Yeah, I hear ya..
I agree ..the Fix is the Concern for any post........

Removed the sensor and let it hang into the passenger compartment. The comp stillcuts out. I am wondering if at this point do I have a bad coolant temp sensor giving the computer a bad reading.??

Hi Saumil. To say there is no change in blower speed with an rpm change would be misrepresenting the facts. There is a nominal change...the same as there has always been.The issue you raise is a good one but on a 1992 400E the compressor will not under any circumstances (according to the shop manual) restart once the sensors have detected a mismatch in speeds without a restart. There is no receck by the computer or a/c system to see if the mismatch was in error. That is why I have not persued that path. Thanks guys for all of your input.

Thats what I though too until I came across someone on the forum who said that his speed sensor was the problem and that the a/c would come on and go-off at times. I asked him, how sure he was about that the problem was the speed sensor, and he replied 100%. I also asked him, that it did not make any sense to start the compressor again once a speed mismatch is detected given that the sole purpose of this test was to protect the belt. I have these conversations in one of my threads, let me look and find out who had this problem and maybe you can contact him and get more details.

Actually, I recall reading that the test is done atleast 3 or 4 times before attempts to start are stopped. And there is an increasing amount of wait after each attempt. One possible way to test this would be to leave it on idle for say 15 mins (hopefully this time is enough to get past the multiple start attempts by the computer), and then see if the increase in RPM turns it on or not. If it fails to come on at higher RPMs, likely to be the speed sensor (assuming 15 mins were enough).

__________________
Saumil S. Patel

Check this page and you will find our conversation regarding speed sensor and a/c on-off.

ac still not working after ac conversion 87 300SDL

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Saumil S. Patel

Hey guys last test and I think I have it narrowed down. Hey Arthur I double checked the aux fan sensor to be sure my flow chart is correct. It's the temp of the freon at the high side receiver dryer that the sensor is reading. It does not turn on the aux fans during in cold weather even though the system has proper pressure on the high side unless the freon temp is excessive. Here is the twist. The car also ran very poorly when the car was at hot temps but I thopught it was unrelated. One of the posts said that high engine temp would turn off comp. The sensor at the computer uses to read that data must be faulty. I tested this theory today by starting dead cold and driving. First the transmission upshift delay did not operate and the car acted as though there was no "choke" activity keeping the engine running correctly while cold. Not a certain diagnosis but I think it might be the problem with both issues. ...Thoughts??

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No..
The reason the aux fan does not come on in cold ambient temps is b/c the high side does not get to fan sensor cut-in pressure...That is the way it is Designed to work..Aux fan is called for when the a/c condenser has a HIGH THERMAL LOAD on it ,which does not happen with cooler ambient temps. It is not temp sensed ..it is pressure sensed..The low fan is not supposed to come on with cooler ambient temps by DESIGN...it is not needed with low ambient temp..it is with high ambient temps.. The aux fans only purpose is to keep the condersers pressure in check when it is being burdened with a High Thermal Load...it is possible to have a perfect running a/c system and never having low aux activation b/c the termal load is just not high enough for the fan to get a sensor activation demand ..quite common with cooler ambient temps.. Remember , it is not the aux fan sensor that causes a comp. to cut-out...it is the rise in coolant temp or high side pressures that trigger the comp. off. The lack of aux fan just perpetuates those conditions.
...if you unscrew that sensor from the reciever/drier , you will loose the refrigerant b/c it is a pressure switch. The temp type sensors can be changed w/o refrig. being lost...that one can not.
As far as your coolant temp sensors being the fault, the simple test is to ohm them out and see if they are in spec with the temp/ohm chart..
The ACC temp input comes from the blue sensor [ 2 wire]..this also activates the aux fan, but, as you know ..High Fan circuit only ..not a/c low fan..low is strictly activated by the drier sw. [ completey different circuit]
That blu one is the one you want to make the ohm/temp comparison on...which is why i asked if you had ever done the resistor mod on that sensor..if one does such a mod and goes too far, that input sig also effects the comp cut-out range b/c it is ACC temp input, not just fan control.
You may also want to check the connection for speed sensor on comp..that harness is short and they have been known to be a problem b/c of their short length..worth a check.

A meter amd some sensor check should track the problem down.
Do you have the schematic??????????????

Yes Arthur I agree. High thermal load. Thermal means temp. Thats what thermal means. It reads the temp of the freon directly. That is why it's in the stream. I have not checked the voltage yet. That is where I have little experience. I do have an aldata on the car and can check out what you have told me to do. Could that be where the speed sensor might be problematic but can restart. By the way I rewired the spees sensor at the plug at the compressor because the fault was at that time the insulation pulled back from stretching and the wires were shorting at the plug. Reinsulated and new motor mounts and I have not had a problem since from that circuit. However I can tell you all from months experience dealing with the circuit it never restarted on it's own. Does this help?

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Well, you are incorrect, but I guess I am not going to convince you that it is a pressure switch, so be it..
Just be aware that the cut-in and cut out of that sw is in Bars , not C, so you might want to think about that a little more.
There are condenser temp sensors ,but they are simply surface reading sensors.. if a sensor is tapped into the system, it is a PSI sensor.

Here is direct Factory Manual Data Spec.
Job # 83-579
Chassis -124.034/1992 E400
[ your car]
S32 -Aux Fan Speed 1 pressure switch.
Cut-in pressure - 20 Bar. +/- .5 Bar
Cut-out pressure- 15 Bar. +/- .5 Bar

You see any temp specs there ??????????????????????????????????????



High Termal Load is a CONDITION, not necessarily a temp..

You should be able to track down a comp cut-out with a schematic and a meter..

I guess I feel a little Like Saumil at this point. Thanks for your help Arthur. I appreciate it

OK arthur. I'll split it with you. Reads the pressure which is a result of the freon being heated when compressed. Explaination from my mercedes authorized shop where the car has been 3 weeks yesterday. He can not find the cause of the problem. They explained it is not a mismatch circuit the comp can not resume without restarting. I don't know. It's been months and over a thousand dollars later with zero success. I am open for input from the brain trust if there are any new ideas. Thanks

Nobody has any additional input. There are so many people saying the comp restarts without the engine being restarted but the consensus is that this is not possible....I'm leaning towards tha motor mount on the passenger side starting to collapse putting pulling pressure on the wire harness at the com.... I will order new mounts and try that

Sorry to all I never realized that I never posted the solution. Yes.......it was the motor mounts. Replaced the mounts, got the extra 2 inches of play in the wire to the compressor...problem solved. While i was under there did rewire the plug as it was pulled enough too much bare wire was exposed and the insulation was no longer protecting them. I hope this helps.