StratfordShark

I'm narrowing down a problem I have with getting full hot air whenever I move the temp slider off the full cold microswitch position.

I've worked through almost all the wsm tests, except for sensor string resistance at the setting motor (console together with int sensor is off just now, but will be simple to hook it up again).

Aircon works perfectly at the full cold position, and the heater valve is new. All vac actuators and solenoids check ok for vacuum.

The external sensor seems fine. At the connector in centre console it's reading about 750 ohm which checks with other readings people have posted here, and also confirms that the connector in ducting is ok.

The pot in the control unit is also bang on spec, moving between around 750-1750 between 18 and 30 degrees.

At room temperature (I have the console next to me now!) the internal sensor reads about 3900 ohm, and this drops a few hundred ohm when I blow on the sensor so in that sense it's functioning.

I found the sensor fan was barely moving, so have a new one on order (I'd have it now but somehow Porsche in Germany sent completely the wrong part sealed in the right box!).

The setting or mixer motor does move between full cold and full hot when I move the temp slider off the microswitch position, and returns to full cold.. With the console inside and the sensor string open circuit effectively I haven't yet done a visual check for the motor moving to an intermediate position. Also I've not yet checked for resistance at the setting motor plug pins 12 and 4 to be moving between 3700 and 4700 ohm.

But looking at the wiring diagram, if the int, ext and temp selector are all in series then I'm already looking at a combined resistance of 5400 ohm, which may explain why I'm getting full hot.

Tomorrow I'll hook up the internal sensor temporarily to the setting motor connector, but in the meantime I wonder if anyone has some sample resistance values from their internal sensor? Also has anyone seen a failure mode where this sensor exhibits higher resistance than normal, but still responds to temperature change?

Actually it would be nice if the problem was just the int sensor, rather than some problem with the comparator electronics in the expensive setting motor.

Oh if it is the internal sensor that's faulty, does it just push out from console? At rear of console it just has a few zip ties around the tube it's pushed into.
Many thanks in advance hvac gurus!

John Speake

Hi Adrian,
Have you read Wally's excellent write up on the HVAC system on the 928SP website ?

Here's a quote from it.....

""TROUBLESHOOTING THE SENSOR CIRCUIT



The usual effect of a bad sensor string is “all or nothing” temperature control. If this occurs, the first thing to check is the connection for the outside air temp sensor under the left headlamp. If that doesn’t help, go to the next paragraph.



Remove the left side panel from the central console by moving the driver’s seat all the way back and down, and removing the two Phillips screws.

Locate the setting unit, and remove the inner electrical plug.

Turn ignition switch on, and move the temperature control lever to the lowest temperature setting. Use an ohmmeter to check the resistance between terminals 4 and 12 - the resistance should be approximately 3.7 Kohms.

Move the temperature lever to the highest temperature setting - the resistance should smoothly change to approximately 4.7 Kohms.

If the resistance is infinite (no connection), the sensor string is broken - the most likely place is the connector for the outside air temperature sensor. If the resistance is much lower than specified, there is a short circuit in one of the sensors or in the control head.""

StratfordShark

Hi John,

Thanks for the pointers.

Don't worry - Wally's write-up is my bible!

What's puzzling me is that this could be a non-usual case where the failure isn't all or nothing, but rather one sensor going out of spec (but must admit I've never experienced a thermistor doing that).

I've checked the external sensor at its connector in centre console and it seems fine.

Must admit haven't checked the whole 'string' yet but only individual components. The slider and the ext sensor seem ok. I'm concerned that the internal sensor resistance seems high. If that's the case then it would explain why the setting motor (I can eyeball the arm driving the mixer flaps) is being driven to full hot.

I feel I'm very close to finding the culprit, but knowing the usual resistance of the internal sensor will help me distinguish between a setting motor fault and a sensor fault.

I may also find that the problem is simply down to the non-working sensor fan, assuming that a working fan would lower the resistance of the internal sensor quite a bit compared with what it is just sitting behind dash with no air being pulled over it. I should have the new fan motor by midweek.

John Speake

I think when the fan is working your internal sensor will be OK.

Are you sure the setting motor circuit is working OK ? The often fail. I have a test jig.....

You could try replacing the string with a variable pot to check that the system behaves as it should....

Mrmerlin

Here are some pictures from the broken temp sensor wires in the external pug, check these first.
Note the side that had the broken connectors was the harness not the temp sensor

Mrmerlin

Here is a link
https://rennlist.com/forums/928-foru...ur+HVAC+system

StratfordShark

Thanks MrMerlin - in fact I was reading your original post last night and was surprised about the pins which just pull out of the connector!

I am testing the ext sensor where it meets the connector under the centre console though, so I'm getting continuity all the way from there to the sensor itself and a resistance reading which is normal for the temperature in these parts, so the connection from the harness to the sensor is ok.

John I'm not certain the setting motor is working ok. I need to confirm whether it can find intermediate positions when fed with normal sensor string resistance. Looking at wiring diagram (Sheet 5) I think I can try a variable pot as you suggest between pin 3 T29, and pin 1 T31 to exercise the setting motor. Feel tantalisingly close to answer!

Alan

Inside Sensor

From Plug T31 4 Pin Console RHS Plug Pin 1 (YE)*
To Plug T29 4 Pin Console RHS Plug Pin 2 (YE)*

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

Alan

StratfordShark

Thanks Alan, that's brilliant to have the measurements.

My sensor is looking very high then with around 4K at room temp. Could well be the problem but need to confirm when the fan is working.

sendarius

Hi Alan,

If I am reading spec sheets correctly, I have found a number of thermistors available locally that are rated as 2.2kOhms at 25C, but not one of them has anything like the NTC curve necessary to get even CLOSE to the figures at 20C and 30C.

I know the curves are not linear (duh!, they are called CURVES ), but those readings give 70 Ohms per degree and 64 Ohms per degree - the stuff I can find is more like 120 Ohms per degree in that region. Any suggestions as to a possible source for the appropriate device?

Alan

I never looked for the inside sensor but the outside sensor is similarly difficult to match with standard offerings... so no not really - I just stopped looking since I didn't actually need one... Pretty reliable actually..

Alan

sendarius

Reliable? Yes, I suppose so. Until the blue smoke escapes.

Alan

Well an option is to create a resistor network - if the NTC thermistors you can find have a similarly shaped (not too non-linear) slope and an approx 2x higher temperature co-efficent then you can try to match with a higher value thermistor (higher than the target nominal value) and reduce to get the correct coefficient (range) via a fixed resistor in parallel to the thermistor while matching the exact mid-point values (if needed) with an additional single series resistor. Its unlikely to be ideal but probably can represent a much better match than you will find off the shelf... (e.g. 1/Rtot = 1/Rfixed+1/Rthemistor)

e.g. a 3K thermistor @ 25C with a 120/C coefficient could be paralleled with a 9K fixed resistor to get close the to the ranges you need...

Alan