need tech advice for W126 HVAC - PeachParts Mercedes-Benz Forum

strelnik · #1 11-02-2018, 03:57 PM

Where is the best explanation of how the W126 "first generation" heater and AC works?

I want to fix the system myself and may need to rebuild it.

I have not found any complete information on a good overall trouble shooting procedure, or even good maintenance to do.

Is there anything out there? I also need to know which FSM applies here.

Thanks, all help appreciated.

Maxbumpo · #2 11-05-2018, 09:46 AM

Chapter 83 of the 126 FSM should contain all the factory diagnostic steps for HVAC. If your 126 system has a push-button unit like a 124 car, then the FSM calls for a special tool for testing all the system sensors and components. I have found that using a good wiring diagram, one can puzzle out which pins on the connectors correspond to the steps in the diagnostic guide, and do all the steps to test the system.

Diseasel300 · #3 11-05-2018, 10:13 AM

1981-1985 W126 uses the same 2-piece ACC-II system that the W123's and later R107's used. 1986-1991 W126 uses the single piece ACC-III system that the W124's used.

The vents and vacuum pods and such are the same between generations, but the control gear and how they work differs. The 1986+ uses a single module in the pushbutton unit to do all the controls electronically. The 1985 and prior models use the pushbutton unit and an external control box to regulate temperature and control the fan speed with relays and a blower resistor assembly.

The functionality is similar, but the execution is different. You need to know which unit you're dealing with and repeatedly state it on this board when asking questions. You can get a lot of overlap that can send you searching for white rabbits if you don't!