I believe you've got a pressure sensor on your car, along with 2 temp sensors - one at the drier and one near the exp valve.

High pressure is caused by compressor running too long, too MUCH freon, condensor coils ineffective, pressure switch inop, lack of PAG, radiator clogged in front of coils, fans switching incorrectly, triffling w/fan switches to make a/c run "better", iced up evap/expansion valve, other clogs in system (like ice, debris), radical 134a formulations guaranteed to be colder, etc.

The answer will jump out at you as you eliminate all the possiblities

< >>

That chassis has two pressure sensors..one for comp cut-in/cut out [ bi-sw., with cut-outs at both low/high pressures] and the other pressure sw. for aux low fan activation..both are on the high side reciever/drier and they are not temp sensors.

hey Arthur..thanks for all the input. The aux fan is getting the reading from the heat of the freon at the receiver/drier. Thats what I meant by the heat. I just want to add I appreciate all the help. Thanks.
Brian

I know at this point that hight pressure is not an issue of any kind. The puzzle is missing a different piece. Just wish I could nail it down. Thanks

That is the point I was making.. The heat of the refrigerant does not trigger the fan , the high side pressure does ..it is done with the S31/1 pressure sw on the dryer ..it cuts fan in at 20 bar high side pressure and cuts back out at 15 bar. So, if there is a high thermal load on the condenser , you get aux low fan, but as the car travels the aux fan is not needed and will cut-out as the high side pressure drops to 15 bar from moving car air-flow. Granted , the refrig is hot in the condenser, but the heat is not the triggering factor for the sensor, it is the high side PSI that triggers the sensor...ie pressure, not temp....

I understand that I think. Did my causing the auxillary fans to run at fast speed without a change in the problem rule out excessively high heat ?

If comp still cut-out at idle with high fan ON , it would indicate that high side pressure is not the problem..which you were thinking anyway...a gauge reading of high side at time of fault would be nice to use in diagnosis..
You should see a large difference in high side pressures with and w/o aux fan activation..that would definately support proper airflow through condenser diagnosis [ which would also eliminate the possibility of debries condition between radiator and cond. ].
You may just want to try that comparison if you are seeing abnormal high side pressure when there is little thermal load. ...

So additional air flow did not fix the problem, then it is less likely to be insufficient condensor cooling. Make sure the fan is turning in the correct direction, it should be sucking in air from outside to inside the engine compartment.

The engine is not excessively hot (<100 degC) when the compressor shuts off, right ? When the compressor comes on again at higher rpm (immediately), is the engine temp noticeably different from when it shut off ?

Did you check the battery voltage at low vs. high rpm when hot ? If it is not voltage, not engine temp sensor and not high refrigerant pressure then the most likely (perhaps the only possible) cause looks to be a speed sensor malfunction (either one: engine or compressor) under heat, which is quite likely.

__________________
Saumil S. Patel

Pull the evap temp sensor out and let it hang there ...if you have low evap blower at idle you can have evap flooding and evap sensor cut-off ...I have also seen the same problem with plugged blower filter..
That would expalin it only doing it after evap has had time to come down on temp/on for a while ...

Blower motor speed changing with RPM is possible if the alternator/charging system is not adequate. Low RPM causes less air flow, and the evaporator freezes because of lack of air flow causing the compressor to cut-off, yes, very good possibility. So do you notice a change in blower motor speed in low vs. high RPM conditions ? Check the battery voltage !!

__________________
Saumil S. Patel

Let the sensor hang and try it..
What does your eyesight look like???????????????

pressure at idle 225/45. Site glass shows flowing feon adequite but I know I have an exact charge. I drew a vacume and recharged it according to the number of oz's called for in the book, 2.2lbs. I will try the evap sensor (which is brand new this project). The blower speed is unafected by enging speed.

Huh !! No change in blower speed with RPM, and no change in air flow with RPMs either ? But try removing the evaporator sensor anyway, since it is a potential source for the compressor cut-off signal. If it turns out to be the cause, the explanation can come later. Everything tested so far has been indirect links between RPM and compressor cut-off, the direct link is still the speed sensors.

__________________
Saumil S. Patel

Why do you keep repeating others posted steps..?

I am just reasoning, along with you all, this is how I am used to brain storming, but if it is bothering someone, sorry. I was puzzled a few days back when someone treated this forum as a competition stage, saying that 'he is the winner' etc., I think we can be lot more productive if cooperation prevails over competition. Just my feeling.

__________________
Saumil S. Patel