The vacuum/heater control system

I've been going through the Buick factory manual I ordered from Detroit Iron Information Systems, reading up on the controls for the heater and A/C unit. Slowly I begin to understand just how the entire thing works (or should work at least).

Now, I won't be giving away the entire manual here, just a page or two in order to illustrate how it all works. If you need the manual - buy it. It really is worth the money, even though it can seem expensive.

The controls in the dash control both vacuum as well as electronics; the vacuum controls all the various doors in the system (there are 5 doors all in all; the defroster door, the mode door, the deflector door, the temperature door and the outside-recirculated air door).

The Fan switch controls the speed of the fan motor electronically as well as being the master vacuum switch. When you move the Fan switch to any position besides the "Off" position the outside-recirculated door is partially opened via a vacuum diaphragm.

The Temperature lever controls vacuum flow to the water valve; at the leftmost position no water from the radiator flows to the heater core and at the rightmost position the valve is fully opened.

Finally, the Selector switch controls the following doors via vacuum; the defroster door, the heater-air conditioner mode door and the outside-recirculated door. It is also mechanically linked to the A/C compressor clutch switch.

Vacuum is provided from a vacuum in the engine and goes through a vacuum tank to the master vacuum switch (the Fan switch, remember). The Temperature lever also receives vacuum directly from the tank so that the temperature of the air can be controlled without running the fan.

This means that if the master switch doesn't work, there'll be no vacuum to the Selector switch. It also means that you should be able to get warm air to flow through the system if you move the Temperature lever all the way to the right as this works without the master switch.

My guess is that if you don't get warm air through the system when driving your car with the Temperature lever all the way to the right with the Fan switch in the "Off" position you might have a vacuum problem. I can't say for sure though as I haven't checked on my own car yet.

You should also be able to measure voltage at the Fan switch in order to see if you have 12 volts at the 'F' pin (Brown wire). Following the wire diagram you should have 12 V out on all the other pins as well depending on what position the switch is in. These lead to various points in the fan blower resistor and from there on to the actual motor.

As for testing the vacuum, the easiest thing would probably be to run the engine on idle and then disconnecting the hose going to the vacuum tank in the engine bay (the 'in' connection on the tank) and feel if you have a vacuum there. If not, I'd guess your vacuum pump is busted.

If you do have vacuum you just connect the hose again and keep disconnecting hoses further down the chain until you find the leak. If there's vacuum all the way to a door diaphragm and the door still isn't moving then there's not much else to do than replace the diaphragm if you ask me.

As for the fan blower motor you should try and measure the resistor before you take apart half the car in order to get to the motor (it sits below the outer fender on the passenger side). There should be a wire running from the A/C box in under the fender from the engine bay. I can't tell you more as I have yet to do this but this is one advice I've been given.

There's a lot more to write on this subject but since I'm very much a beginner when it comes to troubleshooting this and keep guessing for most of the time I'll leave it be for the moment. Hopefully someone is helped by this information.

Feel free to post corrections, suggestions and ideas in the comments.