My a/c cut off after I have been driving awhile. Also the auxiliary fan does not come on. I jump the leads to the auxiliary fan sensor and the fan runs. I’m thinking that the sensor is bad. This is the lower unit on the receiver dryer. IT’S CALLED A A/C TEMPTURE SENSOR. Will I have to evacuate the system? If I remove this switch will the Freion escape?

This is the switch:

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I've been questioning that switch myself. I think if you do take it out it will let all your freon out and leave an ozone hole over your car.:p

That switch runs the fan. There's a lower pressure switch that will cut out the compressor to prevent damage if you have next to no freon.

What caues the voltage to the compressor/prussure switch to turn off. The problem is the voltage to the low freon switch cuts off wile I'm driving.

That's the same sensor I bought for my 560sel (by appearance). You cannot change it without evacuating the freon first. From very helpful people on this forum - I have learned that the sensor causes the aux fan to come on low speed when freon pressure from the air conditioning running switches it on. I was all set to condemn my sensor (even bought a new one) when a couple of people on this forum advised me that if your freon is a little low - the switch will not work. When I get my car out of storage in a week or so (we just got 4" of snow!)- I will try to top it up and see if they are right. If not - I will convert to something (R134 - freeze12- whatever - haven't decided yet)and replace the sensor at that time. If you are really low on freon the compressor will not run to prevent it from getting damaged. Check the freon in your sight glass - if you cannot see any - that's it. If it is foamy - you need a top up.
Just my experience to date
Maurice

Thanks Maurice,

The first thing I did was have the refrigerant top off. I was as happy as a pig in Hollywood until I went on a long drive. The a/c went from nice and cold to warm and muggy. Looking for an answer I started to check out the sensors. I ruled out the both the pressure sensor and the fan sensor showed in the pic. The problem is the voltage to the low pressure sensor. It must have something to do with the climate control unit. What where and how? What can be done beside replacement? Where is the climate control unit?How can I check it?

Sorry - can't help you there - haven't even thought about working on CCU system yet. I would suggest you get a set of manuals first - CD or otherwise. My son just got a CCU manual for his Porsche and even though it is only 100 pages - It is indispenable for troubleshooting as it has a flow chart - if this does this - check this - if that does that check that - really makes checking his system a piece of cake - would not even attempt it without the book. Maybe someone else on this forum can help out - how about it guys?

PA_Joe:

If your system was blowing cold air and you had additional refrigerant added and now it's not blowing cold air, then the system has been overfilled.

The high pressure side is detecting this and shutting off the compressor. Your compressor is running for a very short period of time because the system has excessive pressure. This is likely why it is no longer cooling.

Thanks Mike,

Before and after I had the refrigerant toped off the condition was/is the same. When the a/c cut off there is no voltage to the low/high pressure sensor. If the system was over filled causing high pressure I would still have voltage on the one side of this switch.

My guess… the voltage from the CCU to regulate the temp inside the car. My thinking is the CCU turns the compressor on and off to maintain the temp. For some reason it decides it cool enough inside the car and turns the compressor off. After the car sits sometimes just a few minuets wile I run in the store it work fine. Is there a sensor inside the car?

PA_Joe:

I'll throw this in. May be worth looking into.

I do not recall the official part name, but on the compressors that are mounted in the M103 cars(serpentine belt model), there's a sensor on the compressor that detects excessive belt slack. If it's determined the belt is too loose, it will shut-off the compressor. In other discussions at this site, people have said that one way to detect this is happeneing is to pull over, turn off engine; then restart. Compressor should come back on. If this is happening, this may be your problem.

In one other post here(way back when-do not remember the date), a pro indicated that this sensor can sometimes deactivate the compressor even with correct belt tension...faulty sensor? Sounds like it.

My 2 cents.

Hey PA-Joe: You're onto something. The pushbutton panel inside the car does cycle the compressor on & off, based upon input from the evaporator temperature sensor. The signal from the pushbutton unit goes out to the high/low pressure switch (the one that's not red) on the dryer. From there the signal goes to the compressor control relay AKA the Klima relay. The Klima relay translates the low-power signal from the PBU to a high-power feed which can activate the compressor clutch. The Klima relay also monitors the engine speed and the compressor speed (via the speed sensor on the back of the compressor) and compares the two. If the two are sufficiently different, it cuts out the compressor until the engine is stopped & restarted. This is the most common failure mode on the w124 a/c. About 99% of the time it's a false alarm, but that other 1% it's the difference between driving home & hitchhiking.

Are you certain the signal to the low/high pressure switch is +12v? Have you measured that while the compressor is on? I think that it should be, but on my 1987 300TDT the signal is ground.

Anyways, if you are certain about the absence of +12v it's time to look insode the car. I'd start with the evaporator temperature sensor. It should have a specific temp/impedance curve. I have occasionally heard of them failing, causing intermittent or hard failed a/c operation.

Assumption assumption…..

No I’m not sure it 12 volts I just assumed … All I did was jump the switch and the compressor did not come on… hmmm tricky stuff. Thanks a bunch everyone... I'll let you know how I make out.

Wait... is it the speed sensor for 97.00... or the compressor relay for 97.00 ...what is the best way to check it.

I have read all that u guys have to say , but my biggets problem is that it all sounds too technical with klima relays ,lo wpressure and hi-pressure sides and all dat.

Is it possible for someone to post a schematic or diagram showing the points wia all these devices are located so a novice like me can go in there and chk things out for myself . I dont want any technician rippin mee off.

thanks all

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1986 230E (revisions on the way)