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I am troubleshooting an always heat issue on my 1988 S4. I should note, the temp regulation system worked prior to my dash removal/reinstallation. The HVAC system seems to be functioning normally except for the full heat. I have verified I have no vacuum leaks. I have verified all actuators are working. My heater valve is a newer all metal unit (sold by 928 motorsports) and operates correctly.
I am zeroing in on the temp mixing flaps adjusting motor. I am conducting the tests described in the WSM (Section 87-113-114). It passes the first series of tests of the temp sensor circuit. I was able to obtain the proper values at the high (4.7-Ohms) and low (3.7-Ohms) temp setting. So I am concluding my sensor circuit is good. Correct?
So I moved onto the next section testing the rest of the connections to the setting motor. All connections tested out EXCEPT terminal 2 on Plug 1. I am supposed to get battery voltage, but I am not getting any reading ignition on, HVAC in up/down position. Furthermore, when I connect the setting motor, I cannot get it to move when adjusting the temp slider. So does this mean the motor is toast or the voltage being supplied is bad?
I can't tell if the voltage for terminal 2 is being supplied too or coming from the setting motor in reviewing the wiring diagram. So I am stuck, the WSM says try a spare motor, as if I had one. Is there a way to bench test the motor? Suggestions appreciated.
Last edited by Michael Benno; 04-18-2018 at 05:12 PM.
Off hand I can not answer the voltage question, however having just done an R&R of the guts of the HVAC box, and then verifying all is working with the console still uninstalled I can say this.
It was a wonder watching all the Flaps and the Mixing motor do its thing while changing the settings on the HVAC head unit.
The Mixing motor and assembly move slowly and do not react quickly to changes in the Head unit settings, (this could be just the way mine is working).
I would make a change and wait, it could take several seconds before it and the center flap actually move.
I did some testing of my flap setting motor and I looks like the range on the setting motor is not aligning to the temp setting on the HVAC controller.
At full cold setting on the HVAC, the setting motor is only 35%. According the WSM the setting motor should be at 0%. Also at 35% the heater valve is not closing, it closes at 20%. So this seems to explain why I have full heat.
The setting motor goes to 100% when the HVAC controller is set to 75 degrees or half way.
Does this mean the motor is toast? Or is there some repair/refurb I can do. I pulled it apart and gave the internal contacts a good cleaning but no change. I sure would appreciate your suggestions.
FYI here are the other tests I have completed and passed:
- tested for vacuum leaks at supply and all actuators.
- tested temp control circuit and conformed 1000 Ohm difference from low to high temp.
- tested blower fan on dash temp sensor.
- I have verified the heater valve closes in the off position (per the WSM)
- I noted the heater valve opens as soon as the HVAC controller is switched on and stays open in any setting. According to the WSM, this is the case when the setting motor is above 20%.
Last edited by Michael Benno; 04-20-2018 at 02:53 AM.
And..do these have intermittent failure modes? Ill have cold AC one moment, the hot as fu** (rear ac is coooold still) and then it will go back on its own.
Open it up and clean the inside contacts. It fixed the full heat then cold when set to max AC issue I was having in my 81. It's also worth looking to see if the electronic components inside have failed. They sometimes burn up.
The motor is located behind the rubber cover next to the gas pedal and attached to the side of the HVAC box using 2 bolts and square nuts. It isn't too bad to remove.
The transistors go bad . One thing to remember when testing is that it used ambient temp to adjust the movement. So if it's hot out, the flap won't respond to full hot etc in automatic mode.
The transistors go bad . One thing to remember when testing is that it used ambient temp to adjust the movement. So if it's hot out, the flap won't respond to full hot etc in automatic mode.
I disassembled the setting motor and gave all the contacts a very through cleaning. Actually several cleanings. This helped a lot. When I first started the setting motor was not moving. In my second post I had cleaned and reinstalled and noted I was not getting full movement. More cleaning seemed to help. Here is a pic of the circuit board the powers the shutoff of the water valve before cleaning. I put it all back together and not I have restored full temperature control.
I learned an important concept in my testing and reading of the service manuals. The water valve is activated (closed) in two conditions. First when the ignition is on and the HVAC is off. The second condition is controlled by the setting motor when it is below 20% (0% is full cold and 100% is full hot on the HVAC temp setting). Subsequently if the setting motor won't go below 20% the heater valve is open. There are two causes for this condition, its either a problem with the temp control circuit (temp sensors, HVAC potentiometer, or broken wire) or a problem with the function of the setting motor (failed transistors or dirty connections).
I have an 89 and in researching how to fix the a/c, I thought I read that the mixing system won't work at all if the outside temp sensor wasn't working or not installed (the sensor that sits in the alternator hose in the front left fender)
Someone should be able to verify this.
I have an 89 and in researching how to fix the a/c, I thought I read that the mixing system won't work at all if the outside temp sensor wasn't working or not installed (the sensor that sits in the alternator hose in the front left fender)
Someone should be able to verify this.
That is mostly correct, they both work together to tell the brain what to do.
The sensors make a "loop". There is a feed out of the mixer motor and a return line...the signal loops through both sensors, and the value fed into the unit is a combination of both units' resistance. My understanding is that as along as there is a resistance in spec that the box sees, it should work. If the values are off (e.g. one of the sensors is not transmitting the correct value), then the motion the motor arm makes may be compromised. The motor takes the data from the requested temperature (slider) as well as the sensors and makes a decision on how much to mix the heated and cooled air to get the desired result. The mixing motor also determines the opening and closing of the water valve as well as telling the solenoids to open and close for distribution. AFAIK, the only thing it does not control is the fan speed - it is completely independent of the motor.
If one of the sensors is shorted out (sometimes common in the outside sensor) or unplugged, then the loop is broken and the mixing motor defaults to either full hot or full cold, as it sees no reading.
I originally thought I had a bad outside sensor and was going to put a resistor with a value approximating 72 degrees F in the loop - the system would see that level of resistance and just assume it was 72 outside and still calculate using the inside sensor, but mine ended up working ok.
04-17-2018, 10:23 PM
Help with testing mixing flap motor
