Climate control gremlins - PeachParts Mercedes-Benz Forum

cirrusman · #1 10-30-2013, 05:22 PM

So I was too happy too soon. My climate control was stuck in the defrost position with the fresh air flap open; I took off the dashboard and center console and when I found the mysterious vacuum leak that was ailing my climate control, I repaired it, re-routed the wrong lines and everything was working just fine. Then, as long as I was there, I took off the cheap alarm system someone had clumsily installed in the past. Apparently that caused a couple of electrical problems which new fuses solved. But now my climate control is back to square one at defrost flaps and fresh air flap open. No vacuum leaks this time, when the ignition is off, the whole system holds vacuum quite well. As soon as I open the ignition, apparently power is ran through all 6 switchover valves opening them and losing all vacuum. Also, sometimes when I run the blower at max speed it blows a fuse, especially if the blower isn't working at at least 3rd speed. I'm at my 3rd fuse today. It's intermittent though; if I allow the blower to reach speed 5 and then I hit max speed, it won't blow the fuse. I don't know if the control unit is blown or if the electronic switchgear could have received any damage, but I don't know how to test these two units... Any electronic gurus have any experience with either one?

cirrusman · #2 10-30-2013, 05:33 PM

Here's a pic of the unit in question:

cirrusman · #3 10-30-2013, 08:46 PM

I just tested all the vacuum and it tests out ok. Not perfect but pretty up to par. The unit I just depicted doesn't sound after all like it would control the switchover valves so my last suspect is the pushbutton control unit. The manual calls for a special tool for testing that thing, anyone knows of an alternative? (also comes to mind the numerous times it blew a fuse when I pushed position 6 on the fan speed button...)

JamesDean · #4 10-31-2013, 01:08 PM

I've worked on a few of the first generation w126 climate controls. They are in three pieces. PBU, temperature regulator/vent control and blower motor control.

I seen a one or two of the temperature regulator units with some chips completely blown in half.

cirrusman · #5 10-31-2013, 01:14 PM

Just opened the unit and it's all good inside. Cleaned it spotless for good measure. The quest continues...

JamesDean · #6 10-31-2013, 01:16 PM

Which unit?

Most of the time the problem is with a cold solder joint. They are often difficult to see visually. Also aged capacitors can be a problem too.

sleepstar · #7 10-31-2013, 01:19 PM

i replaced that temperature regulator unit when i was getting nothing but full heat and it solved the problem, but it wasnt causing any of the symptoms you describe, fwiw

cirrusman · #8 10-31-2013, 02:43 PM

I dismantled all 3 components. No minute cracks on cold welds; didn't test any capacitors though but at least they look okay. I'm testing the vacuum switchover valves now.

SD Blue · #9 10-31-2013, 09:06 PM

Which fuse is blowing?

From your description, it sounds like a blower motor is on it's way out. Have you pulled it and inspected the brushes and commutator?

I've also read here of the electrical portion of the ignition switch causing intermittent problems. Have you noticed anything else giving intermittent problems?

Junkman · #10 10-31-2013, 11:07 PM

This might be useful. Scroll down to "Other Items".

http://tank.windwireless.net/~jimc/

My 85 intermittently blows cold at speed. I've changed the mono valve & swapped a working control unit from the 84SD.
Perhaps the temp sensor up near the interior mirror??

cirrusman · #11 11-01-2013, 12:11 AM

I found the culprit. I was testing the vacuum with the ignition off, so of course I want getting any vacuum leaks with all the switch valves closed. The culprit was the vacuum actuator for the windshield defroster whose rubber diaphragm has a small rip, probably caused by never being used up until just recently, worked fine a couple of times and then tore apart. I don't want to replace all the actuators in the car, but I know they're on their way out. I dismantled this one and used electric tape to patch it up to see if there were any more vacuum leaks and it worked fine. No more leaks. I also know that tomorrow morning my patches will be gone. I've been googling about mending rubber diaphragms and found a couple of good solutions. I'll see of I can find a can of liquid rubber, or rubber in a can or whatever it is that they call it and treat all of the diaphragms this way to prevent also future leaks, see if that works. I'll be posting here how that goes

cirrusman · #12 11-01-2013, 12:15 AM

Quote:

Originally Posted by SD Blue

Which fuse is blowing?

From your description, it sounds like a blower motor is on it's way out. Have you pulled it and inspected the brushes and commutator?

I've also read here of the electrical portion of the ignition switch causing intermittent problems. Have you noticed anything else giving intermittent problems?

Fuse #11 I think it was. Blew it 3 times, and that is still a mystery to me... A PO had installed a cheap old security system and it was causing a lot of shorts and battery draining so I just got rid of the whole thing, hoping that that will remedy the shorts i've been seeing...

JamesDean · #13 11-01-2013, 08:28 AM

Quote:

Originally Posted by cirrusman

I found the culprit. I was testing the vacuum with the ignition off, so of course I want getting any vacuum leaks with all the switch valves closed. The culprit was the vacuum actuator for the windshield defroster whose rubber diaphragm has a small rip, probably caused by never being used up until just recently, worked fine a couple of times and then tore apart. I don't want to replace all the actuators in the car, but I know they're on their way out. I dismantled this one and used electric tape to patch it up to see if there were any more vacuum leaks and it worked fine. No more leaks. I also know that tomorrow morning my patches will be gone. I've been googling about mending rubber diaphragms and found a couple of good solutions. I'll see of I can find a can of liquid rubber, or rubber in a can or whatever it is that they call it and treat all of the diaphragms this way to prevent also future leaks, see if that works. I'll be posting here how that goes

My dad's SD had his defroster pod go partially bad. It took disabled the center vents. I would just replace the pod. They are not horribly expensive. The defroster pod is $60. Even if you mend it in one place, it could still fail in another spot. I suppose you could just liquid rubber the entire thing...not sure of the ramifications though.

SD Blue · #14 11-01-2013, 10:37 AM

Quote:

Originally Posted by cirrusman

Fuse #11 I think it was. Blew it 3 times, and that is still a mystery to me... A PO had installed a cheap old security system and it was causing a lot of shorts and battery draining so I just got rid of the whole thing, hoping that that will remedy the shorts i've been seeing...

F11 powers the blower motor controller and blower motor. I would be checking the blower motor and brushes. It is real easy to access on your model. Pull the front passenger's knee panel and IIRC it's only held in with 4 screws.

JamesDean · #15 11-01-2013, 10:41 AM

On my dad's SD, i found that the clutch wire for the a/c compressor was damaged and arcing to the chassis. This blew the fuse for the blower motor. Didnt realize this until after I replaced the blower motor.

Also there is a set of resistors under the brake master cylinder that are used for the blower motor. They should be next to the aux fan resistor.