Heater Fan Selector Switch Electrical

[Charlie]

Still Under Construction ... but there's some new info here as of 12/11/2014 @ 9:35 PM
- Comments welcome!

I've relabeled the diagrams with some new information gleaned from studying the schematic and the switch a little closer.

The pins are stamped differently than the Bakelite switch face:

I noticed that the Bakelite face of the switch has been lightly inscribed (barely visible) with M, L, H and B for pins that are stamped as 1, 2, 3 and what I was calling “C” (a random misnomer since I had not intended it to be confused as “Common”)

This seems more logical seeing that M, L, H and B seem to stand for Medium, Low, and High speed, with “B” for the 12 volts to drive the blower?

Here is the latest version of the schematic diagram:

Corvair Heater Fan Switch Schematic.jpg (58.69 KiB) Viewed 1619 times

It is possible that the "M" coil is used in the "High" position to absorb the "spark" of the direct connection from "B" to the "H" lead that otherwise seems to just shunt the "M" resistor coil? (Subject to confirmation?)

Heater Fan Switch Positions:

Heater Fan Switch Contacts "Truth Table":

[Wyvern]

Here's a picture of the back of the switch.

[bbodie52]

My analysis...

1. A single wire feeds the fan motor DC Voltage. Ground return is via the motor case to chassis ground.

2. Two resistors of different values in the resistor pack reduce power to the motor, to provide two lower speeds.

3. The fan switch sends power to the fan motor either directly, or diverts power through the resistor pack in the trunk... through Resistor 1 only, or through both resistors in parallel. The reduced power then returns from the resistor pack to power the fan motor at the selected lower speed.

The first diagram below is an attempt to show the basic heater fan wiring, while eliminating the remaining Corvair wiring from view. The BROWN wire is the power source from the fuse block (battery). The BLACK/ORANGE wire is the main supply line to the motor. The DK. GREEN wire is simply a voltage tap to feed the backup light switch. It has no relationship with the fan circuit.

Moving the fan switch from OFF to HIGH establishes a direct connection from the power source (FUSE BLOCK) to the FAN. No resistors or trunk wiring are in the circuit. The two reduced fan speeds are obtained by bypassing the direct connection to the fan motor, and routing the power to the resistor pack in the trunk. The Fan Switch selects a circuit to pass current through either Resistor 1 only (LOW SPEED) or Resistor 1 plus Resistor 2 (MEDIUM SPEED). Power passes through the selected resistor(s) and then returns back to the fan motor via the BLACK/ORANGE wire, and the motor operates at a reduced speed, as determined by the current limited through the selected resistor value.

Late Model Heater Control

Corvair Trunk Resistor Module

Heator Resistor (4).JPG (25.17 KiB) Viewed 1688 times

This schematic shows the entire 1965 Corvair wiring diagram...

Left-click on the diagram with your mouse to enlarge for improved viewing...

1965 Corvair Schematic Diagram

Wire color codes in EM Corvairs may vary with the model year, but the principles of operation remain the same for all model years.

[Wyvern]

I've taken a switch apart, and this is what I've learned.

The terminals on the switch have two markings. There's one on the blade, and one on the insulator.

Code: Select all

Insulator  Blade
   B         2
   L   (center blade)
   M         1
   H         3

This is what it looks like inside. The "bumps" are contacts, and the rest of the surface is insulator. I know it's hard to tell from the photo. The slider is copper.



You can see as the slider moves (up in this case), it connects B to first L, then M, then H. However, after L, it connects to two output terminals at a time; i.e. medium connects B to L and M, and high connects B to M and H. This is certain to cause confusion if any troubleshooting is attempted in situ. This will also cause grief for some that connect a separate hot through a relay to power the high-speed wire to the blower.

[bbodie52]

Connecting "B" (Battery Power) to resistor "L" (Low Speed) or to "L" plus "M" (power through BOTH resistors to produce Medium Speed) is not confusing. But moving the switch all the way to apply power to both "M" and "H" might seem to be confusing, unless you realize that a direct connection between "B" and "H" is a direct short from the battery power to the motor "H", which effectively bypasses the resistor "M". The fact that the contact is still touching "M" is irrelevant, since power is applied directly to the motor input and the resistor "M" has no impact on the circuit.

[Charlie]

Right, Brad ~ The "M" contact on "High" may be irrelevant, but may indicate that GM used this switch "off the shelf" instead of designing a new one?

Is it possible that the "M" coil is used in the "High" position to absorb the "spark" of the direct connection from "B" to the "H" lead that otherwise seems to just shunt the "M" resistor coil? (I added this comment to the top of this page, so I hope it's correct?)

[bbodie52]

As far as I can tell the resistor pack used in the Corvair was used for decades in a wide range of heater fan controls for many GM cars. Many different GM switch designs were probably adapted to fit styling variations in a wide range of instrument panels while all used the same resistor pack to regulate fan motor speeds. The switch itself was mechanically designed to fit the unique style of the 1965-1969 instrument panel shape, but internally it had to accomplish multiple connections from the power source to one resistor, both resistors, and then a direct-connect between the battery source and the motor (bypassing the resistor pack). This design accomplished the single and two-resistor connection in sequence. The fact that the final connection reverted back to a single electrical connection that also overlapped one of the resistors in the resistor pack was electrically sound. It was not necessary to design a more complex switch that would physically disconnect from the remaining resistor connection.

By comparison, in the engine compartment, engaging the starter solenoid creates a direct tie from the battery to the ignition coil to provide a boost in spark voltage during engine cranking. The resistor wire that is powered by the wire from the ignition switch remains present. It was not electrically necessary to disconnect the resistor wire circuit while power was supplied to the coil from the starter solenoid. The higher voltage simply made the resistor wire circuit irrelevant while the engine was being cranked, but its presence continued a steady flow of power at a lower voltage to the ignition coil when the starter solenoid was disengaged.

I doubt the switch sequencing from low to medium to high current provides any significant added longevity to the switch contacts. The switch cycles are infrequent and the design of the contacts are simple and long-lasting.

I was glad to find your picture and illustration showing the internal switch contacts. I had a picture of the resistor pack, but it was unclear which circuits were active at low or medium speeds. I wasn't sure if the medium speed used one resistor while the low speed used the other resistor, or if both resistors were in the circuit at the medium fan speed. Your dissection and analysis of the fan switch made everything clear, in a way that could not be determined by looking at the schematic alone.