I don't know if anything is more annoying while driving than to have a malfunctioning climate control system, my personal favorite is when it is below freezing in the car and the heat is in the vents but not blowing through. The solders on the circuit board are known to have problems from time to time, this is probably the number one reason why people buy a rebuilt unit. However with this DIY rather than paying the $150 or whatever it is with the little time to take this piece apart and a $10 soldering iron you can fix it yourself for good!

Before I go on to how to fix the unit itself I'd like to elaborate on symptoms of climate control issues and the most popular problems associated with them. First is the center vents don't seem to work right, perhaps while the a/c is on the windshield starts fogging or frosting up near the bottom even though defrost isn't on. That is a bad vent pod probably caused by a bad vacuum diaphragm or a vacuum leak. The replacement diaphragms can be found many places, here is one I know of off hand. Another common problem is that the fan will only kick on if the climate control is set to defrost first with max fan speed, those are bad blower fan brushes. While you can buy a new motor for about $100 I find it much more appealing to fix it myself for $6 instead. Another common problem is that the climate control only seems to work on max or min values, quite often this can be attributed to a disintegrated foam hose which runs from the dash to the temperature sensor for the climate control. Finally, the one that we are covering here. If your blower fan turns off and on intermittently, A/C is running when it shouldn't be, and several other very odd problems then bad solders could be at fault. On my own W123 my fan would work sometimes and sometimes not, but if I hit the center console near the climate control it would sometimes turn back on.







CCU, ACC, Climate control unit, Automatic climate control, Temperature, Heat, Blower, Brush, Armature, Monovalve, Defrost, Air Conditioning

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

This is a good writeup. You have shown that by doing a little work yourself, a lot of money can be saved. It also shows the manufacturers are making these devices on the cheap. Not enough solder is used and breaks after a few years due to vibration.

One question though, why use flux paste? Solder comes with flux in the middle. Heat up the solder, the internal flux does the same thing.

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Regards

Warren

Currently 1965 220Sb, 2002 FORD Crown Vic Police Interceptor

Had 1965 220SEb, 1967 230S, 280SE 4.5, 300SE (W126), 420SEL

ENTER > = (HP RPN)

Not part of the in-crowd since 1952.

I only had to use it a couple times, sometimes when melting the solder an oil or something would get on the pin and it'd repel the solder. I don't believe my solder has flux in the middle, also I'm primarily reusing the current solder rather than replacing the old solder. Another thing I don't think I've added to the page yet is that with a ~13W soldering iron if it is held at the end of the pin for about 15 seconds the solder which holds the pin is melted, I add another 5 seconds just to be safe. This way the boards don't have to be pulled to resolder them.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

The problem is actually a cold solder joint here and there, not just too little solder (I'm sure they're wave soldered anyway, even back then.) Mfg speed would be the culprit I suspect. Also, rosin core solder is great for clean boards, but with old joints, some oxidation, oil, crap, etc., dousing with flux helps get a good joint despite the odds against.

Tracked down a cold solder/vibration problem that caused an intermittant failure in a radar unit on a military transport back in the early '80's (anyone else lose a few fillings on a 130?)

Everything was to mil spec, checked joints, signed off, everything, but the vibration just took its toll, despite an originally perfect joint. Things can happen even with perfect mfg (especially in a diesel, known for it's "smooth" running!)

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1984 300D Turbo - 4-speed manual conversion, mid-level resto

1983 300D - parts car

1979 300TD Auto - Parts car.

1985 300D Auto - Wrecked/Parts.


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"If you don't know where you are going, any road will get you there". Lewis Carrol

Speaking of cold solders I actually did the same thing to my tach amp but due to the pita of removing the silicone I figured nobody else would be interested in pictures of it. It hasn't gave me any problems since.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

When I start climbing a hill and the engine temperature starts to rise, the heater comes on.

Why would the inside temperature control care or even know what the engine temperature is, or even if its rising.

Yep, that makes sense.

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1984 300D Turbo - 4-speed manual conversion, mid-level resto

1983 300D - parts car

1979 300TD Auto - Parts car.

1985 300D Auto - Wrecked/Parts.


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"If you don't know where you are going, any road will get you there". Lewis Carrol

I now have two units that I am looking at trying resolder. I notced that they both have several pins that do not have any soder on them at all.
Is this s design thing that they should not be soldered to the board?
Should I go ahead and solder them up? None of them appear to be connected to the traces so I'm guessing that this is per spec that they should not be soldered.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique

Hmm, they have no solder at all on them? So this means the pin moves freely around then right? I can't imagine any of them not having any solder on them, and if they were really meant to not be soldered to anything then if they did have an output it wouldn't matter. Have a digital camera?

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

Since you are fortunate enough to have two boards, just compare the two to see if they both are missing solder in the same spots. That might confirm a manufacturing flaw.

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Mr Goodfahrt (pronounced FAIRT... means "good trip" in German)

1982 300D Turbo Pastel Gray (off-white) 200k+(?)
newbie but willing

I'll try to get a photo of the board posted later today.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique

Good write-up Jeff!

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Never a dull moment at Berry Hill Farm.

Thanks Randy.

Ever come to a conclusion Fisherman?

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k

thanks.

Mine: only works after putting on defrost, then hi.
only works at max/min values.

You gave me two great ideas on fixing these!! Thanks a ton.

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1983 300D- silver blue, tan leather, 205,000.
1975 Datsun 280Z. Stock, ready to restore.
1983 San Juan 23. Dry docked.

I had to run down to SF for some business. I've got a couple of versions that I will shoot pics of cuz they are all a little different. If you want I can email to you also...

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique