Solved but at great cost.

So I've been struggling with my 85 300D automatic climate control. I've posted around a bit and I've tried a few things. Prior to these issues I was restoring my AC.
Here are some of the items I replaced.

New Monovalve. New R4 compressor. New r135a expansion valve. New parallel flow condenser. All new rubber hose (reused fittings). New AC o-rings. Some new solenoid switchover valves to replace 1,3 which were burned out. Some new vacuum actuator pods.

The symptoms were sometimes I'd get very hot air for no reason...well, the reason was there was no voltage on my monovalve sometimes. I think I had a bad pushbutton unit. I'd wiggle around my temperature wheel and then the voltage would come back and set the monovalve.

Also, my blower wouldn't turn on when I pressed EC. It does in my SD so I was wondering what gives. If I pressed defrost or AC first then I could get the fan to run then I'd switch over to EC and it would run. For awhile. Sometimes it would run continuously with fresh air coming through the vents, then sometimes it would quit after 30 seconds.

I read around and various posts here and on Benz world said it was the push button unit, the silver temperature controller or the blower relay box.

So being a retired electrical engineer, I tore the pushbutton unit apart and the silver temperature control box apart. I resoldered weak joints, I replaced op amp chips (LM2904), tested and replaced electrolytic caps and tested and replaced the Darlington transistors in the Temperature control box. Lots of new parts and lots of soldering.

the board


tools: soldering braid, solda-pult and a soldering iron (not shown)


remove components from board


testing






burned heat sink on a transistor. I pulled transistor and its gain is below spec. damaged






The system behaved differently but would often go nuts and put out a burst of hot air for no reason. Also, EC button would not always trigger the blower not to mention keep it running. I basically lost it and got frustrated. I pulled my credit card out and ordered a Programma pushbutton unit and a Programma silver temp control box. I put them in after diligently testing my water pump and monovalve.

Now I'm in about $350. And the system still doesn't work. I'm getting some temperature regulation, and varied fan speeds with heat and AC on AUTO but I'm not getting the blower every time I hit EC. I trigger the blower using DEF or AC buttons then go to EC...and sometimes the blower turns off after thirty seconds.

I was getting really frustrated. I was wondering if the Programma stuff was defective. What else is there. So I started looking at the diagrams again for the climate control and reading my 300D owners manual. This section about the blower not coming on when the engine was cold started looking suspicious. If you trace the diagram it looks like the car has a thermo switch that closes when the car comes up to temp so you don't get a blast of ice cold air in your face when you start up after leaving your blower/heater on EC before shutting down.

But I have no idea where this switch is. Maybe it's defective? That may explain why I can get blower in DEF or AC but not always in EC. I think I stumbled on a post in Benz world where they said it was the sensor on the thermostat housing. Really? I thought that was water temp for the instrument cluster. Then I start testing with my meter. Indeed, the thermostat housing sensor is a thermoswitch and the ones on the side of the head are the glow plug and water temp sensors.

And I find the switch is working fine. Bummer.

Then I pull on the wire and get the sinking feeling as I pull it up and it is just dangling with a broken end.

Sensor



Severed wire:



other end going to the harness near the receiver drier



Reconnected the wire and everything works fine just like my SD. Hit EC and the blower blows fresh cold air. No DEF or AC button press required. And it stays on. I feel like an idiot spending a small fortune on Programma parts but at least I know they are refurbed and they'll last me awhile. And it works.

Yeah, the unnecessary money outlay bothers me. Maybe I needed the programma stuff. Perhaps my repairs weren't complete. I guess I should be cool. I'm still way ahead of my neighbors making payments on their Hondas and Toyotas.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles