'85 300SD Climate Control - Intermittent Heat - PeachParts Mercedes-Benz Forum

Brian77 · #1 12-22-2010, 12:45 AM

Hello,

The problem with my climate control started just this evening in my '85 SD. I cannot operate the blower motor for the heat unless I first push the defrost button. Heat will blow thru the windshield at maximum speed. Then, if I push the automatic button, the automatic climate control will kick in, and the heat will then go through the lower vents at a regulated speed. If I first push the automatic button, nothing blows. I am thinking that this could be a climate control board issue with the push buttons. Before I dig into this, I am wondering whether anyone else has had this problem.

Thanks,
Brian

mbdoc · #2 12-22-2010, 08:40 AM

Could be related to the cold engine lock-out switch on the t/stat housing.
That switch must close at 40C for the blower to operate in any position except defrost.

toomany MBZ · #3 12-22-2010, 09:54 AM

If you're sure the engine is warm enough and still have an issue, a routine failure item is the ACC unit itself.

Over the years cold solders pop up and once repaired, you're back to normal.

The only way to check is to pull the unit and inspect the printed curcuit board.

vstech · #4 12-22-2010, 10:10 AM

I agree with MBdoc,
unless this is occurring later in the drive. the CCU holds air back until heat is registered. keeps you from getting a blast of icy air.

micalk · #5 12-24-2010, 07:16 PM

I'm not clear about what's going on. Which button are you pressing after you press the defrost? Are you calling the Automatic button the center control button (ac / heat)? or the automatic fan button? What happens when you push the high fan speed button and ECON? Is the engine warm or completely cold? Fan bearings can get gunked up and stick, but may start turning with the high speed fan setting (same speed as defrost), and keep turning once moving, especially once they get warmed up. Hard to say what the problem could be until all the pertinent info is divulged.

#6 12-24-2010, 08:36 PM

Quote:

Originally Posted by toomany MBZ

If you're sure the engine is warm enough and still have an issue, a routine failure item is the ACC unit itself.

Over the years cold solders pop up and once repaired, you're back to normal.

The only way to check is to pull the unit and inspect the printed curcuit board.

I agree, that is a common failure mode of push button unit. After playing around for a while, I replaced mine with a rebuilt unit about 200k miles ago.

Brian77 · #7 12-26-2010, 12:58 AM

Thanks everyone for the suggestions. The heat has miraculously cured itself and seems to function fine. Which leads me to think that it is something to do with the CCU. The problem was that when I pushed the auto button, nothing happened. No blower motor and no heat. When I next pushed the defrost button, I got heat, except it the blower motor was on maximum speed and only to the windshield. I then pressed the auto button again, and the automatic climate control would regulate the temperature fine.

Best,
Brian

#8 12-26-2010, 01:05 AM

Quote:

Originally Posted by Brian77

Thanks everyone for the suggestions. The heat has miraculously cured itself and seems to function fine. Which leads me to think that it is something to do with the CCU. The problem was that when I pushed the auto button, nothing happened. No blower motor and no heat. When I next pushed the defrost button, I got heat, except it the blower motor was on maximum speed and only to the windshield. I then pressed the auto button again, and the automatic climate control would regulate the temperature fine.

Best,
Brian

Yes, that is exactly how mine worked for several months before it died completely.

Call these guys when you need a rebuilt unit:

http://gdl-online.com/begin1.html

micalk · #9 12-26-2010, 02:27 PM

Just to be clear, the 'AUTOM. button only regulates the fan speed. All the pods and temperature control are still controlled automagically by the ACC PBU depending on which function is selected and the other controlling inputs/sensors. When you select defrost, the fan speed is alway maximum (same as top fan button). If you never tried the top fan button, only the defrost button, then you haven't isolated to the PBU. It seems like once the fan gets moving it stays moving and the ACC works without issue. But the fan won't start without the extra oomph from a max speed control signal. Max speed is straight battery voltage, no blower motor resistance group action. This tells me the fan bearings are the real issue here. Has the weather warmed up recently or has some other change occurred, i.e., parking in a garage? Fan may work when warm, but could be sticking initially when very cold.