'84 300CD ACC works only if you hit DEF first ---> Where to look first? - PeachParts Mercedes-Benz Forum

nhdoc · #1 12-25-2006, 10:00 AM

My recently acquired '84 300CD has a strange issue with the ACC. Upon start-up the ACC will do nothing unless you first press the defrost button then switch over to other modes which seem to work OK, including ECON which is the mode I use most this time of year, for heat.

I have a spare temp controller (the metal can under the dash, not the pushbutton unit in the center) that I know is good so I was going to try to change that out but if anyone has any tips on what could be causing this can they please post it? I don't think it is the pushbutton unit since it seems to be related to starting the unit from power-up each time. Once it is working it seems to function more or less as it should until the next time you shut the car down, then you need to push DEF again to get any of the other settings to work.

engatwork · #2 12-25-2006, 10:17 AM

I've seen this issue before and vaguely remember that the fix was swapping out the pushbutton unit.

tangofox007 · #3 12-25-2006, 10:37 AM

The problem is with a relay in the push button unit. There are two relays that do essentially the same thing but are energized by different buttons. Most likely the relay that is energized by the EC button is bad. When you press the Defrost button, the other relay is energized and remains powered so long as OFF is not selected. (It is possible that the problem is not the relay itself, but the associated solder joints in the push button unit.)

nhdoc · #4 12-25-2006, 10:45 AM

Great, thanks for the help guys, I will pull the pushbutton unit out and check the traces and relays...I'm certain I can find it if it is one of those issues.

Regards,

Marty

#5 12-25-2006, 11:21 AM

I agree, the defrost button is usually the last part of the ACC unit to fail for some reason. It will probably stop working at some point. I just replaced mine with a rebuilt unit when it started acting up.

Motorhead · #6 12-25-2006, 02:11 PM

Check the solder joints at the relays and all others. I fixed one that way.

nhdoc · #7 01-04-2007, 05:36 PM

Just following up - it was indeed the pushbutton controller at fault...that and the stupid foam hose that connects the sampler vent to the sensor hosing that had fallen apart are now replaced and the ACC is working fine once again...thanks to all who posted replies!