How do you test defrost grid and switch when removed from truck?

[Littlebeefy]

I have a rear window for my 84 Bronco with the defrost grid and a defrost switch that I removed from other trucks (separately). Both look to be in good condition but I want to confirm. I see how to run the test with a multimeter on the grid when it’s installed in the truck, but I would like to test both of them BEFORE I install them (crazy, right?).

For this test I have the items below including the switch, a pigtail plug from the wiring harness, the window (obviously) and a 12v 5amp battery. See picture:

I am referring to the EVTM for 86 which matches the wiring color scheme from the pigtail I’m using:

In my mind this should be as easy as hooking 2 wires from the pigtail to the window (1 on each terminal) and 2 wires to the battery (1 positive, 1 ground). Based on the wiring diagram in the EVTM I’m not sure how one would do that. Can anyone explain how to set up the circuit with the switch attached in order to test it? Also, there is another connector that comes out of the switch that isn’t shown on this page of the EVTM. What is that for? Does that need power for the test to work?

Thanks in advance,

Chad

[Littlebeefy]

Here’s my guess:

Yellow wire and White/Purple wire goes to positive end of battery.

Blue/Brown wire goes to one of the defrost terminals.

Black wire goes to negative terminal of battery.

Other defrost terminal also connects to negative battery terminal (via independent wire).

Am I right?

[Gary Lewis]

Chad - I'm not familiar with this circuit nor the components, so hope someone that is will chime in.

However, the thing on the far left of your pic looks like the speed control amplifier. Do you know that it is something to do with the defroster?

From looking at the diagram it looks to me like the switch houses the timing circuit. So I think you provide power to both the yellow and w/p wires, ground the black wire, run a jumper from the br/lb wire to one connector on the window and ground the other connector on the window. Then press the On button on the switch and it should work.

HOWEVER, that battery is a 5 amp-hour battery. And Fuse 6 is a 15 amp fuse, so the window may draw something like 10 amps. Given that, in theory your battery should last 1/2 hour. But I doubt it will, so you may only have one shot at this before having to recharge the battery.

[Gary Lewis]

Here’s my guess:

Yellow wire and White/Purple wire goes to positive end of battery.

Blue/Brown wire goes to one of the defrost terminals.

Black wire goes to negative terminal of battery.

Other defrost terminal also connects to negative battery terminal (via independent wire).

Am I right?

Yes

[old55pete]

Gary is right in the wiring as per the diagram. He is also correct in that there is a timer in the switch that will only let the window defrost for five minutes then shut off. It is my understanding that if it goes much longer, the window will shatter.

To test the grid on the window, set a multimeter on ohms and hook it to the two spade connectors and it should show 0 to 25 ohms. That means that the grid is good in the window.

[Littlebeefy]

Chad - I'm not familiar with this circuit nor the components, so hope someone that is will chime in.

However, the thing on the far left of your pic looks like the speed control amplifier. Do you know that it is something to do with the defroster?

From looking at the diagram it looks to me like the switch houses the timing circuit. So I think you provide power to both the yellow and w/p wires, ground the black wire, run a jumper from the br/lb wire to one connector on the window and ground the other connector on the window. Then press the On button on the switch and it should work.

HOWEVER, that battery is a 5 amp-hour battery. And Fuse 6 is a 15 amp fuse, so the window may draw something like 10 amps. Given that, in theory your battery should last 1/2 hour. But I doubt it will, so you may only have one shot at this before having to recharge the battery.

Hi Gary,

Thanks for the info. I hooked everything up like below:

The first test I ran was output from the switch (the brown/blue wire). When I turn on the switch, I get a solid 12v. So that seems good. However, when the switch is off, it is still outputting .5v. That’s half a volt. Does that mean the switch is bad?

Thanks

Chad

BTW, you are right about that other part hanging off the switch. I think it is part of the cruise system with a horn relay attached. I removed it.

Also, that other connector that sticks out of the defrost switch is to power the little light on the switch.

[Littlebeefy]

Hi Gary,

Thanks for the info. I hooked everything up like below:

The first test I ran was output from the switch (the brown/blue wire). When I turn on the switch, I get a solid 12v. So that seems good. However, when the switch is off, it is still outputting .5v. That’s half a volt. Does that mean the switch is bad?

Thanks

Chad

BTW, you are right about that other part hanging off the switch. I think it is part of the cruise system with a horn relay attached. I removed it.

Also, that other connector that sticks out of the defrost switch is to power the little light on the switch.

Here’s what I found on the grid:

First test I checked every line on the grid and found they were all at 12v at the window midpoint. That meant to me that either every line had a break (unlikely given the condition of the glass) or there is a problem at the ground. I found a small break in the bar that runs from the ground clip to the other side of the grid. I soldered that break:

Once I did that, I ran the test again and found that every line of the grid dropped to around 7-8 volts at the midpoint. Great except for the one that was showing 12v at the midpoint. If I kept checking down the length of that line I eventually hit a point where the voltage was zero. Bingo! It was basically impossible to tell visually that there was a break in the line. Once I knew exactly where it started, I could see a very slight color difference in about .5cm of that line. I thought it would be much more obvious. I got the Permatex repair kit and followed the instructions for fixing the grid. It’s definitely a different color, but if it works who cares. Likely the copper will oxidize and darken over time. I might hit it with a brown Sharpie to make it less noticeable:

I need to give it 24 hours to dry and then I’ll post back to let you know how effective the Permatex product is.

Anybody know why the defrost switch puts out half a volt when it is not turned on? Is it supposed to do that?

[Gary Lewis]

Here’s what I found on the grid:

First test I checked every line on the grid and found they were all at 12v at the window midpoint. That meant to me that either every line had a break (unlikely given the condition of the glass) or there is a problem at the ground. I found a small break in the bar that runs from the ground clip to the other side of the grid. I soldered that break:

Once I did that, I ran the test again and found that every line of the grid dropped to around 7-8 volts at the midpoint. Great except for the one that was showing 12v at the midpoint. If I kept checking down the length of that line I eventually hit a point where the voltage was zero. Bingo! It was basically impossible to tell visually that there was a break in the line. Once I knew exactly where it started, I could see a very slight color difference in about .5cm of that line. I thought it would be much more obvious. I got the Permatex repair kit and followed the instructions for fixing the grid. It’s definitely a different color, but if it works who cares. Likely the copper will oxidize and darken over time. I might hit it with a brown Sharpie to make it less noticeable:

I need to give it 24 hours to dry and then I’ll post back to let you know how effective the Permatex product is.

Anybody know why the defrost switch puts out half a volt when it is not turned on? Is it supposed to do that?

Cool! Or maybe HOT! But well done, nonetheless.

[Littlebeefy]

Here’s what I found on the grid:

First test I checked every line on the grid and found they were all at 12v at the window midpoint. That meant to me that either every line had a break (unlikely given the condition of the glass) or there is a problem at the ground. I found a small break in the bar that runs from the ground clip to the other side of the grid. I soldered that break:

Once I did that, I ran the test again and found that every line of the grid dropped to around 7-8 volts at the midpoint. Great except for the one that was showing 12v at the midpoint. If I kept checking down the length of that line I eventually hit a point where the voltage was zero. Bingo! It was basically impossible to tell visually that there was a break in the line. Once I knew exactly where it started, I could see a very slight color difference in about .5cm of that line. I thought it would be much more obvious. I got the Permatex repair kit and followed the instructions for fixing the grid. It’s definitely a different color, but if it works who cares. Likely the copper will oxidize and darken over time. I might hit it with a brown Sharpie to make it less noticeable:

I need to give it 24 hours to dry and then I’ll post back to let you know how effective the Permatex product is.

Anybody know why the defrost switch puts out half a volt when it is not turned on? Is it supposed to do that?

The verdict is in. The Permatex grid repair product definitely works.

Some tips for application:

Mix well before applying

Use blue painters tape, not the stencil they provide

Remove and reapply the tape between each coat (3 coats recommended)