Climate control questions ...

[78 450sled]

I have a 78 450 Sl and when I bought it the climate control servo was out of the car and broken. I recently purchased what appears to be a good one. It looks fine inside and out. I have installed it but I still get no air except for defrost which I understand is normal. When I turn the car off I can hear the motor inside the servo working. If I look into the center vents I can see them opening but the fan will not blow when I push any of the AC buttons. I have read about an amp behind the glove box but I'm not sure what to look for once I find it. . When I turn on the AC switch the compressor does not work either. It seems to have vacuum to the unit but even when the engine is at 3000 rpm it the fan will still not come on.

ANY ideas would be appreciated.

[KCM]

If you can see the vents moving when pushing the buttons, and hear the servo running, I would almost say the rest of the system is working. If I'm reading the schematic I have correctly, the defrost system has a seperate relay that runs full voltage to the motor and bypasses the blower resistors. Otherwise power runs through a series resistor block, where the voltage is controlled by the servo unit. Looks like the servo has a switching device that connects or bypasses the resistors to control blower speed. Before power reaches the blower and after the resistors, it goes through a master vacuum operated switch with a green vacuum line. I would check to make sure you have power to the resistors, power out of the resistors, and to make sure power is getting through the master vacuum switch. If you have power to the resistors but none out, then you either have a bad connection, bad resistors, or a bad servo. Looks like there are total of 6 connections at the resistors, one being power in, one power out, and a tap between each of the 5 resistors in series (which connects to the servo), so you can unhook it and check for continuity and resistance in the block. Could also be as simple as a broken vacuum line to the switch, which is the very first thing I would check.

Also, when in heat mode, the blower will not operate until the engine water temperature reaches a certain level (except in defrost).

I believe the resistors are in the blower motor compartment under the hood at the rear under a plastic cover. The vacuum switch should be behind the glove box.

As for the compressor not running, I would check the Freon level before assuming something is wrong. It may simply be too low to activate the Freon pressure switch. Or maybe the pressure switch is bad. I would think the fan should run though with the dial set all the way to cold and in Auto mode. The compressor circuit also have some vacuum switches and misc as well.

Maybe this will give you a path to diagnose the problem.

[ctaylor738]

It would be really odd for the resistor pack to go - those things are built to last. I would bet on the amplifier, which is a common failure if the servo dies.

Another issue might be a vacuum leak or a bad vacuum element or a bad micro switch. The flaps have to be firmly in the AC position for the compressor and fan to come on.

Lastly, DEF should engage the compressor. If it doesn't then you may have a compressor or low freon problem.

Good luck.

[78 450sled]

I have the glovebox out and I found the amplifier. I opened it up and on the bottom of the circuit board are 2 brown circular spots that could be burns. Think it's fried ???

[ctaylor738]

That's certainly suspicious. But their normal failure is that the solder joints crack. You might as well replace it and move on to the next likely culprit. If it hasn't failed, it will fail soon. These things don't have a long life span.

Did you ever determine if the compressor is working?

[78 450sled]

I haven't figured out the compressor problem yet. I'm going to dive into it on Sunday. One other thing I noticed is when I push any button on the climate control panel none of the little lights on the buttons lights up. I don't know if that makes any difference or not.