Some climate control questions (300SD) - PeachParts Mercedes-Benz Forum

deniss · #1 06-09-2007, 05:36 PM

I have a couple of questions about the automatic climate control system on my '84 300SD.

1) where is the electrical connector to the monovalve located? a mechanic recently replaced the monovalve insert because the solenoid was sticking and putting out heat through the defroster vents. but now the problem seems to be back, and i am thinking it might be a corroded electrical connection?

2) the air blower seems to have the mind of its own. it sometimes doesn't come on when i pressed the switches for the desired settings. or it would come on, work for a while, and shut off, and then after a while come back on. there is usually a "click" associated with that. where is a switch located that controls the fan operation?

Thanks!
Denis

LUVMBDiesels · #2 06-09-2007, 07:04 PM

On my '81, the monovalve connector is right on top of the valve. It is a small two wire socket that fits onto the plug on the valve. There should be no voltage on it when you want heat and 11 ~12 volts on it when the heat is off. Mine needed to have one wire re soldered to the connector. Try doing that. The blower should not come on right away after you stat the car. It waits until the coolant is 35 degrees C before activating. If you push the high fan button it will come right on. When you think it is going off it might just be going into low speed. In low it is nearly silent.

Hope this helps...

deniss · #3 06-09-2007, 08:02 PM

Quote:

Originally Posted by LUVMBDiesels

On my '81, the monovalve connector is right on top of the valve. It is a small two wire socket that fits onto the plug on the valve. There should be no voltage on it when you want heat and 11 ~12 volts on it when the heat is off.

I found the connector and measured the voltage across the two wires that make up the connector - it was zero! And that's with thermostat setting to MIN. So what could be wrong in this case? (I did measure the wires, not only connector sockets, to rule out badly soldered connector).

Quote:

Originally Posted by LUVMBDiesels

The blower should not come on right away after you stat the car. It waits until the coolant is 35 degrees C before activating. If you push the high fan button it will come right on. When you think it is going off it might just be going into low speed. In low it is nearly silent.

Well, I put my hand up to the windshield to see if the blower is on or not because you're right - it is quite quiet on the lowest setting.

I put the blower on the HIGH fan setting, not AUTO, and it still doesn't come on. What gives?

arew264 · #4 06-09-2007, 08:11 PM

Do a search for resolder CCU. The solders in the CCU overheat and crack over time and you have to resolder them.

deniss · #5 06-09-2007, 08:40 PM

Quote:

Originally Posted by arew264

Do a search for resolder CCU. The solders in the CCU overheat and crack over time and you have to resolder them.

You mean behind the ACC switch board?

Is the thermostat responsible for setting 12V at the monovalve? Is the thermostat basically acting as a variable resistor between 0 and 12V?

arew264 · #6 06-09-2007, 08:47 PM

Well, I just looked back and realized I may be completely wrong... Dunno about the 300SD.

I'm referring to the push button unit (if your car has one).
The relays for the fan and whatnot are in there, and the connections can fail.
The one thing you can try to confirm this is try turning on the blower using defrost mode, and then switch to the mode you want.
There are two blower relays, one for the right two modes, and one for the left two.
However, once the blower is engaged with either relay, the unit keeps it engaged with that relay until you turn the ACC off.

That way you can see that one of the relays has problems by trying the other one (the economy/normal mode one typically has trouble while the other one is fine).

deniss · #7 06-09-2007, 08:55 PM

Quote:

Originally Posted by arew264

Well, I just looked back and realized I may be completely wrong... Dunno about the 300SD.

I'm referring to the push button unit (if your car has one).

Yep, I got a push-button assy. Exactly like this:

Quote:

Originally Posted by arew264

The relays for the fan and whatnot are in there, and the connections can fail.
The one thing you can try to confirm this is try turning on the blower using defrost mode, and then switch to the mode you want.
That way you can see that one of the relays has problems by trying the other one (the economy/normal mode one typically has trouble while the other one is fine).

Right now, I can't seem to get the fan to come on with any buttons on the board.

Oh and my ambient temperature sensor on the dash went out as soon as my "heat on the windshielf" woes have begun today... Is happened last time my monovalve was malfunctioning as well. I'm not sure if there's any connection, but I thought I'd mention this.

arew264 · #8 06-09-2007, 09:02 PM

That sensor shouldn't stop the fan from operating.
Beyond there, I can only tell you to search or wait for someone else to chime in. I just know you probably have bad solder connections, a bad blower, or a bad blower speed controller.

deniss · #9 06-10-2007, 12:47 PM

So can anyone tell me if the electrical connector on top of the monovalve is connected directly to the thermostat wheel in the cabin?

saumil · #10 06-10-2007, 12:56 PM

Erratic blower motor behavior can be due to the ignition switch, it is a cheap part, replace it if you have not replaced it ever.