Symptoms of bad CCU - PeachParts Mercedes-Benz Forum

#1 04-04-2007, 07:42 AM

1983 300CD. No response from CCU - no blower motor, no heater, no A/C, no defrost. No nothing.

I've checked the common-sense stuff. Fuse is fine - not blown, and made sure that it's making good contact w/ the bus. Blower motor is fine. Nothing jammed in the squirrel cage, and when I remove the blower motor and jump it to 12V from the battery, it works just fine.

No response from the CCU regardless of setting - doesn't work at low temp, high temp, low fan speed, auto fan speed, high fan speed. Nothing.

Is this symptomatic of a bad CCU? What else do I need to do to troubleshoot before I order a rebuilt unit? Not sure if the one in the car is original - I suspect that it is.

tobybul · #2 04-04-2007, 07:58 AM

typically, even with a bad ccu, the defrost (or left-most button) should turn the fan on.

Did it ever work b4?

Bruce Kennedy · #3 04-04-2007, 08:08 AM

Sounds like its not getting power from the key switch. My td's cc only works if I turn the key back just a little after starting it.

#4 04-04-2007, 08:09 AM

Yes - it's worked for as long as I can remember.

This happened a couple months ago - I removed the blower motor, tested it, checked the fuses, etc. and the problem went away. Not exactly sure what it was, probably bad contact on the fuse (?) but this time I have checked, re-checked, double, and triple-checked that damned fuse and it seems to be fine.

tobybul · #5 04-04-2007, 08:26 AM

Have you tried jumping the fuse holder just to test? Make sure you remove the jumper right away after done testing whether you get it to work or not. If it works when you jump fuse then it must be a bad fuse.

If its a bad fuse and you put a new one that makes it work but blows again, theres a problem somewhere... like something is shorting out.... or locking up. Remember, the fuse is a good thing. It protects the circuit from damage.

You didn't really explain how this started. Did it just kick out suddenly? exhibited symptoms first? you messed with something that my have caused it?

#6 04-04-2007, 08:58 AM

I do need to try jumping the fuse holder to test. You are right that I need to find the root of the problem if it turns out to be the fuse - the CCU shouldn't routinely blow fuses. If it is, there's a problem and the fuse is protecting my CCU and blower motor...

No symptoms exhibited. Went to turn blower motor (vents) on yesterday morning on my way into the office and... ...nothing...

hitwtom · #7 04-04-2007, 09:02 AM

Almost exactly the symptoms my '82 300D had. I removed the CCU, resoldered all of the connections on the circuit board and re-installed. Has been working fine since.

tobybul · #8 04-04-2007, 09:20 AM

Quote:

Originally Posted by hitwtom

Almost exactly the symptoms my '82 300D had. I removed the CCU, resoldered all of the connections on the circuit board and re-installed. Has been working fine since.

I may have to do this on my 84D. The automatic temp system works intermittently but considering its mileage, its understandable I guess.

Was the resoldering work pretty challenging? I've not seen the inside of the ccu yet let alone take one down. Did you have to use a marnifying glass?

Did you use a soldering gun or iron? How much did you have to do?

Magoo · #9 04-04-2007, 09:47 AM

I resoldered mine a couple of months ago with an pencil-tip iron. The hardest part was prying open the plastic casing without breaking it. It's not a multi-layered board like most things have today, and the traces are rather large, so it was pretty easy to just remelt everything.

dannym · #10 04-04-2007, 09:50 AM

Could it be this:
http://catalog.peachparts.com/item.wws?sku=R2028-15177

Or maybe this?:
http://catalog.peachparts.com/item.wws?sku=R2022-50062

I happen to have both of them available if you need them.

Danny

hitwtom · #11 04-04-2007, 10:37 AM

Quote:

Originally Posted by Magoo

I resoldered mine a couple of months ago with an pencil-tip iron. The hardest part was prying open the plastic casing without breaking it. It's not a multi-layered board like most things have today, and the traces are rather large, so it was pretty easy to just remelt everything.

What Magoo said, use a pencil type iron (not a big Weller gun) just touch the tip to each place where a wire comes through the board, when it gets kind of glassy looking (solder becomes liquid) keep holding the tip where it is and just barely touch a new piece of solder (rosin core) to the spot. Remove the tip of the iron from the spot, let cool and proceed to the next. As I recall, like Magoo said, the hardest part was getting to the board. To get the plastic cover off I had to remove the fan speed switch housing which requires slight prying in 2 directions at once. Picture #1 - CCU removed from dash. Picture 2 - What you're going to be working on. Picture 3 - Soldering. Hope this helps some.

SD Blue · #12 04-04-2007, 01:46 PM

A very likely cause are the solder joints where the boards connect. They are arranged in an "H" pattern and very often the solder develops cracks where the boards meet. Most definitely use a pencil iron.

tobybul · #13 04-04-2007, 02:34 PM

Quote:

Originally Posted by hitwtom

....Picture #1 - CCU removed from dash. Picture 2 - What you're going to be working on. Picture 3 - Soldering. Hope this helps some.

I thought the ccu is behind the glove compartment ... not the on behind the dash that you show....

SD Blue · #14 04-05-2007, 12:42 AM

To split hairs........what was shown in the first photos is the ACC Pushbutton Controller, while the second photo is a Temp. Controller .......in other words neither of them is a "CCU" (according to the schematics I have)

tobybul · #15 04-05-2007, 07:57 AM

Quote:

Originally Posted by SD Blue

To split hairs........what was shown in the first photos is the ACC Pushbutton Controller, while the second photo is a Temp. Controller .......in other words neither of them is a "CCU" (according to the schematics I have)

Yeah... but I believe the part in question is the one behind the glove compartment....