I have an '87 300D whose climate control system has a weird latch-up problem:
- start car cold from overnight rest ( outside temp less than approx 65 F )
-user has no control over system operation..system delivers full-hot air via outboard dash vents, blower does not run. Air movement from vents is due only to movement of car..stop car at traffic light, air movement ceases.
-after driving approx 5 minutes, turn off engine and restart, system now normally controllable, blower runs.
-after the restart, sporadic relay cycling can be heard in dash cavity..diminishes with time as car temperature increases.

If the overnight outside air temp is above approx 65 F, problem does not occur.
Anyone seen this problem?

If you have a similar CCU as in a push button unit, most problems such as yours are cold solder related. The unit will send intermittent signals and all sorts of oddities occur.

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83 SD

84 CD

Bearings in blower motor are probably near gone or all gunked up. When the interior gets a little warmer they loosen up and start working. I'd be surprised if cycling the CCU a few times didn't result in the same effect as restarting the engine. No difference to the blower motor. Try pushing the high speed fan button and cycling the CCU off/on a few times. It is possible to disassemble the blower motor and clean up the bearings from what I've read here. May just need a few drops of oil. Do a search.

Relay cycling inside CCU is normal. It's reacting to temp changes, energizing vacuum solenoids / vent pods, blower speeds, etc. If your system is acting normally when warmed up then the CCU is probably fine.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

There is a 5 volt regulator on the lower printed wiring board within the control assy. This regulator probably supplies the 5 volts for all the digital logic ICs in the control assy. A 33 uF capacitor on the output side of the regulator had gone bad (capacitance much too small) due to age, temperature extremes, etc. The regulator needs a minimum of 22 uF to work properly, per the manufacturer's data sheet for the regulator. I replaced the capacitor and another one just like it that was found on the upper printed wiring board...they both had the same bad appearance on visual inspection. My control assy part number is 124 830 18 85. Assy works fine after repair.

This is good information. Power supply capacitors are the bane of all older electronic equipment. Did you happen to take any pictures of the disassembly/repair/re-assembly process.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

hope to get more information on this. I seem to be experiencing similar problems

I wanted to bump this thread and say thanks.

I was having the exact same symptoms as described in the first post. I found the 5 volt regulator and googled up a data sheet for it so I would know which leg was the output. Then followed the output to the capacitor. It was labeled 33/16 along with some other markings and was a brownish plastic cylinder. I replaced it with an aluminum electrolytic capacitor of the same rating although the physical size was a little bit smaller. It seems to be working correctly now.

I've been away from this forum for quite a while and just saw requests for more info. I'll be glad to help if folks are still needing help.
//steve

Cool!

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What Would Rudolph Do?
1975 300D, 1975 240D, 1985 300SD, 1997 300D, 2005 E320 , 2006 Toyota Prius

Any chance someone can expound on this a bit? My 87 wagon is doing the same thing!

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1987 300TDT - 195,000 (Original #14 head)
1993 190E 2.3 - 105,000
1981 300D - 250,000

Glad to hear you found the problem.

But I would check the fuse to make sure the tabs holding the fuse have not melted the fuse box and embedded themself into the plastic. This causes the fuse not to seat properly causing insufficient power to get to the climate control unit. After I gently pryed the tab from the plastic on my 81' 300TD, the climate control worked fine saving my some bucks.

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81' 300TD
Peter

Too much capacitance on the input side is no harm. You could put 22 and 47 in parallel if you're in doubt.

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Cheers!
Scott McPhee

1987 300D

A single 47 uF cap will be OK. Regarding polarity, the positive side of the cap must be connected to the trace that leads back to the voltage regulator integrated circuit. [LM2940]

Recently, the ACC in my 87 300TD (124-830-18-85) would not function at all until up to 20-30 minutes into a car ride. I knew the temperature sensors were good, and everything would function properly after it eventually kicked on. After months of freezing temps and no way to warm the cabin up, I finally had the idea to bring the ACC unit inside overnight thinking that maybe it had become sensitive to the cold. Sure enough when I plugged it into the car in the morning it worked fine. The next night I left it out in the cold plugged into the car, but hanging out of the console. In the morning I went out and confirmed that it didn't work. I then used a heat gun to warm the unit up and within 45 seconds the unit kicked on and worked as it should. A quick google search led me here and now with two new capacitors, everything works perfectly. Thanks smorelen!

Also, my brother has the same car and ACC unit with exactly the same symptoms. His capacitors (and mine) looked exactly like the one in your photo.

Might have to pull the ACC and see how my circuit board is doing as well.
Time to do some research

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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.