W126 CCU erratic heat, 1986 300SDL
 

Hi, I have a 1986 300SDL that I purchased 8 months ago. The person that I bought it from rarely drove it and it is in almost mint condition. When I purchased it in February of this year, I noticed that the heat did not work. I live in the Atlanta metro area and Winter is from January 26 at 1:00 pm until 6:00 pm ( a joke)..... So, I wasn't concerned at the time and by March I found myself working on AC system as summers here can be brutal..... Fast Forward: My AC is ice cold and the car is now a daily driver that my wife adores... Up until a few weeks ago, I had no issues with the climate control system..... In fact, during the summer, I would periodically cut the heat on and it worked every time...... My assumption was that because the previous owner had hardly driven the vehicle, a lot of the quirks were caused because the car was sitting and not being used as a daily driver...... I assumed that I would have to either rebuild or replace the monovalve as stated above that the heat did not work when I first purchased the vehicle..... So, for the last few weeks, when I put on the AC it still gets nice and cold but suddenly the heat will come on without me calling for it by changing the temperature adjustment on the ccu unit....... My question is: Can the monovalve be sticking in the open position?...... I have had many diesels and on my 300SD, I had to replace the ccu unit..... also have a 1995 E320 wagin which I disassembled the ccu unit and resoldered the board joints and it now works like a charm...... Any suggestions on troubleshooting and determining if in fact it is the monovalve or the ccu that is causing the erratic heat activation?..... Thanks in advance!

They do stick. As we just started to get cooler in the AM I had to do the fall "Bump" with my rubber mallet on the 86's monolvalve. Maybe not the purist fix but that all it usually takes and much easier than disassembly. If you haven't taken it apart its probably a good idea to, and inspect the diaphragm. Also check that you are getting voltage to the valve.

Remove four screws and inspect the monovalve diaphragm. Do this before wasting any other effort. If it checks out, then check for voltage at the two pin plug.

86+ climate systems are far more simple than the 85 and older in the W126. PBUs are not a big failure point on these in my experience.

Do you not have the W126 service discs? It shows how to repair everything on these vehicles. You need to be able to use a multimeter and understand basic schematics.