Hi all -

This is my first post. I've read so many threads and consumed so much information just in general - it's been great, so thank you in advance.

I've followed some threads on the rear defrost switch, but think I need to get a little more technical. 1) for general knowledge and 2) to hopefully fix my problem.

My 76 defrost switch (from what I've read - only made 1 year, 2 switch toggle ON/OFF) - with Red Light. The 76 has NO blower in back, but is the electro-grid in the window.

The window is fed by 2 wires. A black ground on passenger side. A purple power on drivers side, which feeds into a black wire connected to the rear window.

I'm at a loss for how the switch works. I've read that it's a "momentary" switch, but have failed to understand.

So, let me tell you what I've done. I have a Brown wire at the Fuse Box that has power in IGN - it runs to the black connector in the picture.

The connector when the car is in Start/Run has power on the Brown & a good ground Black. However, the Purple wire is what feeds the rear window.

I think the switch is good, because if I bench test and ground + 12 v to purple the light turns on.

However, the toggle ON/OFF switch only goes between the GROUND and the BROWN wire. When toggled OFF it's in the GROUND position (red dot in picture), but when toggled ON it moves to the BROWN wire (green dot in picture).

I've failed to see how when toggled ON the power moves to the PURPLE wire to feed the window.

Any info and help would be apprecaited!

I don't know if my switch is missing a part or I just understand.

 

Welcome to the forum - an invaluable tool is a wiring diagram for your car. There are several types to choose from (I have an 11 x 17" color "plasticized") as well as several forum members usually offer various year free downloads.

 

Switch is on/off , Brown is 12v in , Black is ground for the light in switch , purple goes to the filaments . Should have 12v on purple wire with switch on . Sounds like your switch is bad .

 

Thanks Redboat.

I agree with your statement. Do you know what the mechanism for Purple to get power is?

When the switch is ON. The toggle switch moves the nub from the OFF black ground (red dot in pic) to the brown power in (green dot in pic).

So, with that said. How does Purple get power.

I don’t understand what gives it juice since you can see it’s not touching any other metal piece in the inner switch workings.

 

Nub shouldn't go to black . The indicator light goes to black and purple .In the off position , the nub should only be at the brown wire . In the on position , it should go between brown and purple . If the nub goes between the brown and black in the off position , the 10 amp fuse should blow !

 

So you know, the red light is lite when the defogger rocker-switch is activated...so another clue if that light is not lite when you toggle the switch.

 

Just connect the brown and purple wires together. Bypassing the switch.
Does the defroster grid start to warm up. Do you get power to both sides of the grid?
If the answer is no to any of the above your banging your head against a post.
Back in the day when these cars were fairly new. Those grids burnt out with great frequency.
They were extremely unreliable back in the 70's.
And the only way to repair them is to install a new rear window. And then in 2 years that one will burn out.

 

There are a couple 1st here...
1. That is not a Brown wire that is a Tan wire...
2. The only purpose for the Black (Chassis Ground) at the switch is for the light at the switch.
That is not a momentary switch that is a maintained switch.
If it was a momentary switch there would be some sort of timer relay in the circuit and I don't see that in the line diagram.
1978 was the first year for that I believe.
Don't worry about the light, I know on the 1977 Defogger switch there is a external resistor for the LED and they can fail.
A failed resistor has no effect on sending power to the grid.
78 switch, extra terminal for reference.

 

Wow! I was out on spring break with the kids and came back to all this awesome info. Appreciate everyone who contributed. You guys were spot on.

1) purple to TAN jumped gave power to the grid.
2) switch was wired wrong. Which is why I could never figure out why it toggled back and forth between black and tan and how purple got power.
3) rewired it and BOOM. Exactly as you guys said it would.

thanks so much!

 

Side note. Bmotojoe - where did the diagram come from? First time I’ve seen something so clearly explained. Would like to get one for the 76.

 

There is a Forum member that has diagrams for most C3 years.
You can ask if there is one available for your 1976.
Here is the link: https://www.corvetteforum.com/forums...or-anyone.html
Good Luck!

 

Awesome! Thanks so much. I’ll look into this.