I have a 1992 300te 4matic w/232k miles and my AC cuts out randomly and will start again when car is restarted.

I have searched and found lots of info for testing a Klima car but I have a MAS and no Klima [I believe]If I should have a Klima and the impressive Klima testing regimen posted by Samuil will work for me where is my Klima?

Does anyone have a testing procedure?

I am leaning toward the compressor speed sensor and have recently replaced the serpentine belt and my tensioner seems fine although from another post I tried to remove the belt [when tensioned] by hand and was able to pull it off the top pulley with two hands. Am I lacking belt tension? The owners manual belt length was 2415mm and the Gatorback belt for my car was 2420mm. Should I tension the belt further past the mark on the housing?

My AC seems to work fine in the driveway but the cooling fans only come on when car is hot,not when compresor engages and I am under the impression that the cooling fan should come on when the compressor engages

My low side pressure is 35lbs at idle and vent temps are around 40 at 2k rpm. I don't have gauges for high side pressure.

Thanks in advance

Excessive high side pressure is what would cause belt or clutch slippage. You REALLY need to borrow some gauges and make sure it's not overcharged.

That said, belt slipping and clutch issues are the most obvious causes of rotational sensor trip. A cause almost as common is when the compressor wears out allowing thrust movement of the innards which will trip the sensor even with no belt or clutch slippage.

Thanks for the attention Larry
I was able to check my low side pressure at 2k rpm and it dropped to 25 after a few minutes.
Am I missing anything else [other than checking high side pressure which I will do soon]
What is the mechanism for high side pressure causing cut-out?

Well the high side pressure causing problems is not due to a high pressure sensor. On the back of these compressors is a rotational sensor. The computer matches the rotation of the compressor to the rotation of the engine. If the computer senses that the compressor is turning less than the compressor it assumes that the compressor is locking up and interrupts the clutch circuit to prevent compressor debris from being strewn through the system.

This system has since been determined to be a bad idea and abandoned in later vehicles. The reason that excessive high side pressure can cause trouble is that it makes more resistance to rotation, thus sometimes making the belt or clutch slip, causing the sensor to trip out. A worn compressor that allows thrust movement of the innards can also indicate to the computer a locked compressor and cause shut down.

Monitoring low side pressure offers no data while troubleshooting this problem. You need a high side gauge in order to determine if there is excessive high side pressure.

OCASIONALLY you can correct this problem by spraying down the clutch with brake cleaner and letting it drip dry. If there is any oil in the clutch, cleaning it out can be just enough to let it operate a little longer. There's no point in trying this trick before testing high side pressure.

I was able to borrow gauges last night and results of this mornings testing are[caveat, the low side gauge on the set is apparently broken,I used the low side gauge from my charging hose] [caveat,I am using an r12 substitute from enviro-safe called Industrial 12A,I am aware that it is inferior to R12 but same cutout symptoms existed last year with R12]

Outside temp 62,temp at front of radiator 85

car off pressure equal at 90

car idling low side 35 hi side 175 vent temp 45,switch to econ gauges slowly toward equal and vent temps 65

car at 2000 rpm low side 22 hi side 180 vent temp 35,switch to econ gauges slowly toward equal and vent temps 65

Am I right to say my delta is 20 at idle and 30 at 2000 rpm?
The reading I have done tells me my pressures are slightly low?[This is supported by sight glass boiling]I have always charged by sight glass,which I know is not perfect,the boiling now may be a result of losses involved with multiple errors when hooking up gauges.

Any conclusions?

Am I correct to expect the electric fans to be running any time the compressor is engaged? Now they seem to only operate with engine temperature.

Speaking of which,my dash temp gauge operates [and has for 80k miles] at 80C,do i need a thermostat and can that affect my AC?

Any feelings about my belt tension being too loose?[New belt,good tensioner set at markings on housing but belt can be removed from upper pulley with two hands straining.

Is there a way to defeat the speed sensor without dropping the compressor?[when vacuuming before charge there was no soiling of vacuum oil,bearings/seals in compressor may not be as new but are not shedding pieces]

As noted my cutout does not happen in the driveway,I have closed my hood with gauges attached and will read gauges next time cutout occurs [It is cool today so it may not happen]

I forgot to mention that yesterday I did spray the clutch with carb cleaner [before your post that recommended brake cleaner] It was not obviously oily

I remembered to check my service records [from previous owners dealer] hoping to find the compressor replaced but no luck. Apparently this is original compressor with 232k miles.
However,belt tensioner was replaced at 130k

Also forgot to mention I may have found my slow leak [charge only lasts about 6 months] The schrader valve in the high side was not tight,I heard a tiny hiss when I removed the protective cap.

I'm not sure if that pressure is low or not since I'm not familiar with the press/temp characteristics of the alternative refrigerant. If it were R12, I would definitely say that it is low.

Yes you can indeed defeat the sensor using an ice cube relay in place of the klima relay if you are comfortable fabricating such electrical items. It is very possible that the compressor is just worn out allowing thrust movement of the guts which is tripping the trigger. There's no way to know for sure, short of replacing the compressor.

The klima relay is behind the battery. Remove the plastic cover behind the battery and it will be at the right side of the row of modules. You really need an FSM, but you might can search the forum and find the wiring diagrams you need.

thanks Larry,
I am under the impression that I do not have a Klima since I have the MAS.

Any feelings about belt tension? or other questions from my post?

Is my definition of Delta correct? and reasonably good?

What is FMS? Sorry FSM

wrooper, I'm not the AC expert, so I'll pass, though it does seem as though the pressures are low. No, the aux fans do not automatically engage when AC is running. That's under the control of a high pressure switch that monitors condenser pressure. If you're certain that the aux fans do not engage with AC, that might well be another indicator of low refrigerant charge, inasmuch as the charge may be inadequate to build up enough pressure to trigger the pressure switch. What strikes me about your comments is that you say that the temp gauge on the instrument panel shows 80C, but you say that only high temp causes your aux fans to engage. The temp sensor should be triggering aux fans right around 105C (unless it's been significantly modified). Could be the instrument gauge sensor is faulty, still showing 80C with the aux fans running.

__________________
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Owned since new and still going strong and smooth
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Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
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I'm not familiar with the MAS reason that you speak of. Can you find a Klima relay behind the battery? You don't have to pull the battery to get at it. Simply work the plastic cover out of the way.

Due to the way that the belt wraps around the compressor pulley on these cars, the belt has to be really loose to cause this problem. Not to say it couldn't be the belt, but give the belt a tug with your finger to see if it has any tension at all. If there is any tension at all, then this shouldn't be your trouble.

Thanks Cal
The fans did not engage this morning but they did yesterday when it was warmer and before I leaked some refrigerant hooking up hoses. It may have been engaging from overpressure yesterday?

My crappy handheld radar? [point and press] thermometer says 180F at the thermostat housing this morning and no fans on

I looked again for the Klima and found 3 large [6 inches long] relays that include the 4matic and 2 others I could not read without removing[they are fixed in front of one another] Also the MAS to the left and the OVP to the right.

There is a small metallic relay behind the OVP that I could not see markings on.

I got the idea [after searching for the Klima yesterday] From another Benz forum that I do not have the Klima because I have the MAS[I don't know what that stands for]

My E class owners bible says the Klima was discontinued in gas models in 1990 and replaced by the MAS [engine control module combines fuel injection,fuel pump,AC compressor control]

Anyone have any ideas about how to bypass the compressor speed sensor on a car with no Klima?[MAS instead]

BTW,Larry I am under the impression from other sources that the speed sensor is on the front of the compressor? The plug on the front has 3 wires which are red[may be brown] white and green and is not removable with my fat hands.
There is a lead going into the rear of the compressor that I may be able to access. Can I replace this speed sensor without dropping the compressor?

Can I drop the compressor without disconnecting the pressure hoses?