I couldn’t get the AC fan to move as much air as I thought it should so I decided to take the switch apart (again) and sort it out. I figured while I was at it I’d document the guts of it in case someone else could benefit from knowing how these things work.
When you take the switch out of the AC case you need to separate it from the temp control switch by removing two screws that hold the two together. Then drill out the rivets that hold the switch together. The switch will come apart in three pieces; he front metal cover, the body and the back plastic cover. Take it apart slowly and nothing should fly apart. On the front side of the body observe the four spring wire strips that function as springs for the four detent positions: off, low, medium and high.
The back side will look like the below photo. In this picture the contacts are in the medium position.
You have two moving contact arms designated by the blue and yellow arrows that contact the fixed contacts designated by the white, black and green arrows. When you rotate the fan speed selector knob the white cams in the center of the switch move the arms up or down. Doing so distributes 12V to the various terminals on the large resistor by the brake bottles which varies the fan speed.
In the off position neither of the contacts on the moving arms contacts any of the fixed contact points.
In the low position the yellow contact arm contacts the green fixed contact only. So, you have continuity between the Brown and green terminals.
In the medium position the yellow contact arm contacts the green fixed contact and the blue arm contacts the white fixed contact. So, you have continuity between the white, green and brown terminals.
In the high position the yellow contact arm contacts the green fixed contact and the blue arm moves down to contact the black fixed contact. So, you have continuity between the black, green and brown terminals.
The most common malfunction is corrosion on the contacts that causes poor or no continuity. Clean the contacts with 1000 grit wet dry sand paper and you usually fix the problem. The blue arm on my switch was also bent so I was getting an intermittent contact on high.
Oh, don’t try to rotate the switch without the back cover in place. That will probably cause the plastic cams to displace. They are easy to put back in position but it’s easier to avoid it…
I reassembled the switch using long countersunk screws and nuts on the back side.
The fan now puts out a very decent amount of air in all three positions.
