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Old 01-25-2002, 01:07 AM
300EVIL's Avatar
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Join Date: Apr 2000
Location: Lake Geneva, WI.
Posts: 1,676
No need to remove the pushbuttons.... Just take the two screws out of the bottom of the wood cover. remove the cover. take out the two screws holding in the ccu. unplug the two connectors from the ccu. remove the "two" screws from the bottom of the ccu,, and your in..... Ahhh. i prefer to use a desolidering tool $9.00 at radio shack to remove the old solider. then take a regular old iron and fill in the blanks. Neatness counts! just try the pushbutton connections first. I was amazed how fast I was up and running.
Good Luck!

Current Stable:
01 ML55 AMG
92 500E (a few mods)
87 300E (lots of mods)
00 Chevy 3500HD Diesel Box Truck
68 18' Donzi Marine
06 GT i-Drive7 1.0 Mountain Bike (with GPS!)

PREVIOUSLY OWNED:83 300SD, 87 420SEL, 88 420SEL, 90 420SEL, 86 560SEL, 86 190E 2.3-16V AMG, 94 E320

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Old 01-26-2002, 09:23 PM
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Join Date: Apr 2001
Location: fort worth
Posts: 363
Adam, Harry, Larry, Dpetryk... I pulled the entire plastic unit out of the dash. Took off the top row of buttons(Rear deck light etc) and pulled the 2 electrical plugs out of the ccu. I took 4 small screws off the thin, plastic bottom plate. There exposed was a green circuit board. I could redo all of these solders but are there more? There seems to be another board deeper. Do I need to access it? How do I get the green one out? Do I have to pull the temperature wheel off to get both boards out? I do not see where the pushbuttons solders are now.

I did check the ccontinuity between the pins for the plugs and the green circuit board.

My local library has internet access to Mitchell's. Will they have good info. Does the MB shop manual have an effective algorithm to check all parts of the HVAC system? Is there any detailed source on line?

Many thanks to all.
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Old 01-26-2002, 10:05 PM
300EVIL's Avatar
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Join Date: Apr 2000
Location: Lake Geneva, WI.
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You don't need access to the other board for this application. I've tried to remove the board in question and it's a pain in the a$$!
What I would do is just resolider the switches and temp pot in the box without removing anything else. Then I would plug it in and see what happens. BTW, if you suspect the aux pump i would take it apart and see if there is a problem. Only takes an hour at the most. Keep us posted!
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Old 01-27-2002, 04:59 PM
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Join Date: Apr 2001
Location: fort worth
Posts: 363
I did use Mitchell's in the library. They had a very detailed algorithm to diagnose HVAC. I do not have 2 MB tester cables but it seems very logical. Touch certain pins and measure resistance to see if each part works. I will likely not buy the cables but I will ask the mechanic if he has them.
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Old 02-01-2002, 01:05 PM
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Join Date: Apr 2001
Location: fort worth
Posts: 363
Final Report

Climate control works. Mechanic replaced the following

monovalve "saw water in the coils"
evaporator temp sensor(near accelerator) "circuit was open"
pushbutton unit -did not open doors and poor temp response
aux coolant pump apparently ran but current draw gradually increase to 10 while he was measuring. When we took apart the pump the bearings turned but not smoothly. The bearings had a notchy feel.

Amen. When the system works I do not notice it.
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Old 02-01-2002, 03:42 PM
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Join Date: Oct 2001
Location: Newton, MS
Posts: 123
Congratulations Lizem, I am glad for you.

Many thanks for sharing your trials and tribulations with us.

86 300 SDL
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Old 04-15-2007, 04:20 AM
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Join Date: Jun 2006
Location: Mooresville, NC
Posts: 344
Originally Posted by Ken300D View Post
Yes. I second Adam's suggestion that you resolder only the pushbutton switch terminals. They are pretty easy to find on the underside of the circuit board. Each switch has two rows of three terminals.

My primary problem symptoms were the fan running in the OFF position, and the AC compressor coming on in the OFF and ECONOMY positions.

The pushbuttons get a lot of stress normally, and a whole lot of stress from people who think violently pushing them in (while angry ) will somehow make the system act right. Once you get your pushbutton switch terminals resoldered - treat them very gently.

It is also worthwhile to resolder the three terminals that connect the round temperature setting dial if you can find them. They are in a row, adjacent to the wheel, on the circuit board.

By resoldering only switch terminals and the dial terminals, you avoid a lot of contact with more sensitive components that are probably still soldered in OK anyway. Its the control hardware terminals that get stressed and cold crack.

You may have other system issues, but this should fix the fan running in OFF.

Outstanding! Almost exactly the problem I have now. A/C will run in EC mode, will turn off when I press down and hold the EC button, kick back in as soon as I let off pressure. And half the time when I start the car, the blower motor will come on as soon as I put the key in Pos II to kick in the glow plugs, have to punch O to turn it off.

Already had the unit pulled out of the car, the bottom cover off, the temp dial removed, trying to figure out how to disassemble it further to get to the switches when I tumbled onto this thread. 3 AM so repairs will wait till tomorrow, but already sounds as if it's going to save me a lot of grief.

A few questions though -

This thread was originally talking about the CCU from an '87 300E. Same CCU in an '87 300DT?

Will a Radioshack ColdHeat soldering iron be adequate for this?

Would cleaning the circuit board and spraying it with a conformal coating after doing the resoldering be any help?

As far as the aux coolant pump - instead of having the pump powered off the CCU, would it be feasible to instead have the CCU powering a standard automotive 12V relay (using the wiring already installed to power the pump), with the pump powered thru the relay from another source (ie, straight off battery) with an applicable fuse in line?

Just say "NO" to Ethanol - Drive Diesel

Mitchell Oates
Mooresville, NC
'87 300D 212K miles
'87 300D 151K miles - R.I.P. 12/08
'05 Jeep Liberty CRD 67K miles
Grumpy Old Diesel Owners Club
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