Cut to the chase, it was the temperature control box ("TemperaturRegular").

The AC in my 1985 300D stopped cooling as Summer faded last year, then I noticed the heat was on full during the winter, though the fan speed stayed low. I first tried swapping a spare push-button box, but no change. I didn't expect so because that box is mostly just relays for the climate door soleniods. The temperature wheel sends an "analog request" signal to the temp control box. But, an easy swap since the wood cover panel was off (to fix the warp).

I recall my next swap was the temp control box (above glovebox, shiny one). No change. I forgot where my spare came from.

A month ago, I played with the AC again, verifying pressures and that it cooled the return tube. I even blocked off the heater hoses on both sides w/ clamps, but never got very cool air from the vents, even on "manual fan high", mostly just ambient temperature. I also checked the hot water servo-valve ("mono-valve"), which was OK, but I changed it just to be safe. No difference.

The temp control box still seemed like the best suspect, so I bought another one on ebay that was claimed "working when removed" ($22 w/ shipping). It worked. Now the vent temperatures vary with the setpoint wheel, even blowing cold in the blue. The fan speed also varies automatically, as it should. Lesson - "if at first a swap don't work, try, try again".

Now I have 2 spare temp control boxes that appear not to work. When I get time, I'll open them up. One guess is old capacitors. Tantalum and electrolytic ones degrade over time.

I was almost questioning if I had caused the problem when I added a relay to power the "blower speed relays" box (above glovebox, black one), which is a separate post. I was smart enough not to mention that possibility to my wife, since that would be placing a target on myself.

My remaining question is why I never got cold air when I blocked off the heater core and the AC was chilling the pipes. I thought the air always went thru the AC evaporator. Now, I am wondering if there is a diversion door, as in many cars. That is a deep dive into the "air box" in our cars, and I have never been there.

Update. Maybe not.
Today, I opened both "bad" TemperaturRegler boxes to inspect and found them pristine inside. No apparently burnt components nor cold solder joints. I plugged each in (easy, since glovebox liner is still out) and both now blew cold air and varied the fan. I had swapped before, but perhaps something external changed (loose wires, a stuck actuator).

Both my circuit boards look much different than the photo by JamesDean in an earlier post (3rd photo, copied here). For experts, the 2 LM224 chips are quad op amps (standard pinout). The 4 NSDU45 are NPN Darlington transistors, which were common power drivers in that day (5 A rating), but people want $11 ea for them today. If you want the same, search for "U45". "NSD" = National Semiconductor. A pinball hobbyist site said a modern replacement is "Central Semiconductor CEN-U45". A more modern IGBT would probably also work, and run cooler. Note the transistors appear to have pin labels for "emitter", "base", and "collector", but I didn't find a datasheet to verify. The 9 electrolytic capacitors may have degraded, but currently "not broke, won't fix".

It would be fairly easy to draw a schematic of this 1-layer board. When I must, I will (unless someone else helps), but appears my problem wasn't this box after all. BTW, you can buy these boxes "professionally rebuilt" from Programma for ~$85 today.

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Bill, on a separate note, how did you correct the warp in your wood cover panel? Two out of three of my cars have this exact condition.

Thanks,

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Will



72 280SEL
85 300CD 288k
00 E320 Wagon 4-matic 100k
11 E350 Wagon 4-matic 45k

73 VW Westfalia Camper
67 VW Westfalia Camper

66 Porsche 912

You assume that I did so. As you found, a junkyard one won't help, since all appear to warp. I did fix one. I put it over a pot of boiling water, w/ lead scuba weights on top. I then let it cool in the garage for ~6 months, w/ the same weights, supported by bricks on the end. It is flat enough now to not keep popping off the dash, but the laquer has some cracks from the "bend back".

You can buy a set of all new woodwork for only ~$550 (ebay, made in Thailand), for those who love their M-B more than I. A recent car show (Car Fix?) replaced the wood trim in a 60's U.S. car by an aluminum piece w/ custom simulated wood applique. Looked nice on TV and was fairly affordable. Not sure exactly how they did it.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans