Hi there,
I'm a kind of experienced to the 928 world (worked on them in the past) but I have finally encountered one that has me perplexed. I posted on the Pelican board and a member there provided me with a somewhat helpful link, but it still hasn't helped me finalize what the fault is. I would greatly appreciate any information that can be contributed. Here's the skinny:

* I have replaced the control unit with a brand new one, from Porsche
* I have checked the Colored vacuum lines and their switching unit for vacuum leaks and proper operation -- it works and there are no vacuum leaks.
* I have tested the directional electrical vent controls and bladders that operate the vents -- they work correctly (after installing new control unit and testing with hand-vacuum pump).
* A/C is fully charged and compressor/electrics for the A/C itself work properly.

So far I know that the HVAC control unit ("head unit") will default to full hot in the event of a fault. I have fully replaced the A/C lines and charged with freon, the A/C blows ice cold when the temperatue setting is set to full cold. The little microswitch on the far left of the control unit I believe tells the system to bypass any temperature sensors and set for a full cold condition. Am I correct so far?

I pulled the inside temp. sensor forward enough to put an ohm meter on the leads. It is not reading full open or full closed, but it will stay at about 5.4 MΩ when exposed to full-cold air from the vent. It will then increase to 8.5 MΩ when exposed to full-hot air from the vent. Are these readings in the correct range? I cannot find anything on our microfiche depicting the resistance range for this sensor. I assume if the sensor were bad it would read open or 0 Ω, correct?

My next question is... When I get the car up on the lift Monday and go to check the outside air temp. thermistor, what should I expect for a normal resistance range? MΩ? KΩ?

Final question is, if we find the outside sensor is good or we replace with a good one, what if the full-cold/full-heat condition continues, where should we look next?

Thank you in advance,

~B

*Edit: Added some steps/repairs already done.

 

~B,

If you find that the outside temp sensor is the problem (sounds like it is- I had the same symptoms on my former 86, cold only all the way left & hot anywhere else) you will probably get an infinite or very high reading on the ohm meter. You're supposed to get I think about 500 to 2000 ohms depending on the temp. What I did to fix mine was just replace the sensor with a resistor with a rating of about 1200 ohms- you figure Porsche was probably designing this sensor to handle extreme temps but with a mild climate a resistor near the middle of the scale allows all the temp adjustment you'll need.

Brian
90GT. Guards Red/Lt Grey Sport Seats, Ott X, GTS clutch, short shift, 68K

 

This article suggests that the thermistors are in series and provides some guidanance on resistance.

http://www.nichols.nu/tip584.htm

When was the heater control valve last replaced?

They are a common failure item and usually fail open regardless of vacuum actuator position. That would explain the constant heat.

http://members.rennlist.com/v1uhoh/hotwater.htm

 

Brian, Borland, thank you both for your input.

I forgot to mention, new heater control valve was installed as well. It was previously stuck open, the new one functions properly. I see now how tedius it can be when someone says "my heater is stuck on full!" chasing down all the little things.

 

The temp setting motor is controlled by the sensor string - the outside temp sensor, inside temp sensor, and temp setting lever are all in series. Any break in the string will result in the problem that you have - full hot on the right end of the lever travel, full cold on the left, and no control in between.

The most common problem by far is a faulty connection on the outside temp sensor. The sensor is located in the alternator cooling air hose under the left headlamp. You can often get to the connector from the bottom of the fender with no disassembly.

It is important that the cooling air hose for the alternator be correctly installed...

If you will send me a request via regular email (not thru this system) to tech at 928gt.com (replace the spaces and "at" with @ - just trying to keep the address harvesting bots at bay), I will send you a paper on the 928 HVAC system.

 

Thank you, everyone. We were able to get this repaired by an outisde air temp. sensor. Funky how these HVAC systems work. Such redundancy and over-engineering. I believe one sensor inside the mixing chamber would have sufficed, but then again, I'm a parts replacer -- not an engineer