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Wacky Mercedes A/C

stim141 on Wed June 24, 2009 2:09 PM User is offline

Year: 2000
Make: Mercedes
Model: CLK430
Engine Size: 8
Ambient Temp: 100F
Country of Origin: United States

This one I can't figure out and it seems the local MB dealer can't either. A month ago had poor cooling and dealer said had low refrigerant and a leaking condensor. Prior to the installation you would hear a slight hiss from the front vent and a low growl at low rpms. After replacement seemed to be gone.

Now - system cools in the morning then afternoon will periodically stop or not be very efficient. Exterior temps about 100F with humidity. Interior center vent starts high then drops to about 64F then starts to spike - this is at idle. I also noticed when it spiked the engine also had a blip in power. If you start to drive it will drop to about 45F then spike up again. When its starting/during the spike you hear the compressor kick in and out repeatedly. You also can hear what sounds like a chirp/squeak which comes and goes.

When you increase RPMS at idle the ac center vents when its at its bottom center vent temps will not decrease as expected. When I had problems with my VW GTI it would do this and in a new C class it also does this.

So ... yesterday the dealer said he can't find anything wrong or can't replicated the problem. Well in five minutes in their lot it happened again - tech writer had me drive to shop with the tech and it started to kick in again - but he noticed the noise and saw the cooling or lack of cooling. The tech said about 45F was ok in these temps but to me that seems pretty high considering the loaner I'm driving gets as low as 38F and my VW 95 with 195K can go as low as 37F with recirc closed and low fan speeds.

Any ideas? They've had it off and on since last Wenesday. Forgot to mention that an extended warranty company will pickup the tab for any replacement parts (dealer or independent) and to me this sounds like a compressor seizing problem or at the very least a bad high pressure sensor although the chirp noise/engine blip is a little odd.

Voyager97 on Wed June 24, 2009 3:49 PM User is offlineView users profile

That chirp/squeak could be a slipping belt. That would also explain the engine blip. When the a/c clutch engages, the engine ECU increases the engine speed to compensate for the extra load from the compressor - if the belt is slipping the engine will 'race' until the extra load catches up.

Vic @ Pennine AutoChill

stim141 on Wed June 24, 2009 9:29 PM User is offline

I thought so but it only rarely happens. I really think the compressor is seizing up causing the noise and lack of cooling. Today the dealer said they replaced the evaporator sensor and everything is cooling. WELLL not really - picked it up or rather got into the car in the parking lot and turned in on. On high started to cool - vent temp at about 65F then it stopped cooling and the temps started to go up and did not go down. Was going to bill warranty company 365 dollars. Asked about clogged expansion valve - said they would have seen that when they read the gauge. Said they had a good vacuum when charging. Either the control unit is defective or the compressor is seizing up. If its the controller it could send out a signal that tries to turn on and off the compressor. Still even when its working the temps will only get to about 43-44 out of the center vents which is a little high. They still have it and haven't billed anything - so hopefully they'll get somewhere tomorrow.

Matt L on Sat June 27, 2009 5:06 PM User is offline

The first thing is to check for AAC trouble codes.

Is that a W208? I assume so, but I am not sure. For the W208 as of MY 1998, here is the procedure for reading those codes through your climate-control pushbutton unit (you do have one, right?).

1. Review prior sections in the manual. Omitted this step for now.
2. Ignition: ON
3. Temperature selector: left: "HI"; right: "LO"
4. Ignition: OFF
5. Ignition: ON
6. Within 20 seconds press REST and EC simultaneously for more than 5 seconds.
7. The led in the "recirculate" button flashes and the display shows "di R"
8. Press AUTO repeatedly until all DTC's are displayed. Record each DTC as it is displayed. (let me add that FF means end of codes.)
9. Each fault is assigned a DTC. DTC table omitted for now.
10. To erase: Can only be performed after all faults have been readout. Press left AUTO and right AUTO buttons simultaneously for more than 2 seconds, and the display shows: left: "d" and right: "FF"
To cancel erase, press the left AUTO button. The current faults will appear again.
11. Select a base temperature setting.
12. Turn ignition OFF, to end test.

stim141 on Sat June 27, 2009 5:35 PM User is offline

Did that - didn't throw any codes. Also dealer has Star diagnostic and said they can't find anything. They still have it and called on Friday. Said they still can't reproduce the problem and its cooling but they didn't say what the center vent temps were so I left it because I know it will still do the same if I drive and pick it up. Was able to reproduce the problem in 5 minutes before leaving the dealership. Turn it on it starts dropping vent temps slowly from 90 to about 65 then stops - turns off cooling then temps rise. Will only go down again when driving for a minute or two but never drops below 45F with ambient at 100F and fan on slow - loaner MB gets about 41F with fan on full - when on super low it will drop to 38F.

Personally I think the compressor is acting up (hear squeaking noise when its not cooling) or the controller that was recently replaced is defective - telling the system to shut down and restart hence not cooling. I wrote down the controller coding before it was replaced and the numbers are vastly different - P1 was 160 and P2 was 15 now P1 is like 235 and P2 is 150 something. Don't know if that could do anything to mess up the AC controls. What is a little strange is the ambient sensor (not the temp on the gauge cluster), it can read up to 145F when sitting at idle and will drop to actual ambient when driving. They did replace the evap sensor and it still does the same thing. They still have to check all the other sensors and maybe the DUOVALVE. If I had the car I can get some of the sensor readings and post them here through the DIAG menu on the controller.

Edited: Sat June 27, 2009 at 5:39 PM by stim141

Matt L on Sat June 27, 2009 5:44 PM User is offline

Yea, it's a complex beast, but nothing is simple any more, it seems.

Do you (or they) know if the clutch is disengaging when the cooling stops? Squeaking is the odd thing here, since adding heat shouldn't affect anything in the cooling system. Or maybe you have a clutch bearing on its way out, and the squeaking is simply the noise you hear with the compressor disengaged.

Which sensor do you mean by "ambient sensor" if it's not the exterior gauge temperature?

stim141 on Sat June 27, 2009 8:00 PM User is offline

Don't know about the noise either - seems ok when cooling - but it may be when its engaging/disengaging. Ambient meaning external temps but not the same as whats displayed in the speedometer. Just what is being sensed by the climate control as outside temperature and is read when pressing/holding REST to get sensor readings. When I get the car back I'm going to enter that mode and check the sensor readings when the system cuts out.

Can't understand why the dealer can't figure this out its a huge Mercedes dealer with tons of service bays. Love it when they say they got it and I can fail it in five minutes just sitting in their lot.

Matt L on Sat June 27, 2009 11:27 PM User is offline

Being very cynical here, but they know that if they do some work, they'll get paid and you don't care. Perhaps they are reverting to the parts-changing mode which seems to trap so many mechanics.

Back to the "ambient sensor" question. I am not positive about it, but I believe that your instrument cluster supplies the ambient temperature to the climate-control pushbutton unit. If so, and if these readings differ, it points to a problem with the data bus, or one of the modules (cluster or pushbutton unit) being bad.

stim141 on Sun June 28, 2009 12:08 PM User is offline

Thanks Matt also posted in - They are different only when the car is sitting or running at idle. When it starts moving both match. We are not talking about a few degrees - gauge reads say 85 and the diag sensor mode can read as high as 135 or more. You can see it move up when the car is sitting in the sun so its reading something. Dealer said the sensor is in the airbox not the small tube at the front bumper. So it must be sucking hot engine compartment air?

Don't think the control button assembly is defective (something else) since it was replaced within the last two months and the old one did the same thing - although the AC was working better even with the leaking condenser and low charge.

The post I received on Benzworld sort of makes sense in that the system may be slightly overcharged. When I drop it off its in the 80's and they claim they can't find anything wrong - assuming they didn't do a evac/recharge to get the correct freon charge. When I picked it up last week the temps in St Louis were in the high 90's 100 so the pressures would probably be slightly higher - enough to trigger the high pressure sensor and stop the compressor. When the car starts moving and there is more air flowing through the condenser it may just be enough to drop the pressure? Just thinking outloud. When I had a VW GTI VR6 that was overcharged - even slightly you can hear the compressor groan at higher RPMS - don't hear the same thing on the Mercedes but the VW was really tempermental when it came to its charge.

Will give them one last chance before taking to Manchester Radiator - the guy there - coincindentally named Matt always seems to get it right in a day. Only took it to the dealer because I have an extended warranty and they have no problems getting things approved.

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