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Sorry about the diagram. The changeover switch will activate the heater fan and AC fan as usual. My light on my compressor thermostat does not light up. I will need to pull the dash to get to the back of the switch to test voltage.. If the dash is too challenging, I'll probably just put a on/off switch under the dash connected to my HotWired contraption by the changeover switch. I already pulled the knob out at the AC knob and can't figure out how to get it back on.

You have been an awesome help already. I'll keep you posted.
 

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Sorry about the diagram. The changeover switch will activate the heater fan and AC fan as usual. My light on my compressor thermostat does not light up. I will need to pull the dash to get to the back of the switch to test voltage.. If the dash is too challenging, I'll probably just put a on/off switch under the dash connected to my HotWired contraption by the changeover switch. I already pulled the knob out at the AC knob and can't figure out how to get it back on.

You have been an awesome help already. I'll keep you posted.
You only need to pull the center console. But that's still a pain!
First, check the fuses in those little aux under-hood fuse-boxes. I believe one or two of them are for the AC. They can look OK but the contacts could still be corroded.

Many years ago, I acquired my first W115 - a 1972 gasser 220. After I got the AC working, I took a trip out to Maryland's Eastern Shore. On the return, it wasn't that hot, but it was extremely humid. So I ran the AC. I had one of those little dial thermometers in the center dash-vent and it was reading well below 32F. I thought "Great! This AC is really wonderful". But after about an hour, I noticed the airflow had completely dwindled to nothing. The evaporator had frozen solid!. So I had to pull over, shut off the engine, then turn the ignition back on and let the AC blower melt the ice for about 15 minutes, until a big puddle formed under the car and the airflow returned to normal. Then I could start the engine and enjoy my AC - for awhile! This went on for about every 40 miles until I finished my trip.
Later, I pulled the center console to find someone had previously been in there and managed to pull the AC thermostat's temperature probe out of it's tiny hole in the evaporator case. After I re-installed it, the AC never froze up again. But it never again got down to 26F at the vents either!

Happy Motoring, Mark
 

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Unfortunately, the component labels on your diagram are nearly illegible on my display and I'm 200 miles away from my FSM this week. so I'm going by memory of how my '72 W114-115 AC worked.
Does your change-over switch activate the heater-fan in the dash, then the AC blower in the console as needed? Do you have power to the AC compressor thermostat? In the OP's photo it's one of the two green wires connected to the square metal device in the console, controlled by the rotary AC knob. When the thermostat is getting power and it's connecting to the compressor, you should have 12V at both wires.

Happy Motoring, Mark
I spent some time and I compiled some schematics and drawings of the A/C system. I opened up the center console and tested out the wires on the back of the A/C rheostat/C O L D E R knob. Some of the terminology on the Mercedes schematic is confusing, but I think I've got it.

I'm looking for input on if anybody thinks I have a bad A/C rheostat /vacuum control/ C O L D E R switch?

The drawing is of the female end of the change-over switch. The Mercedes wiring diagram draws the circuit in an out of order orientation to the switch. So this is helping me out. I have 12 volts at location 4. Nothing else is energized.

The vacuum system appears to be working properly.

I do not have 12 V at the A/C rheostat knob. I tried to energize the switch with 12 volts and nothing happened. I may have a bad switch, or bad wiring. I am pretty sure the 6-pin change-over switch can only be inserted one direction. I tried to flip it around and the plug really didn't fit.

As you can see in the pic I have a floating wire from the A/C control light. I am not sure where that goes. Any help there?


Hopefully we can get this figured out.
 

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OK, so you have power coming into the switch. Now remove the back of the connector, being careful not to let the wires come out. Put the connector back on the switch, and turn the AC on, and see if you get power out to the blower and other "consumers."
 

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I tested all the connections. I found out where the loose wire went for the A/C control light. Hooks up the a ground spot on the speaker fader knob.



With the A/C knob turned OFF and blower switch turned on max.

I am referencing the plug numbers on change-over switch below. The change-over switch is plugged in.

1: 0.0 Volts: Goes to A/C Fuse PassSide, A/C Thermo, A/C clutch
2: 0.0 : Blower Switch
3: 13.8V: AC temp control knob/Blower Motor Preresistance-
4: 13.8V: 12V hot
5: 0.0 V: AC Control Light
6: 3.7 V: A/C Fuse PS, A/C Blower Motor for air conditioning, Blower Switch

With the A/C knob turned ON and blower switch turned on max. THe change-over switch is plugged in.

Everything is dead except for plug 4/ 12Volt hot

I have considered the "new" change-over switch may still be faulty. what else can been seen by these values?

I am able to get my compressor and air to work by jumping 1,4,6 on the changeover.

It also came to mind that I may not have enough R12 and the A/C thermostat circuit at the Drier may not be closed. I am out of R12 so no way to charge it up right now, but can I bridge the thermostat for testing? I have cold air blowing so I know I have some R12.

Thoughts?
 

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1982 240D, 1984 Euro 300TD
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I tested all the connections. I found out where the loose wire went for the A/C control light. Hooks up the a ground spot on the speaker fader knob.



With the A/C knob turned OFF and blower switch turned on max.

I am referencing the plug numbers on change-over switch below. The change-over switch is plugged in.

1: 0.0 Volts: Goes to A/C Fuse PassSide, A/C Thermo, A/C clutch
2: 0.0 : Blower Switch
3: 13.8V: AC temp control knob/Blower Motor Preresistance-
4: 13.8V: 12V hot
5: 0.0 V: AC Control Light
6: 3.7 V: A/C Fuse PS, A/C Blower Motor for air conditioning, Blower Switch

With the A/C knob turned ON and blower switch turned on max. THe change-over switch is plugged in.

Everything is dead except for plug 4/ 12Volt hot

I have considered the "new" change-over switch may still be faulty. what else can been seen by these values?

I am able to get my compressor and air to work by jumping 1,4,6 on the changeover.

It also came to mind that I may not have enough R12 and the A/C thermostat circuit at the Drier may not be closed. I am out of R12 so no way to charge it up right now, but can I bridge the thermostat for testing? I have cold air blowing so I know I have some R12.

Thoughts?
After studying the AC wiring diagrams in my FSM, my first thought is a corroded fuse in the AC fusebox located on the PS firewall, near the rec-drier. One of those is supposed to feed 12V power to terminal #1 on the change-over switch. With the engine running, check for 12V at both ends of both fuses.
My FSM also shows an AC relay located under the bracket where the AC fusebox is mounted. You may need to check that if there's no power going to the AC fusebox.

Happy Motoring, Mark
 
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