a/c relay operaton

[mytrplseven]

Are there any electronics techs or people with experience in understanding the function of the A/C relay? I have recorded the volts on all the pins with the A/C switch off and on and frankly, it doesn't make a lot of sense. I was trained in digital electronics in the Air Force and what I'm seeing doesn't make any sense. Bottom line is the A/C relay doesn't energize and the volts I'm seeing on the other pins are a puzzle.

[djnolan]

I pulled out my relay to take a look. The pin numbering is different than the wiring diagram, however the circuitry appears to be the same. If you have a DME jumper you could try to jumper the load pins to see if the compressor engages. Might have to take the relay cover off to see which are the load circuit pins vs. the actuator pins. With the cover off and relay plugged in you could even just manually engage the relay to check the compressor.

As far as what drives the relay, I have lost faith in the wiring diagram.

[9FF]

Also was an electronics engineer in the Air Force back in the 1980's

You may want to have the wiring diagrams in front of you when reading this. Hope it helps:

When you operate the AC (snowflake) switch a 12v signal is sent from terminal A8 of the switch to terminal AC of Coolant Fan Relay (G10), which will turn both radiator fans on low speed (the fans being supplied with 8v instead of 12v as they are fed through 0.9 ohm ballast resistors on the firewall).

A 12v signal is also sent from terminal A8 of the AC switch to one of the terminals on the Icing Protection Sensor, this sensor is normally-closed (N-C), so the 12v exits at the other terminal of the sensor and enters at terminal 3 of the AC Relay (G17).

The AC Relay is also N-C, so the 12v signal will exit at terminal 4, go through the N-C Refrigerant Pressure Switch and to the AC Compressor where it will engage the clutch to start the cooling cycle.

The 12v signal from the AC Switch is also fed to terminal 7 of the AC Relay which is the +ve side of the relay coil. The -ve side of the relay coil terminates at terminal W of the Coolant Temperature Sensor on the cylinder head.

The Coolant Temperature Sensor on the cylinder head has an additional switch on cars with AC. While terminal G of the Coolant Temperature Sensor controls the coolant temperature gauge in the instrument cluster, when terminal W closes (at 240 degrees F), it grounds the -ve side of the AC Relay coil which causes the AC to switch off to avoid excessive thermal loads.

All in the manuals

[mytrplseven]

I appreciate the suggestion. In actuality, I have checked from the relay to the compressor and it all works. It's just that the input signal to actuate the relay from the A/C switch on the Climate Control panel is not there.

[mytrplseven]

aaahh, the manuals (which I don't have). This is a great explanation of the circuitry dynamics and voltages. Tomorrow, after walking the dog again, I'll put the diagram in front of me and sort this out. It's a process. Thanks again, as always.

[9FF]

Originally Posted by mytrplseven

aaahh, the manuals (which I don't have)....

You just know I'm going to keep on at you for that

I'm sure you have the wiring diagrams but I've posted them anyway in case others find this thread and have the same issues. The wiring spans from B22 to K49 so I've circled that transition for clarity.

[mytrplseven]

That's great. I was wondering where the other end of that crucial link was. Thanks again.

[mytrplseven]

Interesting that the coolant temp sensor was included in the circuitry. It also drives the temp gauge and that was an "aha" moment I think. My temp gauge was reading lower that I felt it should be and now this. So, to be on the safe side, I've ordered a new sensor and will see what the overall effects this has on the a/c issue and temp indicator. Not a good place to work in on the 951 but what else is new?

[djnolan]

9FF, Thanks this is very instructional and helps me to understand how to read these diagrams. Also I think this means the current to drive the A/C clutch comes from the TCU and not from the relay board main electrics. I guess the clue was the relay was shown as closed in the diagram instead of open which I missed. I wonder if there is a code to how the different diagrams link together such as the temp sensor "K49" link to the other diagram.

[9FF]

Originally Posted by djnolan

9FF, Thanks this is very instructional...I wonder if there is a code to how the different diagrams link together such as the temp sensor "K49" link to the other diagram.

No problem. It’s quite simple really, the codes in rectangular boxes like “K49” refer to the numbers and letters along to top and down the side of the wiring diagrams, so you’ll see that K49 refers to the intersection between the K and the 49 in the other diagram and right at that intersection you will find another rectangular box “B22” which refers you right back to the K49 box in the first diagram.

I have a good reference to these wiring diagrams saved somewhere, I’ll try and find it and post it back here.


Edit: Here is a link to a couple of articles on the wiring diagrams on our cars. One is 928 specific and quite in depth but everything also applies to the 924/944/968 models. The other covers all German cars and more concise.

...and here is a great Wiring Diagram Video Primer by Van Svenson.

Hope this all helps someone out now or in the future.

[mytrplseven]

Thanks for the continued help.

[mytrplseven]

Update: Since I was concerned that the coolant sensor might have been the culprit (based on inaccurate engine temp readings) I replaced the coolant sensor and it's still not energizing the a/c relay. Now that I'm overwhelmed with electric diagrams and schooled on how to decipher them, I'm going to try to find out where the output line from the a/c snowflake to it's connector is. I also believe that the engine temp gauge needle may have slipped on it's shaft so I'll have to fix that too.

Is there any way to check that the "snowflake" switch is functioning (pins to check for continuity). The fans are turning on so I suspect that the switch must be ok but not 100% positive. Checking continuity through the freeze protection circuit shows NO continuity and if it's normally closed like the diagram shows, then I may have found the problem.

[mytrplseven]

Ordered a new Freeze Protection sensor (titled Temp Sensor at some parts outlets) and like all others, will have to come from Porsche warehouse in Germany, a 2 to 3 weeks wait. Summer weather here in Florida is regularly in the mid to high 90's so that sucks a bit.

[TheSecondChris]

Where is this sensor located, exactly? I'm gearing up for an AC fixing expedition soon.

Are you able to jump the sensor and see that everything works? If so, then it seems you found the problem and it might be fine to run the AC with the freeze protection switch jumped for a couple of weeks until your new component arrives.

[mytrplseven]

It's right above the gas pedal on the side of the HVAC unit. I thought about doing that but it's so darn hot here and since I'm not as flexible as I was in my 20's I would pull the seat out to have better access. I don't want to leave the car like that until the replacement arrives. It's ok....I'll just drive my wife's Boxster.
Here's a picture of the unit.

[mytrplseven]

The new freeze protection sensor came in and there is a potentiometer on the module. I know there should be an adjustment procedure at for this but I can't seem to find it anywhere. I think it might be outlined in the 944 manual section 87 but can't find that online to download. if anyone has a link for that particular adjustment procedure, please let me know......thanks as always.

[mytrplseven]

I just decided to set the adjustment to mid-range and install it. I NOW HAVE AIR CONDITIONING!
What a journey this has been. Many thanks to all of you who helped me sort this out.

[djnolan]

Great journey, I learned a lot from this. Glad you solved the problem.

[Chalt]

9FF, thank you so much for the links to those documents. Very helpful.