Rear Defog Problem

[Konos44]

Hey All,

I have a problem with my rear defog in my '92 MTX. It simply does not work. The orange light goes on when you push the button, but the window never heats up.

I've checked the leads with a voltmeter. With the ignition on and the defog on I still get 12V across the two terminals (from the passenger side terminal to the driver's side terminal).

Is there an easy way to trace the break?

 

[projectSHO89]

Use your voltmeter. The traces are conductive.

If none of the traces are heating up, look for an open near where the connection terminal is at.

Otherwise, check each trace.

Steve

 

[Konos44]

Well, its weird because I get 0 Ohms when I measure from post to post.

I also measured across all the "wires". I pulled the plastic rear covers over the small windows behind the rear doors(on the inside). They're not actually individual wires. Its more like some kind of tape thats just one piece that runs up and down each side of the window. No matter where I measure I get 0 Ohms.

I also get 12 volts when I measure from the right post to the left post.

Am I even doing this right? Or could something else be causing my problem?

 

[luigisho]

That's possible if the break is in the conductive material on the window. Do you see any physical breaks in the continuity on the window? I'm not sure if you can get partial defrost if the material is disrupted. I would think it's possible. One of the metal tabs where the wire plugs in is broken on my rear window so I haven't been able to use mine at all.

 

[Konos44]

Well, that's the really weird part. Its not like I have partial connectivity. It doesn't defog whatsoever. Even though there's no resistance across that tape at all. And it measures 12V! I just don't understand electrical stuff enough to test it properly. I could do it if someone could tell me exactly how and where to put the probes.

 

[Rockledge]

Well, since I endeavored to repair my SHO's rear defrost grid today, I figured I'd post a little summary of how the troubleshooting and repair went. This thread seems as good as any. wink

Now that winter is here full blast in New England I've been using the rear degogger much more, and several of the grid lines I noticed were inoperative. So this morning using my multimeter, I tested all of the grid lines and was able to track down breaks in 4 of the lines (a total of 6 breaks in all). Several of the breaks were so tiny that I couldn't find them even after very close examination by eye. I actually repaired 4 of the breaks first, and then by further testing, found 2 more breaks.

I used a Permatex Window Defogger Repair Kit I picked up from AutoZone (about $9) to repair the breaks, and now the whole grid checks out fine! I know it works by the voltage #s I am getting, and by tommorrow (Sunday) it'll have it's first test in real life since more snow and ice is being forecast.

Once again, my digital multimeter shows how much of a friend it can be! For those of you who are interested, here is the diagnostic procedure I used:

Heated Back Window Grid Wire Test

1. Using a strong light inside vehicle, visually inspect wire grid from outside. A broken grid wire will appear as a brown spot.

2. Run engine at idle. Set heated back window switch and light to ON. The indicator light should come on.

3. Working inside vehicle with a 12-volt DC voltmeter, contact broad red-brown strips on sides of back window glass. The meter should read 10-13 volts. A lower voltage reading indicates a loose ground wire (pigtail) connection at grounded side of back window glass.

4. Contact a good ground point with negative lead of meter. The voltage reading should not change.

5. With negative lead of the meter grounded, touch each grid line of heated back window glass at its midpoint with positive lead. A reading of approximately 6 volts indicates that the line is good. A reading of zero volt indicates that the line is broken between midpoint and the positive side of the grid line. A reading of 12 volts indicates that the circuit is broken between midpoint of grid line and ground.

<small>[ December 13, 2003, 10:12 PM: Message edited by: Rockledge ]</small>

 

[luigisho]

Wow, this is an old thread. I have since fixed the broken tab on the rear window. To add to the info in the previous post:

I bought a kit from a company called frost fighter. It's more expensive than the old Permatex kit but I don't even see those in the stores anymore and the FF kit has a higher conductive compound. The repair works like a charm. If you lose the oem metal tab that comes as part of the rear glass, the kit comes with several sizes to match your application. I found the original tab from the window and reused it. Everyone else in the auto glass industry I talked to wanted to sell me all new glass. bs

The autoparts store repair kit for broken grid lines should be adequate for that job.