1977 Buick Skylark Heater Controls Not Working Fully?

Hi multiburon. Welcome to the forum. You have this exactly backwards. The relay is in the circuit that IS working. The resistor assembly is used to develop the lower speeds. It is bolted to the heater box inside the car and has a thermal fuse that typically fails. Because of the relatively high current, the switch contacts can overheat and become burned but most of the time that will just cause one of the lower speeds to quit, not all of them. When all of the lower speeds quit, it's USUALLY caused by the resistor.

The relay is used to handle the really high current of the highest speed. That takes the strain off of the highest speed switch contact, so that one rarely fails. That relay is bolted on top of the heater box under the hood on the passenger side. It has four wires in its connector. When it does fail, it is most often due to corroded contacts from the environment it lives in. A defective relay causes the fan to not work in only the highest speed.

Caradiodoc

The resistor assembly has no idea whether the car has AC or not. It is strictly for fan speed. The resistor can be physically different depending on whether the switch is for a three speed or four speed system.

The resistor in your photo is for a three speed system. Each resistor is switched in for one of the lower speeds. The relay bypasses the resistor block completely for the highest speed.

Caradiodoc

Is it possible the fourth terminal isn't used? There might be a blank spot in the electrical connector where one wire is missing. Changing the resistor shouldn't be necessary when adding or deleting air conditioning. The resistor must be matched to the number of settings on the switch. If you end up with a different switch because it is part of the heater / ac controls, then you WILL have to change the resistor to get the correct number of speeds.

There are two different ways the resistor can be wired by the manufacturer, and depending on how they do it, there will be different symptoms. The most common way is to put all of the resistors in the assembly in series. On the second highest setting, one resistor is switched into the circuit to lower the current flow. On the next lower setting, a second resistor is switched in to add to the first one, thereby lower the current flow even more. In this configuration, a burned switch contact will cause the fan to revert to its lowest speed when the dead speed is selected. All other speeds will work properly. It is possible for all of the resistors in the assembly to be the same size and value. This is what GM used so the fan would always when the switch was turned off, as you have noticed.

The second method is to use resistors in parallel that are different values for the different speeds. A wire from each switch terminal feeds a different resistor. With this configuration, a burned switch contact or defective one resistor in the assembly will cause that one speed to be dead, but all other speeds will work normally.

The number of terminals will match the number of speeds on the switch. The highest fan speed terminal on the switch goes to the relay. The total of the number of lower speeds, (usually 2), means each of those has a wire that goes to a terminal on the resistor assembly, and the third wire from the resistor goes to the motor.

The fan will always run on the lowest speed when the ignition switch is turned on, unless you select a higher speed. That was a GM thing. They called it "Flow Through Ventilation".

Caradiodoc

Black is nice. :)

I'm not familiar with the vacuum controls, but there is a check valve on newer vehicles. On '90's Chrysler products, the check valve is about the size of a stack of three nickels. Some people experience a problem where the system switches to defrost during prolonged heavy acceleration such as when going up a long hill. To address that, there is an updated replacement valve with a built-in vacuum storage canister. That valve is about 2" long and 1 1/2" in diameter. Some Ford owners are having the same problem and the Chrysler valve works on them too.

If you have a vacuum canister on your car, it should also have a check valve built in, otherwise it won't do anything during periods of low manifold vacuum.

Caradiodoc