AC to Intermittent/Random Heater Mode - PeachParts Mercedes-Benz Forum

BayouFlyFisher · #1 07-21-2014, 01:38 PM

Here's my AC story: - It has worked flawlessly until yesterday afternoon. I was driving along and 10 minutes into the trip the AC suddenly went to Heater Mode - HOT air and no flow out of the two center vents. This failure only lasted about 30 seconds and then the problem seemingly corrected itself. About 20 minutes later, the same failure occurred - full on Heater Mode. This time it took about 5 minutes to fix itself. No amount of button pushing, turning on and off, etc. would make it work right until it fixed itself. About 10-15 minutes later, it repeated the failure mode and did not fix itself during the rest of the drive home (10 minutes or so).

This morning it is working exactly as it should. Being brave and daring the South Louisiana heat and humidity, I ran my morning errands in the car. In the two hours of stopping and restarting (about 40 miles altogether) the AC did not miss a beat - no heater mode. I am now living in dread of this problem resurfacing while; A) I am a long way from home and/or B) I am on my way to a meeting/event for which I should be all hot and sweaty upon arrival.

My first thought was the vacuum actuator that operates the heater flap. But then I started to think that it was more likely to be an electrical issue due to the sudden failure and the sudden "self-repair”. Plus since my Cruise Control recently failed in exactly the same manner (sudden and random failures with "self-repairs") and that issue was only cured by having the Cruise Amp repaired/re-soldered by "JamesDean JamesDean" (LOVE the actor and the Eagles song), I am leaning towards the Climate Control Unit needing the same type of re-solder/repair. Right now my best option seems to be to pull the CCU and send it to “JamesDean JamesDean” for the re-solder/repair.

Opinions/Options/Suggestions??

uberwasser · #2 07-21-2014, 01:54 PM

This is a W123 with the horizontal push button? Since you don't have your car in your signature it is helpful to include it in your post.

It is sounding like the push button unit is the likely culprit. Flow of coolant is not controlled by vacuum, there is no heater flap per-se. Air is always flowing over both the A/C evap and the heater core - temperature is controlled by the flow of refrigerant and/or the flow of heated coolant. Your A/C may still be operating when you're getting full heat, but it cannot overcome the heat from the coolant especially if the monovalve is suddenly allowing full flow.

Since you mention that the center vents are closing, my best guess from afar is that the push button unit is putting the system in to defrost. This closes the center vents and opens the defrost vents and brings on the heat. Even in the winter, when in this mode on purpose, the A/C compressor would run to help dehumidify the air in the cabin and as noted you'd still have hot air because the heater core can easily overpower the evap.

BayouFlyFisher · #3 07-21-2014, 02:05 PM

First, thanks for pointing out I didn't have my car info showing. I fixed that.

Yeah, that's sort of where I'm at too. My car has the vertical push buttons and a temp wheel to the left of the buttons.

Thanks for the help!

Air&Road · #4 07-21-2014, 03:33 PM

Before replacing the ccu, remove the glove ox and make sure the horizontal tube connecting to the sensor in the middle of the dash is in place.

uberwasser · #5 07-21-2014, 04:35 PM

OK, that may be a different story then. You have the "dreaded servo" and therefore no monovalve. A good example of how not knowing the year can affect the diagnosis.

I believe the climate control push button units on these are more reliable (at least, the normal diagnosis and repair does not include opening up this unit for repair like it does on the later climate control) and it's more likely the problem lies with your servo or climate control amplifier.

As mentioned above it's not a bad idea to replace that foam tube if it's indeed gone or split in half or something similar. With it missing the dash temp sensor doesn't get reliable readings of interior temp.

BayouFlyFisher · #6 07-21-2014, 07:34 PM

Quote:

Originally Posted by uberwasser

OK, that may be a different story then. You have the "dreaded servo" and therefore no monovalve. A good example of how not knowing the year can affect the diagnosis.

I believe the climate control push button units on these are more reliable (at least, the normal diagnosis and repair does not include opening up this unit for repair like it does on the later climate control) and it's more likely the problem lies with your servo or climate control amplifier.

As mentioned above it's not a bad idea to replace that foam tube if it's indeed gone or split in half or something similar. With it missing the dash temp sensor doesn't get reliable readings of interior temp.

Thanks! I will be checking the foam tube tomorrow morning.

My concern still centers around the climate control Amp. I can understand that the Evil Servo could fail and over power the AC with heat, but could it account for the "no air through the center vents" portion of my problem. Wouldn't it take a signal, real or faulty, to switch those two vents off, increase air flow out of the two outboard vents and turn on the heat?

I borrowed this picture from an earlier thread (2008?) about the evil servo:

Dr. Evil Servo