w126 air conditioning problems again - PeachParts Mercedes-Benz Forum

SCOTTFISH · #1 07-30-2004, 12:10 AM

I know I have probably worn this issue out, but still have not figured out what my problem is on my wife's 89 300se air conditioning. The ac has been charged and when working is extremely cold. But, one day it will blow cold from all vents and maybe the next time she gets in to go somewhere, it is just ambient through all vents. I have had a person or two say that it very easily could be the control button panel needing to be resoldered. I am not real handy resoldering things. Is it possible that a relay of some sort is not working correctly? I have checked fuses and there ok. I can't really tell if the compressor is kicking on or not. I did at one point have it recharged and the mechanic stated that the compressor was in good shape. Can anyone advise me on what steps I can go through to maybe track down my problem? I am very novice on test meters. I have also checked the mono valve and it checked out ok. Any clues you can give me would be greatly appreciated. Again, one day the air may work great all day long and the next 5 days it may just blow ambient air. If you move the dial to heat, it gets hotter than the hinges.

sixto · #2 07-30-2004, 12:27 AM

Things more or less related to AC compressor nonfunction:

- oil or dirt geting into the AC compressor clutch -- if it causes the clutch to slip, the ACC controller or Klima relay (if equipped) will think the compressor is binding and cut power to the clutch

- faulty engine rpm signal into ACC controller or Klima relay (if equipped)

- faulty cabin temp sensor

- faulty engine temp sensor

- low refrigerant pressure

- faulty solder joints in ACC PBU

- faulty solder joints in Klima relay (if equipped)

If your car has a Klima relay, you have an intermediate point at which to probe whether the problem is upstream or downstream of the Klima relay. BTW, the Klima relay considers compressor rpm and engine temp to determine whether to pass the ACC PBU AC compressor on signal to the compressor. Hook up a test light to the AC compressor on pin at the Klima relay to localize the fault. Then take it from there.

If there's no Klima relay, I don't know where to begin besides reflowing the ACC PBU solder joints. Which BTW is a PITA to take apart.

Sixto
95 S420
87 300SDL

#3 07-30-2004, 09:06 AM

Check for threads that explain the function of the door (behind the glove box) that controls fresh air intake. As I recall the defalt is full open. Your AC can be fine but cant overcome all the hot air coming into the cabin. I had the same problem but mine was due to a satellite antenna wire I had ran that jammed the mechanism open.

#4 07-30-2004, 09:12 AM

Check for threads that explain the function of the door (behind the glove box) that controls fresh air intake. As I recall the defalt is full open. Your AC can be fine but cant overcome all the hot air coming into the cabin. I had the same problem but mine was due to a satellite antenna wire I had ran that jammed the mechanism open.

#5 07-30-2004, 04:43 PM

Since it is cooling one day and not cooling the next, it could be a deteriorated tube that draws air past the cabin temp sensor. I am not intimately familiar with the 126 chassis so I can't tell you where to look.

Maybe the 126 hounds can tell you what and where to check.

Good luck,

#6 08-02-2004, 02:48 PM

Sensor is located to the left of the sunroof control. I believe the test is to find enough vaccum to hold a piece of tissue to the sensor when the systems calling for A/C?

#7 08-02-2004, 02:55 PM

Sensor is located to the left of the sunroof control. I believe the test is to find enough vaccum to hold a piece of tissue to the sensor when the systems calling for A/C?

stevebfl · #8 08-02-2004, 03:12 PM

It all boils down to the fact that you are going to have to have someone show you how to tell whether the compressor is turning or not. There are a billion senarios but your symptoms require this first bit of info: Does the compressor turn when the cooling is poor? All other diagnostics wait the answer.

crowesnest · #9 08-16-2004, 08:21 PM

The a/c on my 1982 280SE began to cool one day and not the next. My guru said he would put money down that the only problem would be 22 year's worth of gunk/crud between the clutch and the compressor body.
He was right. He cleaned it up and it hasn't missed a beat since.
Crowesnest.

engatwork · #10 02-04-2011, 07:46 PM

Try swapping in a known, good pushbutton unit to see if that helps.

mak · #11 02-05-2011, 09:39 AM

the 89 300se 126 does have a Klima Relay , Mostly intermittent operation is rectified by cleaning its pins and re-installing . Next replace the 8A fuse for the ACC control system if its old one ,(# 5 or 6 )

mak
300 se 89

tinypanzer · #12 02-08-2011, 12:33 AM

There are LOTS of possibilities, but start with a simple test that will tell you a LOT. The fresh/recycle flap is located just behind the glove box. It takes all of 5 minutes to pull out the glove box.

Once you have it out, observe the door behind there. This door selects if you are drawing fresh air or recycled. A/C on a hot day requires this door to work correctly. If the two pink vacuum pods in front of it are shot, then you will never have reliable cold air. Check out this video: http://www.youtube.com/watch?v=rRP892ytqXg

The door should function like this when you apply vacuum to the green line. It should also do this when you press the recycle button on the console, or when it's hot and you put the A/C on. If the pods are shot, replace them. If the pods hold vacuum but you still don't have the door working correctly with the recycle button, the next likely cause is a bad solder joint on the climate control vacuum solenoid unit. I posted here about that somewhere, with pics.

Once you have verified this door is working as it should, if you still have problems then post back. Check this first, it really is the most likely cause of poor A/C in these cars, and if those vacuum pods have never been replaced then they are almost certainly shot.

Good luck, post back!