air conditioning puzzle
 

Here is the latest puzzle in the life of a 92 400e. Air cond works great til car is hot. ..then the ac comp cycles frequently and is not sufficient to cool the car. High side gets near 300lbs and the low side around 50 or so. When the car is cold the preassures are about 225 high side 38-45 low at idle. ambient temps around 98 so this is still normal. compressor has 186 k on it but makes no noise at all and makes cold air on cold start up...Ideas?
Thanks for any help with our puzzle.


P.S. The comp stays running at fast idle. when i let it return to normal it cycles at high side 225 low side at 60. New expansion valve, receiver dryer.

Brian

Is the aux fan/fans coming on ? If they dont come on, the pressure at the high side will remain high and that would cause compressor cut-off/cut-in cycles. Did you recently charge the system ? It could be slightly overcharged. If the aux fans do come on at the right times, I would try to remove some refrigerant from the system, get the high side down to between 185-205. Another possibility is that you have air in the system, is there a leak in the system ? Have you charged the system without evacuating ?

Hi Saumil.Thanks for replying. I've had more time to check out the car. The auxillary fans work great. The expansion valve is done, the receiver drier, and now the evaporator temp sensor has been replaced. The compressor stays engaged when the rpm's are maintained at acceleration but when I let it idle after the car is hot the comp disengages and will not reengage until I raise the rpm's. This is only true after the car is warmed up. When cool or cold the comp stays engaged full time. I"m scratching my head. Any input will be greatly appreciated.

Brian.

What is the high side pressure when the car is hot and compressor just cuts off ? Belt is nice and tight and the compressor pulley is oil/grease free ? Battery/Alternator voltage is o.k. at idle when warm ? Is there a difference in the battery voltage when cold vs. warm ? The cold vs. warm and low vs. high rpm tests you did were with the car stationary right ?

225 on the high side and cooling perfectly...no slippage on the belt...the compre3sser can be made to rengage the compressor without a restart so i know it's not the crank match mismatch circuit. I don't know about the voltage and yes the car was stationary....does this help?
Thanks...Brian

PS..When I replaced the components. (changed out the expansion valve because of high sides over 300 low sides in the upper 20....clogged or failed expansion valve I think) Drew a vacume to -30lbs for 4 hours. It held off the pump with zero leaks over night. Charged exactly 2.2 lbs R134 as the book calls for. I'm in florida so at 180-205 high side at idle with the outside air temp at 98 would I believe be undercharged. Could a bad relay cause this?

I would suggest making sure the fins on the condensor are clean and clear of bugs and debris. The condition you are describing is consistent with over-pressure due to the condensor not cooling sufficiently at idle. If the system is really not overcharged then that is where I would look. The reason it kicks back in when you rev it is because the fan speeds up and cools the condensor more and pressures drop below the cutoff (300 PSI). Clean out the condensor and between it and the radiator and see if that helps. Sometimes you can find quite a mat of crud between the condensor and radiator which blocks airflow. Cleaning it out will help both the A/C and the car's cooling system so it will not be time wasted even if it does not cure the immediate problem.

While at idle, pull the high fan connector from the blue temp sensor to default to high fan and see if the comp then holds IN...

...and give me a coolant temp reading when condition exist

I remember someone mentioned that in his case, the problem was with the speed sensor (either engine or compressor, I dont remember) but the strange part was that the compressor would come on without a restart, and I was puzzled, but what I am trying to say is that just because the compressor comes on without a restart would not rule out the speed mismatch based cut-off condition. Atleast check the connections to the speed sensors, make sure they are clean. The compressor speed sensor should produce an a/c voltage, on my 87, 260E compressor (assuming the compressor is similar in our cars), about 0.3V at idle.

The pressures are not that high right before the cut-off, your pressure cut-off switch could be malfunctioning. Try bypassing that switch, just jumper the two wires.

The fact that you can make it work with a higher rpm I think rules out the coolant temperature sensor based cut-off, because the temperature is not going to change instantaneously when you change RPM. I am assuming you have sufficient coolant in the system.

<The fact that you can make it work with a higher rpm I think rules out the coolant temperature sensor based cut-off, because the temperature is not going to change instantaneously when you change RPM.>

His chassis uses thermistor sensed ACC Panel temp input and they change very fast..these is not a mechanical sensor as the older chassis use.

His posts have some contridictory information. In the first post he says when cool high pressures are 225 and when hot they reach 300 which would trip the high pressure cutoff...in post #5 he says 225 is the "hot" pressure when it would cutoff which would not be high enough...so which is it? That is pretty critical.

Right.
Which could be fan ..
So, a high fan at idle will tell us if it is an airflow/condenser based problem..and coolant temp will lead us into ACC temp input data. [then next ? will be if he has sensor resistor mod, which effects comp cut-out on termistor type circuits.]
Specific info is needed ..and "Fan works fine" could be taken many ways by someone diagnosing the problemlike..is the fan cycling when the pressures are different???????????????????????.....

Thats true, the temperature sensor in his car could change faster than in old ones, so it could still be lack of engine and or condensor cooling. Hose the condensor with water when it cuts off at low rpm, if it is insufficient condensor cooling causing the problem, the compressor will come on after it has cut-off without any change in RPM.

Which brings it right back to my post of defaulting the aux fan into HIGH FAN ...this should give more than enough added airflow to diagnose the problem..ever notice the big floor fans we use in front of the car at A/C Service centers ????
Added flow will keep high side in check at idle condition...........

hey guys thanks for the input. Let me try to clarify a few facts. The first post says the pressure reaches around 300..at fast idle...the compressor stays engaged...when I let it return to normal idle the pressures return to around 225/45...at those numbers the compressor operates normally for as long as 10-15-even 20seconds after returning to normal idle...then cuts out and won't come back on until I rev the engine...this only happens once the car is hot. When the engine is cool it is not a problem. I tried causing the aux fans to run at high speed to see if it corrected the problem at idle..it did not. I don't believe it's the high pressure cut out ..not at 225..With regards to the fan works fine inquiery I will have to see. The low fan comes on when the temp sensor on the dryer reads the heat there but I don't know if the fans stops after the compressor disengages. I will check. I think I noticed the fan turning off but that is just a guess.

<The low fan comes on when the temp sensor on the dryer reads the heat there but I don't know if the fans stops after the compressor disengages. I will check. I think I noticed the fan turning off but that is just a guess.
>

The low fan is high side pressure sensed , not temp...
Parts cat. says temp, but that is incorrect on that chassis.