor pin 4: 2,910 Ohms. Also none is grounded / shorted
6. Checked for ground or power on all pins of Setting Motor connector and Head connector : all OK

So, it seems I am good with all these.

The suspect then becomes the Setting Motor, since when we check test 1 above, it excludes it. Maybe someone can tell me if test 4 above indicates something too high / too low as it includes the setting motor on that measurement.

I opened up the Setting Motor and cleaned the contacts using a electric degreaser and cotton swabs. The board appeared fine, but got some gunk from the transistors solders. I am pretty clueless to try to resolder or test the things inside.

I have not reassembled the Setting Motor back...it is getting pretty late.

Any suggestions?

Thanks