I used to have the all too common "defrost default mode" on my A/C where the heater was on full blast, but the defroster only worked with the heat button tilted to max heat.

Many on this site suggested I look into the pushbutton unit (some even called me crazy, lol). I recalled seeing some posts on here that the cold solder on these pushbutton unit circuit boards were prone to failure after 15 years of wear and tear.

I am very lucky in that I have an Aunt close by that has worked for Digital for 25 years assembling, testing, and fixing circuit boards for different applications. She took the circuit boards off my pushbutton unit and checked andresoldered all the connections, checked the diodes, and cleaned all the contacts. The only thing she could not do was check the IC's since she didnt have the propietary connectors/diagnostics to test these. Keep in mind that any breaks in the contacts cannot be seen by the eye, but what is even funnier is that my Aunt, aided by whatever microscopic, solder viewer she employs, could not see a single break or area that might be faulty. She resoldered every single solder on that board and brought it back saying she found nothing at all wrong with the board but she did the work anyway.

Well lo and behold, I reinstalled the module and it worked perfectly! Albeit my A/C needs to be recharged. Now I hope my condenser, hoses, etc are all intact, but at least I have gotten to step 2!

I see so many posts on here about this problem. It would behoove many of you to do the work yourself if you know how. I dont think it is too difficult, I just lucked out with my Aunt.

By the way, my cruise control was intermittent also, I had her resolder all the connections on this also, and lo and behold it works 100% of the time now!

Cheers.

Hi SpineDoc
I read your other post on re-soldering the CCA board (cruise control amp)

I have 3 questions

1 is your car a W124 ?
2. where is the CCA board, and what does it look like?
3. is it the same on a diesel (I don't really expect you to know this one)

Hey Jom,

Welcome to the forums!

The post you replied to is an old one and I'm not sure if spinedoc posts here much anymore. You may get an answer from another member though.

Cheers,
C.

The CCUs are known problem components for 107s, 126s - basically any car with the push button/thumb wheel climate controls. They just wear out. What happens first is the solder joints between the thermostat wheel and the fan controls become intermittant and finally they just quit. Problem is that it is intermittant - so you can drive yourself CRAZY trying to diagnose.
The Benz Store out of Georgia has rebuilt CCUs with a 3 year warranty for $135 - $155 (depending on model) + $45 -$75 core. Best money I spent - got a working AC, vent control came back and shiney new buttons:D!
If anyone is having AC problems, replace the CCU first! Cheap insurance for something that is known to be problematic.

On the 300E the cruise amp is under dash to the left of the steering column, in a rectangular silver box. You must remove the lower dash cover.