How to test defrost rear windshield heat wires

[jgurnari]

How can i test each defrost wire is working in a rear windshield glass prior to installation?
1981 windshield

[wazzz]

I would power it and spray some water or steam on it. That way you can visibly see that each element is heating up.
You cannot isolate elements as they are all wired in parallel.

[jgurnari]

power it how? not much of a genius with electric

[WMichelsen]

Interesting timing. I'm contemplating the same. My rear Targa glass is out while my car is in repaint. I had a broken tab that I repaired, and am pretty sure 1 of the elements has a break, that I am repairing.

My plan is to leave it out over night. That's when the air temp drops and the humidity will condense or frost on it (depending how cold). But, that requires humid air and cold night temps, (likely in various locations this time of year). Then, hook up to my 12V battery and see which lines are working. There are 3 sections with multiple elements each. Do each section separately.

My question: Is direct 12V from the battery OK, or does it go through a line resistor somewhere in the circuit? Should I fuse it, just in case?

Wayne

[jgurnari]

yes, wondering the same thing… how do i hook up the 12v? just touch it to the tab? use wires, pos and neg?.. and where?

[wazzz]

Yes, just power it any way, works in both directions, no bother with plus and minus. 12 V battery directly connected to terminals, no need for a resistor or whatsoever. Just be careful not touching both battery wires with one another or you'll get sparks, but you know that. No risk of electric shock like in bad movies, however, as it is only 12 volts.

[dannobee]

Forget all that nonsense.

Get a 12v test light. Hook the alligator clip up to a ground (seat frame, door striker, etc.). Then start the car and turn on the defroster. With the pointy end of the test light, touch each wire terminal on the grid. One side should light the test light brightly. The other side should have no light. If your test light is off at both sides, there's a problem with the circuit and you'll need to get out the wiring diagram. If the test light is bright at both terminals, the ground is bad. Now carefully and lightly touch the test light probe on each of the lines on the defroster grid. Each line should go from bright to less bright to dim to off as you sweep across each wire, going from one side to the other. If you come across a line that goes from bright to off, there's a break in that line at that point. The breaks can be repaired with a kit available at the auto parts store.

[john walker's workshop]

Just breathe on the lines to create fog and see if it dissipates along each line.

[WMichelsen]

Originally Posted by dannobee

Forget all that nonsense.
Get a 12v test light. ...

I think your point is that the light will dim because of cumulative resistance the longer the path from the probe to the tab. I've tried with an ohm meter, and I can't detect any significant difference along the element wire, wherever I put the probe. Maybe my meter is not sensitive enough. So, I question if it's enough to dim a light bulb.
(ie. Left-Tab to A is lower resistance than Left-Tab to B.). While that's true, my crappy meter can't distinguish the difference.

[jgurnari]

I need to know how to rest with the glass out of the car

[john walker's workshop]

Battery or jump box. 12V on one side, ground on the other. Use alligator jumper wires if needed. Warm up a minute or so, then get close and breathe on the glass with your mouth and fog up an area over the heat strips. If the fog clears over the line, that line works. Test them all. I go for the center of the glass usually.

[dannobee]

The 12v test light works great. Use alligator clips and a 12v battery if it's out of the car. Don't make this harder than it needs to be.

https://www.youtube.com/watch?v=R0ZKoK_x6XM

[wazzz]

Originally Posted by dannobee

Forget all that nonsense.

Thanks!
Just read what the OP says. Glass is out of car and he wants to test heating elements.

[WMichelsen]

Had a chance to do tests today. The steam iron was a good idea. I thought about the test-light (don't have one) and figured I should be able to do similarly with a volt-meter.
Next up, buy defogger element repair stuff (Permatex?). For now, I hope this helps others:

My setup using my other car (as my 911 is at repaint)


Using steam iron to mist steam onto the window (don't touch iron to window, just mist with it). You can see which elements are working, or not.


Using a volt meter, you can track down the break. In my case, there was a big difference in voltage right near the edge. I think this first one was when a shop R&R'd my Targa glass during a repair (it looks like a tool scrape).



This second break looks maybe like corrosion. Both of these were along or under the glass seal. I don't think I would have found them if the glass was installed.

[jgurnari]

wow great help!

wish you could fix my mirrors! i get panning only, no tilt...not matter with direction i toggle the controller