Replaced monovalve, no improvement! - PeachParts Mercedes-Benz Forum

BWatson · #1 02-13-2003, 04:26 PM

Just replaced the monovalve with a repair kit from fastlane and no improvement. The control unit seems to work ok because the airflow definitely redirects and I can hear all the vacuum servos changing direction when I move the wheel from hot to cold and the pipes coming away from the AC compressor are ice cold, so we can rule those out I think. The problem, heat only. Heat, heat, heat, no matter what setting. New monovalve = no improvement! Any ideas?

surfblau · #2 02-13-2003, 06:36 PM

If you have the CD and an ohmmeter, I think it outlines the specific resistances that the temp sensor is supposed to read for a given set of temperatures.

Maybe the wire going to the sensor fell off.

You can check the air sampling tube that is known to decompose on 123 chassis cars when you are checking the sensor.

Those seem like 2 high probability failure points- plus the pushbutton control unit....

Those first two are cheap and easy. There are some good threads on using 1/2" IIRC pipe insulation for a replacement tube if yours has gone bye-bye.

alec

BWatson · #3 02-13-2003, 07:13 PM

Thats a good start anyway. If the control unit was bad, would it still redirect the airflow? It does redirect the air to the center vents when I move the temperature wheel so I know something is happening. Could all this happen if the control unit was bad?

lrg · #4 02-13-2003, 08:43 PM

I'm not sure you can always tell with the CCU. On my W124 the unit first showed a problem by running the A/C all the time, even on the Econ. setting. Everything else worked fine. Months later it wouldn't run the fan consistently at high when set on defrost, but still everything else was OK. A few more months and then the temp was 100% heat, the fan would change speed but it was full heat. A few days later it announced it was done for good with some wisps of smoke coming out from behind the buttons. I later discovered my circulation pump was frozen which has been known to cook these units. (You might want to check that by the way.) Even so, my problems started months ago and were all cured with a rebuilt unit. Check everything else first but if everything else checks out the CCU is probably the culprit. If you need to buy one buy rebuilt, it's a fraction of the cost of a new one.

terminator · #5 02-13-2003, 10:13 PM

I had the same problem with my 84 300SD. My problem was solved by a little silver box (computer) in the passenger side of the dash. I tried every thing from the push button unit to the mono valve to the temp sensor nothing worked but this. I'm not sure if your car has one or not. But I bet it has something like it. After I replaced mine, my climate control system worked great. From what I hear, these don't go out often. I'm sorry I don't know what its called.

Terminator

02 C240W 13K
89 300 SEL 163K
83 300TD 190K

Larry Delor · #6 02-14-2003, 01:32 AM

I had something like this happen once a few years ago in my 300D. After turning the fuses around, it worked normal again. I dont remember what fuse it was, but I think it was an even numbered one.

Fisherman · #7 02-14-2003, 04:23 AM

terminator was that the amplifier?
Square box mounted on a two screw mount on the passenger side?

just trying to help.

Thanks to jcd via larry bible (I think) I have an acc trouble shooting guide that i converted to a word doc. It is pretty big but if you send me an email address via a PM I will shoot it off to you.

It has the ohm values for checking various parts, tested at the main test plug.

jhal · #8 02-14-2003, 11:27 AM

There are two "computers" that control the climate control system on that car. They are above the glove box inside the dash. The silver box, with some cooperation from the pushbutton unit, controls the temperature, a/c compressor, and vacuum solenoids. The black box, in conjuction with the resistance box under the hood, controls the blower speeds. Quite a complicated system even by Mercedes standards. Great when they work correctly, a pain in the a$$ when they don't.

terminator · #9 02-14-2003, 12:35 PM

FISHERMAN

IT WAS CALLED THE AMPLIFIER, IT WAS LATE LAST NIGHT AND I COULD NOT REMEMBER WHAT IS WAS CALLED. BUT THAT IS WHAT FIXED MINE, THE PART WAS NOT CHEAP.

TERMINATOR

02 C240W 13K
89 300SEL 163K
83 300TD 190K

BWatson · #10 02-14-2003, 12:46 PM

Thanks for the input gang! This weekend I will check all the fuses and look for the amplifier and if that doesn't help, there's always dynamite...just kidding.