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.

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

<<along with 2 temp sensors - one at the drier and one near the exp valve.
>>

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.

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 !!

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.

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.

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).

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

http://www.peachparts.com/shopforum/diesel-discussion/158591-ac-still-not-working-after-ac-conversion-87-300sdl-7.html

New twist a/c puzzle.
 

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??

<<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. >>

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?

<<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. //

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.