I just wanted to mention that I had great success overhauling the Automatic Climate Control pushbutton unit in the '87 300D.

This unit can be removed from the console relatively easily, and it is all electronic - no vacuum connections. There is a large electrical connector on each side of the module that needs to be carefully pried off. Disassembly of the module is by removal of four screws on the bottom and then carefully prying off the plastic bottom cover - watch for the catch tabs.

Removal of the bottom plastic cover reveals the solder side of the larger of two printed circuit (PC) boards. All of the pushbutton switches solder onto this PC board, and the solder connections also serve as the mechnical support. I suspect that years of using the pushbuttons (sometimes violently in anger - trying to get the climate controls to work) tends to break the solder connections.

So, I suggest you resolder all of the pins on each pushbutton switch. There are six pins (three each in two rows) on each switch. I also did some resoldering on the large pins that connect to the external electrical connectors - but I did this with the PC board removed from the case so as not to melt the plastic case.

While I completely disassembled the unit, and cleaned the PC boards off - I believe the majority of problems can be resolved simply by removing the bottom plastic cover and resoldering the pushbutton pins.

AND TAKE IT EASY ON THE PUSHBUTTONS.

If violently pushing the button helps in some way, then it probably has a loose solder joint.

The symptoms I was trying to cure included the AC compressor coming on in the OFF and ECONOMY positions, and the fan coming on in the OFF position. Seems to be completely fixed now.

Hope this saves someone $250 in parts and $100 in labor.

Ken300D

This is common repair attempt at dealer. They say it's a cold solder joint, your theory of the button actuation causing the failure seems more plausible.

The other common failure of these units occurs when the auxilliary water pump gets crudded up and draws too much current. If the auxilliary water pump draws more than 1.3 amps, replace it before it blows an active component in the pushbutton control unit.

Have a great day,

That's a good point. It sounds like a reasonable preventive modification would be to put a 2-amp fuse (maybe a 2-amp slow-blow fuse) in the power supply line to the auxiliary water pump. I can see how you'd just about ruin a control pushbutton box if it directly controls the auxiliary water pump and the pump shorts.

My fix pretty much only helps if the pushbuttons are intermittant. Fortunately this is the root of all my ACC problems so far. It was rather disturbing to have the AC compressor come on in the OFF or ECONOMY positions. Kinda cuts the fuel mileage a bit.

So far its been working perfectly. This also fixed a spasmodic flap somewhere in the interior of the dash that was actuating back and forth randomly. Maybe a fresh air vent or a high-low air redirector.

Ken300D

I believe the best fix would be to take the controlling wire from the aux pump and kick a relay with it, then operate the pump with the relay contacts.

Have a great day,

Do the 300D's with out auto climate control have an electric aux water pump for the heater cores? I never noticed one, and would presume there would have to be a switch for it unless it would be always on.

Always looking to replace something.
TIA

Adam

I don't know about the 300D. I thought that all the US 300D's had automatic climate control. I've had several 123 240D's, none with automatic climate control and none had the auxilliary pump. I personally see this as an advantage. The KISS principle, you know, Keep It Simple, Stupid!

Have a great day,

Does anyone have any good "removal" advice for the unit? The one in my 300D seems to be erratic (at best) and the backlight has a mind of its own.

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1960 MB 190Db w/ full sunroof
1985 Toyota Pickup
2006 Honda Odyssey EXL R&N
2001 VW Jetta GLS TDI

I'm struggling to rmove the console.
How can I get to the Climate contol unit. What do I have to remove?

You don't need to remove the console to remove the ACC unit. On the 1987 300D, you just pop off the wood trim piece and that reveals the screws you need to take out to get the ACC out of the console.

Carefully pry the wood trim piece that goes around the buttons near the corners. It has little friction tabs that hold it on.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

when removing the wood trim around the ac controls in a 87 300d, first remove the 2 screws just above the radio on the "bottom" of wood trim.

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Tony L.
87 Merc 300D
97 Toyota T100
00 Honda Superhawk

GO Dawgs!

Ken300D-

Is the pushbutton unit on the '87 the same as the one on the W123's? On my 1985 CD, when I push the Economy button, the fan will come on for a minute or two and then turn off by itself. All the other buttons work as they should, but I don't like to run the AC all the time. If I go to an AC button, then to the Econ. button, the same thing happens. Occasionally I can also hear a buzzing, like a short coming from the unit.

The unit I have was replaced by the PO with a brand new MB unit (from a dealer) about 1.1 years before I bought the car. Out of warranty

How do I check the auxilliary water pump?

TIA,

Craig

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1994 E420, Pearl Black/Black. 2.82 rear diff., AMG front spoiler, painted lower half. SOLD

1972 & 1974 BMW 2002tii's.

I think your CCU circuit board is probably the same an 1989 420 SEL CCU board. Can you tell me how you were able to remove the circuit board from the case? I have removed the bottom plastic cover on the CCU, and pulled the pin from the temp. control wheel but the pushbuttons seem to be retaining the circuit board in the case. How would one disconnect the pushbuttons from the circuit board and is there anything else that is retaining the board in the case?

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1989 420 SEL

Dcraig:
The 123 and 124 CCUs are very similar but not interchangable. Ken300D's fix should work on either one though if that's what's wrong.

IMHO the best/easiest way to check if your aux pump is working is to unplug it and test it with a direct feed from the battery but others here may have other suggestions.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold

How would you do this, Larry? Thank you.

I know that my aux pump is working because if the heater is on I can hear, and feel the pump vibrating.

I thought that for some reason the pumps tend to draw extra current and fry the ACC.

I just bought a multimeter, how would I use this to check out the aux pump?

Craig

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1994 E420, Pearl Black/Black. 2.82 rear diff., AMG front spoiler, painted lower half. SOLD

1972 & 1974 BMW 2002tii's.