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

First, thank you for posting in such detail including pictures.

Second, I'm glad that you found the fault - as demoralizing as it sounds.
Reminds me of this: When we choose to bravely acknowledge our errors or oversights in life, it can have the effect of giving others a sense of safety in doing the same to whatever degree when their inevitable time of eating crow arrives.

Third, your travails reminded me to first be open to the simple solutions because sometimes they are the right course.

Yeah, I figure if I post it up here I can save somebody. This forum has saved me dozens of times.

There were some posts saying if the Fan turns off under EC after thirty seconds it’s probably the pushbutton unit. Well, the switch at the thermostat and it’s wire are another thing to check.

I tried all the common problems mentioned on the forum like bad solder in the pushbutton unit. Bad traces in the thermo controller silver box. Somehow the fan cut out switch got past me for the last few weeks. I wish I’d noticed the wire was broken.

So the switch usually shorts the wire to ground. My wire was frayed and dragging all over the firewall. It was probably alternating between high and low temp signaling the controller.

I’m just happy I got it fixed. I try to tell myself that maybe my silver box and pushbutton still needed replacement. It makes me feel better about spending the dough. One thing I can say is the plastic on my pushbutton switches was disintegrating under the buttons and the springs had nothing to hold on to. The buttons were held on with tape.

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

Thanx for sharing this ! .

I've bought few Programma PBU's and wasn't really impressed .

Be sure to come back next Summer and up date .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

LOL. Oh, life can be cruel. That was a bit painful just to read. Whew.

I'll look at that. I also noticed a fan relay behind the glove box when looking at the FSM. I haven't looked behind the glove box yet.

__________________
85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

Uh oh, what was bad about the programma units? Do they burn out?

I didn’t have to wait for summer. How did you like the weather in SoCal today? It hit 91 in Laguna Woods when I went to lunch. My fully functional AC hummed away and worked flawlessly. I almost didn’t want to go home.

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

I've repaired so many Programma "rebuilt" units, not only for Mercedes. They seriously suck. Not sure what they actually "rebuild" on some of them because I often can't find anything. They certainly don't replace the capacitors, and that's usually what's failed. When I do find "repairs" that they've done, the quality is usually absolutely shocking....like a high school Freshman did it. Unless what you have went up in smoke, you're far better off just rebuilding what you have. Significantly cheaper and if you shelled out for quality components, you'll get the rest of your lifetime out of it.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

As mentioned it looks mostly cosmetic in nature .

Never any capacitors replaced and after all these years (decades !) that should be the most important part to replace, first .

Sunday 11.18 it was 92* F in down town Los Angeles ! .

Whew .

I'm glad I did the AC in all three W123's I own .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better