New CCU, New Problem - PeachParts Mercedes-Benz Forum

Uncle Possum · #1 07-12-2007, 02:28 AM

I had been having trouble with my 82 300SD AC unit. When I got the car, the AC compressor was fried, and most of the vacuum pots were shot. I had a reputable Mercedes Mechanic (Sig of Sig's ******** here in Vegas) completely go through the whole system, replaced the compressor, drier, flushed and purged, replaced vacuum pots, etc. Everything except the CCU was fixed. The AC would work great, blowing air out all vents, controlling fan speed in auto, etc. Then it would randomly just stop working. I could get to start working again by pushing the off button and restarting, or just by jamming on the middle button.

I thought the problem was the CCU so I bought a used one off of EBAY that supposedly had been tested. The replacement CCU now only blows cold air from the side vents and dash and only with the temperature wheel at minimum setting. As soon as the wheel is moved off the lowest setting I get hot air from the side vents, and dash. I get no air from the middle vents at all anymore.

Did I get a bad CCU? Or is this a symptom of a bad Aux water Pump? Or, even though it looked straight forward, could I have screwed something up? From other posts I have read, may be a vacuum issue, but air was coming out of the right places before I changed the CCU.

Thanks for this forum me. Three weeks ago before I joined this forum I wouldn't have had a clue on even how to change out a CCU.

tangofox007 · #2 07-12-2007, 09:23 AM

Quote:

Originally Posted by Uncle Possum

Then it would randomly just stop working.

Could you be more specific about the meaning of "stop working?"

Uncle Possum · #3 07-12-2007, 09:30 PM

Quote:

Originally Posted by tangofox007

Could you be more specific about the meaning of "stop working?"

No air would come out of the vents, I couldn't tell if the AC compressor was still on, but it felt like it was.

Thanks,

tangofox007 · #4 07-12-2007, 10:20 PM

Sounds like a bad solder joint in the first CCU could be the problem. A bad relay in the CCU is also a possibility, especially if the blower will not come on when the EC mode is selected, but will come on when a/c is selected.

But if the blower just randomly "turns off," my money is on the solder joints in the CCU. The odds are better that three to one!!! For Vegas, that's not bad.

babymog · #5 07-13-2007, 01:34 PM

First of all, unplug the aux pump, it's summer and you can eliminate that variable.

The CCU could be bad. Try the max. fan speed with the temp wheel not on full, see if it is just a fan-speed issue with the CCU. I had one that had a shorted aux pump, blower wouldn't run except as you mention. The CCU was damaged, opening it revealed an overheated TO-220 component (transistor or other).

Tested doesn't mean much on ebay, how many people are really set up to test the CCU?

SD Blue · #6 07-13-2007, 06:10 PM

With the way the ACC controller is designed, it could have easily been damaged in shipping if not packaged very well. I agree with Tangofox, sounds like solder joints in the controller.