ac/heat control unit stops working

There have been a couple of threads about this lately. I myself have had this happen to my 348. The problem is that there are bad connections inside of the a/c control unit. This can be either a bad solder joint or a corroded connection between a pin and the socket between two of the circuit boards in the unit. It could also be one of the pin connections for the main connection of the unit. I had taken mine apart to change out the buttons for some that were in better shape. To do this, I had to pull the boards apart. When I put them together again, my issues as you described were gone. The a/c stayed on the selected input and the lights stayed on bright. It is a frustrating problem for sure.

 

i just got this done over the weekend and its solved....used to be blinking, switching the compressor off at its whims and fancy and lets not even get to the clock!

simple solution.....these boards are actually welded pretty well....mine was....i'v seen boards on similar time period german cars like merc S classes with worse soldering.....

take the back cover off...

then inside the unit, there are 3 boards (actually mine had a 3+1 board) connected together with these long male connectors....these 3 sets of male connectors need to be cleaned and i used a fine metal brush, then sanded it with grade 2000 sandpaper...

the male connector which connects to the cars was cleaned with Autosol as i couldn't get the plastic head off the circuit board (if someone knows how this is done plz help)....its hard to get into the pins so use a tweezer and a very small piece of cloth and go through the pins till the cloth (keep changing the cloths) is not brown or black anymore..... as an alternative, a grade 2000 sand paper could also be used. and finally gave a dash of WD40 (just a dash into the female connectors before i connected the unit back to the car)



ps: the first set of long male connectors inside the unit (the ones nearest the buttons/displays) are easy to remove as it just needs to be pulled apart...the second and third set of male connectors need to be pulled apart after these 8 black "plugs" are pulled out (four will be facing you and four more will be in between the boards....these plugs connect and hold the 2 top most boards together.......

 

Nice write up. I just wanted to add that when you are pulling the two main boards apart with the most pins connecting them, take your time and use even pressure while doing it. You want to make sure that the boads come apart in a straight line as to not bend the pins.

 

thanks mate. I forgot to add that that was my 2nd attempt!....my first attempt was just by using contact cleaner and that effort was in futile! these connectors are 20 years old or thereabouts and contact cleaner alone will not cut it especially for those units which never have been opened up before/serviced!.....
the connectors need to get scrubbed!!!.....

be careful not to use a lower grade/ coarser sandpaper as you don't want to leave too much "scratches" on the male connectors ( this will be visible if you use such a sandpaper and viewed under a high powered magnified glass). what happens is, these micro grooves or "scratch marks" will attract dust particles over the years (the dust particles will get stuck in those "grooves") and screw up the connectors/contact once again...Autosol does the job pretty well if you can't find super fine sandpaper.....