Well, I had been under the impression that the '96 and on c280 had a very good A/C system. I had mine charged this spring. It had no trouble passing a vacuum holding test, but this last week, I felt that the A/C was a little weak, so I took it for service. It was very low on refrigerant, but was still capable of running. The protection had not yet kicked in. It still passed a vacuum holding test, but the miniscule leak was sealed and all was well for 2 days. Yesterday the compressor started making horrible noises and today it will run but not load. Here goes just over $2000.00 for a new compressor (installed). Total mileage on car - 100,000km, almost exactly. Obviously the compressor's protection is inadequate and running the climate control system on "auto" for a month with lower than normal refrigerant level is not good for it. The new one is supposed to go in tomorrow so hopefully I won't cook as it gets warmer this week.

Unfortunately with the spring we have had, it was impossible to tell that the system was low. Having had it charged in the spring, I would never had expected it to be low so soon. Had it been warm, I would have caught it much sooner. Yes, I am in Canada, and MB Canada has no rebuilt compressors. $1400.00 for the compressor, some installation and taxes, brings it to about $2100.00. I really hope there's a warranty on the new one too.

Not if I want to drive the car for the next month, it's not an option. I rely completely on this vehicle. And it's not the dealer changing it. They don't do A/C. As for catching the leak, 2 separate shops did vacuum holding tests and pronounced it sound.

Hmmmm, I just reread my last post and I don't like the tone of it. It was rather abrupt. I really don't mean to be short with anyone. I have a very terse writing style due to occasional technical writing.

My thanks always to those who take the time to answer. I resigned myself yesterday to the expenditure. I'm not mad at my car, just a bit bummed about spending major $.

I guess the lesson learned is to keep the "EC" button pushed unless I deliberately want to run the compressor? I reread my owner's manual and it does not really mention this. Should I treat this system a little less carelessly? I have had it on "AUTO" for a couple of months now.

Hi John,

In your post "One reason the C-Class sompressors are so reliable has to with the fact that they run most of the time." does it mean the compressor always engaged when AC on? This is what I found on my 94 C220. It works not like those Japanese cars keep cycling the compressor on and off. I check the PBU item 7 by depressing REST a while then press FAN-UP to item 7, the reading goes up from 11.x to 16.x, then goes down to 11.x, repeatly. Seems pressure is OK. I wonder is there another internal switch in AC system to regulate the pressure?

Thanks in advance

CC

I heard that the compressor is always on unless the climate control is turned off or the EC button is turned on. This applies (compressor on) even when you are heating the cabin.

Charles, what does PBU stand for? I use the N22 read out for information like that also.

I am guessing item number 7 is the pressure in bar, correct? I don't have the list off hand, it's in my visor in my car. Now, can we tell if there is a leak or not by monitoring the pressure? What seems to be the "normal" pressure range and what pressure should we be concerned with? Anyone know? Hopefully Steve B will come and help us out.

And Benzfan, I agree with John S. on trying to put the blame on one of the AC shops.... though I'm assuming that would be rather difficult, but it's definitly worth a try and some persistence (for the $2100!)!

David, Thanks for your quick reply. Yes, the AC compressor behaves just the way you describe.
PBU stands for climate control's Push Button Unit. Sorry for not clearly state it. Item 7 should be pressure reading, I have no idea what should be correct value either. When in compressor off mode (EC) the value is about 4.5-5. Maybe it could be a clue to system leak. Need expert like Steve B to answer for us.

CC

Charles,

I will check out the ratings tomorrow and take not to them, and then post them here. I will do it with EC on and off. I'll try LO, 72 degrees, and HI, just to get rough ratings (and differences). I'll report them tomorrow night!
PBU- interesting... always heard it referred to as N22 Read-Out Values!

BTW, check out your ratings too so that we can compare.
(Isn't that feature just amazing!? I love it.)

PS: I'm still trying to figure out how to change the KM/H to MPH!

Whoah! Some of this is leaving a vapor trail over my head. Do you mean to say that I can monitor system pressure through the PBU on a '96?

Just got home from the shop. The new compressor is in but is circulating liquid. The vapor phase does not occur. Apparently the compressor took out the expansion valve too. Back to the shop Friday to have a new one put in. In the meantime we disconnected the new compressor electrically to ensure nothing happens to it for the next day.

Benzfan,
Yes, you sure can! I just did a search to give you a link to the thread that discusses this, but cannot find it?
Oh well... I have the codes for a 1995 C-class. They should be similar to your car. But for now, I'll give you the basics...

With car and climate control ON, press and hold REST for about 5 seconds (or until a new screen appears). There should be a 1 on the left side and a value on the right side (this value will be in-cabin temp.). I know how this system works on a dual climate control system, but not on your single mode... so I cannot be sure how it is laid out on your PBU screen.
To scroll up and down, you press AUTO (and whatever's on the other side where AUTO would be on dual climate). Sorry these are rather poor instructions, but you should be able to figures it out.

I can report to you what each value means later on this evening when I report my Pressures in Bar! Isn't this the best feature!? :) :) :)

I just gave it a try. The fan control scrolls through the parameters. If I read correctly, there are 23 of them. Most dont mean anything to me right now because my compressor is disabled and the system evacuated. I would love to know what all these are for the '96.

OK, first of all Benzfan, I will list off what the different codes mean. I had to improvise off of 2 different sheets and figure out what was mine since i could only get them for 94 C and some other model, but together, I think I got it pretty accurate. It might even be a LITTLE different than yours, but shouldn't be much different, or too hard to figure out.

1. In-Car Temperature Sensor
2. Outside Temperature Sensor
3. Heater Core Temperature Sensor
5. Evaporator Temperature Sensor
6. ECT Temperature Sensor (Coolant Temp in F)
7. Refrigerant Pressure in Bar (see results below)
8. Blower Control Voltage
9. Software Status of A/C Push Button Module
15. Selected in-car Temperature
20. Version Code
21. Engine Speed (RPM)
22. Terminal 58d, Battery Voltage, % of (ex: 48 = 48 of battery voltage, ie 13.1 volts)
23. Vehicle Speek (Km/H)
51. Current Number of Slip Recognitions of the Poly-V-Belt.
52. Stored Number of Slip Recognitions in the Poly-V-Belt.

Yours might vary a little, but this should be pretty right on.


Ok, now the results of the bar pressure (# 7).

I did a few different readings... wish I could make a graph, but this will have to do for now.
________________

In Car Temperature selected to 72 degrees.
-with engine idling and vehicle stopped, pressure seemed to be in the range of 15.4-14.2. With the car traveling at roughly 20 mph, pressure seemed to go down a little... between 10.1 and 15.

In Car Temperature selected to LO degree.
-with engine idling and vehicle stopped, pressure seemed to be in the range of 15-15.8. With the car traveling at roughly 20 mph, pressure seemed to be between 14 and 17.

In Car Temperature selected to HI degree (full heat).
-with engine idling and vehicle stopped, pressure seemed to be in the range of 12.8 - 17. With the car traveling at roughly 20 mph, pressure seemed to be a lot lower at 10.0 - 8.7.

Now I did some readings with the car in NEUTRAL and revved the engine to 3500 or so, the pressure seemed to slow go down as the revs increased.

Oh, almost forgot! When in EC, Economy Mode, pressure goes down to about 5, between 5.1 and 5.8 were my readings.

There is no consistency with these results, or any pattern that I can see. Maybe the pressure just stays somewhat consistent and varies upon many factors. I'm not sure. But I hope Stevebfl will come along and look at these results!!

BTW, why don't the car rev past 4000 RPMs when in Neutral?






[Edited by David C Klasse on 06-13-2001 at 09:25 PM]

Benzfan,
You did the right way to disconnect compressor. As my understanding fluid do harm the compressor since it is not compressible, where air is compressible. That's why the system needs a dryer to filter moisture and evacuate system to get rid of moisture before recharge.

Just my .02

CC

David, What a nice test and answer you have done. Bravo!!
For item #22, I believe it's compressor RPM on my car. When in EC mode it's 0, in AUTO mode no matter temp set to HI, LO or 72 degrees it's 9.3 which can be interpret as 930 RPM, at the same time the engine speed, item #21, is in idle 760 RPM. And it goes up along with engine rev up.

I only did the test in idle and recorded one data only. I'll try again tonight to record the pressure range as you did. The value of #7 I got is:
EC/temp LO: 6.8
AUTO/temp LO: 13.6 (varying)
AUTO/temp 72: 12.7 (varying)
AUTO/temp HI: 14.0 (varying)

By the way, what is the reading of #50? It's 0 degrees C at both EC/LO and AUTO/LO, 21 degrees C at AUTO/72 and 68 degrees C at AUTO/HI.

The ECU will cut out fire when engine reached 4000RPM in P & N gear for protection. Well, BMW does not do this way. You can experience body shudder in such condition, very similar to over RPM protection when driving. Don't worry about it.

CC

I was in such a hurry writing down all of the info, I skipped 50. It means nothing. I have it listed as empty or something like that. It will always say 0. So that's normal.

As for your # 22, it could be? I believe I have the papers that list a pre 6/94 production 202 N22 read out values. I can double check. That would be interesting...

As for the rev in N and P. Is this for transmission protection? Why wouldn't BMW do this? But the engine just sounded so silky smooth revving up at 3800 rpm....! And the rev limiter was very smooth and sustained. I wasn't sure what was happening because the few times I encountered the rev limiter was in 2nd gear (manually in) and I forgot to switch it out!! It's not pleasant, quite jerky.... must just be 'cause the car's in motion!

Anyway, I'm going to compare the values of our pressures now. I hope others can give us their data too for comparison.

I guess I have been sleeping. Actually the laptop I take home is getting its brains reworked so it can attach to my home DSL line and I haven't had it for all this week.

There are a couple issues here that I will comment about since no one has writen technically about this post.

First issue is temp control. All MB systems till around 1997 used compressor cycling to control evaporator icing. All climate control versions uses system fixed, cycling temperatures whether they used a mechanical switch (as in 123/107/126 chassis - early) or electronic temp probes and logic control. The concept here is to allow the evaporator to reach close to 32 degrees then shut off the compressor and restart when the temp has risen about five degrees. If measured at the center duct the temp is usually from 35-45 degrees when cycling occurs (this often is affected by the temp of the ducts if one is measuring).

I guess the seond issue is what happens around 1997. At this point variable displacement compressors started being used. Its hard to explain this simply. I suppose I will have to write a book. Let me first describe how constant compressor systems have worked since the sixties as all of this is based on the thermodynamics of change of state and pressures ounder the ideal gas laws.

In the cycling example (above) the pressures inside a R12 system drop to around 28psi on the low side. At this pressure freon is boiling and the temp is below 32 degrees. The compressor is developing 180- 250+psi head pressure (depending on outside temp and design). The compressor is shut-off and the low side goes up and the high side goes down. The high side is the pressure that comes over the PBU. In early GM systems a device called a STV valve (suction throttling valve) was placed at the exit from the evaporator. It allowed the pressure to be capped inside the evaporator and as a result the temp was stabilized right above the freezing point of water. In other words the boiling freon which would thermodynamically continue to drop the temp, has its boiling capped similar to a pressure cooker reducing the boiling of water. The later POA (pilot operated absolute - suction throttling valve) valves were used on a number of domestic and Jappanese cars.

But leave it to the Germans to do it differently (actually GM did it first), they control the temp in the evaporator by reducing the pumping capacity of the compressor ). In this case as the evap temp hits 30psi the wobble plate in the compressor is driven by the internal POA valve to a near verticle position. In this position the pistons of the compressor hardly move. The point is to use the least amount of effort to achieve an evap temp just above 32 deg. This amount of effort changes with rpm and air temp and the amount of heat needed to be removed.

Anyway if the pressure on the PBU is cycling then the car has an early compressor. If it slowly changes or just stabilizes at a given road speed then you have the new compressor.

Lots more could be said and some of it I have said. Here are is an articles I wrote on MB A/C for the trade publication "Import Car". There is another that I will try and give an address to but it didn't work here so I removed it:

http://www.continentalimports.com/ser_ic60046.html



[Edited by stevebfl on 06-14-2001 at 12:32 PM]

John,
Well, # 23 is vehicle speed on mine, as I said, they probably will be slightly different. And thanks for making me aware that the exterior temperature plays a role in the pressure! It makes perfect sense. I am 'beginning' to understand...

Steve,
Thanks for helping us out on this thread! So you're telling me that I have compressor cycling on my 1995 C280, and anything 97 and later has variable displacement compressors? But it seems to me that my pressure via PBU changes slowly...? Well I for sure will have to read this one or two more times but I've got to get going for now. Thanks again Steve for coming! :)

I am at a loss for words seeing the response to this. I thank everyone who took the time to reply.

My '96 has variable displacement, as this is the year MB made everything OBDII compliant (fully). '96 seems to be the break year on the compressor part #. My readings from the head unit are quite different from those posted here and when I get things working right, I will post a list of them. The only one that has me truly puzzled right now is 43, which in my car reads 'bEn'. ?? The rest I think I can figure out. Except instead of 9 in mine, it is a -9 and the reading is in degrees Celsius. Between 51 and 54 is a reading labelled -2 which says 10 degrees Celsius. A little playing may tell me more this weekend.

Benzfan,
No problem! I'm glad we could help... we even helped ourselves along the way. :) I will get out the sheet that Steve faxed me some time back and see if I can figure out what those numbers are for you. I have quite a few lists, 3 I think. Maybe I can peice together what your other codes might be.

Send me your name and where you are from and your year and model and a fax number and I will send a list for your car.

I will do that for the first ten people that reply to limit my time and long distance costs.

Remember, like all other testing on the road, one person should drive and the other monitor the readings. I would not suggest doing this while talking on your cellular phone on the Santa Monica Freeway (bg).

PS: unless of course you are stopped, which I guess is more often the case on the Santa Monica Freeway.

Steve, thanks for your insightful technical explanations. So my 94 C220 have a fixed, cycling compressor. Back to my original question, the compressor seems never visually disengaged once AC is on but the PBU #7 reading does change back and forth from 10.x to 16.x in engine idle condition. Besides, the lowest evaperator temp is only 39. It's not as cold as you said 32. Does it mean I have to flush and recharge the system then the compressor may work the way it should be.

Thanks a lot

CC
94 C220

CHarles,
Your readings should be between 35 and 50 degrees on LO, depending on fan speed and engine coolant temperature. 32 seems a little low.

Wow, a fast reply in 3 minutes. What a quick service. ^_^
Thanks a lot, David.

I set the temp to LO, fan speed to level 2, coolant temp is about 88-90 degrees C. So 39F is OK for evaperator.
I feel more comfortable.

CC
94 C220

Charles,
YEs, your evaporator sounds perfect! I wouldn't worry about a thing. Your temp sounds fine. Now, how to detect leaks in the system! I seem to have a general feel for it now so I will monitor it occasionally.

BTW, anyone know how often we should have our AC system checked?

The 32 degree temp is what has to be avoided. Ideally the evaporator would be held just above the barametrically calculated freezing point of water (32 deg F at sealevel). When your evaporator is at 33 deg the air blowing over it is never made as cold as the actual metal of the evaporator. The sensor is measuring the temp of the air right as it comes through the evap. I would guess that in your case where the air is 39, that the evap is very close to 32 if not cooler.

This relationship (the increase of temp related to distance from the evaporator)is further realized if one watches the calculated temp on the PBU and also monitors it with a digital thermometer at the center vent. If you are cycling at 39 then you will see 42-47 degrees at the center vent. This changes as the car cools off. The first cycle from at dead start with a hot car can sometimes look like 50-55 degrees. That is because that 39 degree air is heated by the 130deg temperator of the dash and ductwork.

It also must be noted that in hot southern climates the A/C may never cycle around town, ESPECIALLY if run in the high blow position.

Thanks master Steve. You clear my mind. The reason why I care the AC system is because my compressor and dryer were replaced last summer and got whole system flushed and recharged. The job was done by my uncle, and he is not so familiar with MB. Through your and David's explanations, I believe I can have a comfortable summer this year. Thanks again to all of you.
BTW, I live in Taiwan and it's hot now.

CC
94 C220

I just retrieved my car tonight, and all is well. The A/C works as good as new. We did, however, have to replace the dryer as a final piece. Total bill: $2700.00CDN. The shop (a well-known chain) insisted on giving me a rental for the weekend. They gave me an unlimited-mileage Lincoln Navigator. Interesting but thirsty vehicle. There is a warranty on parts and labour for my A/C and they are going to try to help me figure out all my head unit's codes and parameters for nothing later this week. I'm happy with their work and service.