[Fourninety2]

I have a 1978 Mercedes 450slc, California rust free car. The heater/ac controller has issues. Defrost and off button are the only ones that work. I replaced the ac relay under the dash figuring this would solve the problem since prior to switching out the relay, the off button when depressed would blow air and the defrost button when depressed would turn the system off. The other buttons worked as well. Now that I changed the relay it only works on defrost and off. Basically I'm trying to get the ac to work to no avail. System is charged with freon, I just cannot figure out why I cannot get all the buttons to work on this controller. I have even replaced the heater ac controller as well and have the same results as the original one I replaced. Anyone know what the fix is for this? Is there another relay in the fuse panel that is bad that is causing this debacle? I want to fix it so I can sell the car, and I live in las Vegas, so it's hard to sell a vintage Mercedes if the air doesn't work.

Thanks in advance for any and all help.

 

[flyboyike]

By "heater controller" do you mean the amplifier behind the gloveblox?

 

[Fourninety2]

I mean the actual control unit in the center of the dashboard, with the thermostat and heat, defrost, bi level hi, bi level low and off buttons.

 

[flyboyike]

Got it. Check the amplifier behind the glovebox. See if it smells burnt (they often do when they fail). Also, how does the servo look? Is it even intact? When you turn the key to "on" (but not start the engine), do you hear the servo "whirr"?

 

[Fourninety2]

The servo (which I believe is the part mounted to the firewall on the passenger side) is intact and when the key is turned I can hear a whirring sound coming from that direction while in the car. I haven't checked the climate control amplifier. Does that control the function of the buttons on the dash board controller? I ask because of the previous post, I replaced the AC Relay that is mounted under the dash because the control buttons all worked except the defrost button didn't work, but when the off button was depressed, it would blow air, as if the functions of the defrost and off buttons were reversed somehow. When I replaced the relay, no only the defrost button blows air. All the other buttons when depressed do nothing, just like the off button.

 

[flyboyike]

I haven't checked the climate control amplifier. Does that control the function of the buttons on the dash board controller?

It acts as the middleman between the control unit and the servo. Give it a good look. The fact that the servo whirrs is a good sign, the amplifier is a lot cheaper than the servo.