Defrost Control

The SLC kit comes with eyeball vents for passenger heat/ac, which can be turned off to force all the air to go through the defrost vents. However, even with the vents fully open, some air will want to go out the defrost.

I live in a part of the country where it can get warm and humid. This means there are days where you want the a/c running, but don't want to blow cold air on the windshield, or else it will fog up.

To address this, I modified the ventilation "anchor" with a kind of butterfly valve, using the following:

- 3/8" steel rod (lying around, but probably came from Home Depot)
- 1/8" steel angle (lying around, but probably came from Home Depot). Could also use flat bar stock.
- 2 Snap ring bushings ($0.12 each) with 3/8" inside diameter
- Epoxy and microballoons (also lying around, but probably the epoxy came from West Marine and the microballoons from Aircraft Spruce)
- Small sheet of steel, aluminum or plastic (I used some plastic which was lying around)
- 2 #6 machine screws, with washers and locknuts (also lying around, but also probably came from Aircraft Spruce)

The steps to build it were:

1) Using a power file (portable belt sander with a 1/2" wide belt), square off the inside of the "anchor" so that a square door will be a good fit. Actually, because of the curve of the piece in this area, it was necessary to apply a slurry of epoxy and microballoons to add extra material to build up square walls (step 2)

2) Mix a couple of squirts of epoxy into a paper cup (I really like the West System for this; you attach their pumps right onto the can. One pump of resin to one pump of hardener gives the right mixture). Pour some into a second cup and add some microballoons, stirring with a popsicle stick. Keep adding microballoons until a thick mixture is obtained that doesn't drip off the popsicle stick.

3) Apply the slurry to the inside walls of the anchor. Use a piece of waxed paper on the inside to provide a non-stick surface to use a piece of cardboard or plastic applicator to smooth the walls.

4) After hardening, sand with a sanding block. Repeat step 2 again. When the walls are square, paint some mixed liquid epoxy (no microballoons) to help provide a smooth, hard finish.

5) Drill a hinge line through the anchor. Drill to match the outside diameter of the snap bushings.

6) Insert the snap bushings. Push the steel rod through them to make sure they are lined up correctly. Make sure the bushings are flush with the inside walls and apply some epoxy/microballoon slurry to the outside, which helps to glue the bushings in place and hold them square.

7) Fit a piece of cardboard into the tunnel so that it is a snug fit. Use the cardboard as a template to cut the "valve" out of the plastic sheet.

8) Grind a flat spot into the middle of the steel rod to fit the plastic sheet. The ends of the rod should be kept round where they will rest inside the bushings. Only the middle section should be ground flat on both sides (one side for the sheet and the other side to make a flat surface for the screw heads). Place the rod into the anchor and mount the "valve" onto it with the #6 screws.

9) Cut the steel angle to make two flat bars. Put one in the scrap metal box and trim the other into an arm. Drill a 3/8" hole at one end.

10) Assemble everything onto the anchor with the arm loosely on the steel rod.

11) Place the assembly on the car and mark where a slot needs to be cut in the top of the car.

12) With the assembly on the car, place a felt mark on the rod and hinge and hold them together by hand while rotating the valve, to ensure the slot in the car is long enough. Determine how the arm needs to be mounted to the rod such that it will travel from 45 degrees forward to 45 degrees backward from vertical (i.e. when the arm is straight down, the valve should be halfway open). Mark the desired position of the arm on the rod with a felt marker.

13) Tack weld the arm on the rod.

14) Take everything apart, weld the arm fully on the rod and then paint it with some rust paint.

15) Put it all together. The arm can be moved with a cable pushrod control (e.g., or a servo motor.

The whole process took about 10 hours, including head scratching. Someone who knows what they're doing could probably do it a lot faster.


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GT40s Supporter
Nice job! That is one of my next projects. I'm planning on using the servo from the electric heater valve that came with the Vintage Air unit.

Do you have any ideas on how to seal the connection from the anchor to the dash? I've been scratching my head a bit on that.

I haven't gotten that far yet, but was planning to use some thin weatherstripping, with tapped and locktited mounting screws just outside the weatherstripping.

. . . I just reread your post. I suspect you mean the connection between the anchor and the instrument panel? I haven't cut that connection out yet, but was hoping that some weatherstripping would work there, too.


GT40s Supporter
Well, the big piece of fiberglass that goes on top of the anchor - I guess that would be the instrument panel. Rough fitting it in, mine rests on the defrost duct part of the anchor in the middle, but there's a pretty big gap on the sides. This is prior to cutting holes in either piece.


GT40s Supporter
I finally got around to fabbing up my defroster control.

I mounted the actuator from the electric heater valve on top of the anchor:

Made a "door" with aluminum, the post from the heater valve, and a some JB weld:



Hi Pete,

Does your instrument panel/glareshield sit down far enough with the actuator mounted on top of the anchor or does it contact the actuator?


GT40s Supporter
It's under the center pod, so there's plenty of room between the instrument panel and the actuator.