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Noticed the last few times the snow/ice wasn't melting off the rear glass despite the rear defrost button lighting up. Took out the volt meter and opened the hatch, then checked the voltage directly where the plug connects at the glass (the actual spade tab that's slightly exposed) and grounded other probe at chassis. Was getting around 14v. I'm stumped. Only thing I can think of is a break in the actual wire embedded into the glass, but I've NEVER heard of that happening before. Glass has NO chips or external damage that I can tell. Ideas?

Edited by Bushwick
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it isnt wire embedded in the glass, it is a conductive material "painted' on the inside of the glass and it does lose its ability to heat up over time. mine is getting pretty weak...

 

frostfighter.com has kits for fixing conductive defrosters among other things. We used one of their kits to fix a broken tab on a 2004 Mercury Sable - worked great! I have a kit for fixing the grid on my rear window, but it needs to be relatively warm out, or have in indoor place to work & let it cure...

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Ah that's right. Completely forgot about it being an issue with removing window tint (razor = bad) as it's actually "on" the glass. Dunno why I was thinking copper strips sandwiched in with the safety material  :wacko:

 

I'll have to look more closely then if something flaked off. I can get creative enough to possibly come up with DIY solution to that scenario. By the way, does the coating ground itself out or is it just a loop? I don't remember there being any other connecting points other than the 12v+ tab. 

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The individual grid lines are arranged electrically in parallel.

That means that each grid line has 12V across it.

One line can break, and the others can still work.

 

If nothing works, then the break is likely in the feed to all of the grid lines.

Check for voltage in the wider vertical feed lines on the left and right.

If you have an aftermarket film on the glass, you may still be able to check for voltage by piercing through the film with the sharp end of an electrical probe.

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I only checked voltage right where the spade tab connects at the window itself. The probe was touching the metal tab. So it's at least getting voltage to there. This car has no window tint. One of my previous cars did, and was just saying I completely forgot about that being an issue i.e. conducting material on glass.

 

Didn't have time to inspect today, but will look later tonight or Sunday.

 

@Fairtax  How would you check voltage at both sides? Just touch probe directly to material on glass at the sides of the rows? Are these rows grounded somewhere from the glass? Trying to picture how it completes the circuit..... aren't the rows heated like the heater wire in the seats? Meaning 12v+ passes through the rows and at some point the end of the circuit grounds out?

Edited by Bushwick
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Yes, there is a tab on each side. One side the supply wire hooks to the tab.

The other side a ground wire hooks to it. Touch the probe to the wide areas at the ends of the rows for basic voltage check. Be sure to ground the meter to body ground, not the hatch.

 

Typically the ground wire for the grid is short (maybe 8-10" length) and will have a screw securing it directly to the body. On the hatch there may be a longer ground wire or there should be another ground wire going from the hatch to the body.

 

Its also possible the tab could have broken off of the window. I've had that happen on two of these cars. Both sedans, which makes them a royal pain to fix.

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It's funny I never noticed the driver-side tab before. During the hatch replacement, only the hot side was removed as that part of the harness needed to be fed back through the donor.

 

 

Seems like something is goofy with the driver-side tab (ground). If I probe the passenger-side (12v+) and body ground, it shows around 13v. If I touch the neg - probe to the ground tab and pos. + probe to the pos. tab, it'll show 14v. If I probe a body ground AND the neg. tab, it won't zero out on the display, which is odd and suggests it's bad, though I dunno why it show voltage across the 2 tabs still.

 

Can I just ground out the neg - tab directly? I followed the wire as best as possible but don't think it grounds to the hatch. Think it grounds out up under the headliner near the rear hatch dome light. Thinking of just pulling back some of the insulation and adding my own (new) ground and be done with it. Both tabs are tight and holding to the window.

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In a properly workig circuit there should be 0 voltage on the ground side, usually you'll see less than 0.1v, but not totally 0.

Voltage is electrical pressure. That pressure will be present in the circuit up to the point of the load or resistance.

As voltage passes through the load (in this case the grid) it drops (decreases). When it gets to the other side of the load, there is no more resistance and the voltage runs out to the ground freely, so you'll see close to 0 volts on the meter.

 

Defog grids are nice because you can watch the voltage drop across the grid by checking in several places. The principle is the same for a light bulb, but you can't check voltage in the middle of a light bulb filament.

 

If you probe the supply side of the grid you should see supply /system voltage or close to it.

On the ground side of the grid you should see close to 0 volts.

If you have high voltage on the ground side, it's because there is resistance in the ground circuit and that voltage is not able to flow out to ground. The electrical pressure is still being held in the circuit.

 

If you have 0 volts (or close to it) on the ground side, either the voltage is not making it there, or the ground is good and that voltage is flowing out of the circuit the way it's supposed to.

The way you can check that is by wrapping a strip of foil around the probe, then you can slide the foil across the grid lines on the window.

Normally you should see a steady drop in voltage as you slide across. If the voltage stays steady , then suddenly drops to 0 after a certain point, there is a break in that grid line where the voltage dropped out.

Do this check on each grid line to determine if any or possibly all of them are broken.

 

If you determine the ground is bad, you can just run a new ground wire to an easy spot on the body. Do not ground directly to the hatch, unless you then also run a wire from the hatch to the main body.

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I'll go out and mess with it. When I was probing the ground tab and the chassis, it never went zero like when you directly short the probes together. I'll try and ground the tab directly then and see if it melts anything or not, then go from there. Thank you.

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After adding a direct, secondary ground, the unit actually started to work. The bad thing was I noticed 2 consecutive rows at the middle of the window, dead-center of course, with broken lines (actually looked like they blew out like a fusible link) and thought, "well, all the others look OK" and the entire bottom of the window where the wiper passes through was actually pretty warm to the touch. Waited a while for some more snow to build up with it on, and discovered only the very top line and the very bottom of the window itself plus a second from the bottom row was all that was working. Not sure what went wrong here as nothing ever touches the window, and seems odd to have all those rows seemingly go out together. Starting to think the unit wasn't working at all after installing though I could swear it was.

 

Oh well, at least the base of the window gets hot and between that and the TWO remaining rows that work will be enough to at least keep heavy ice from forming until the cabin temps are up. Thankfully these things have rear wipers. Thanks again for the advice everyone.

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if you replaced the rear hatch, there is no telling what happened to the grid. Glad you got it at least partially working.

 

Mine is the original hatch & the grid all works, but is pretty weak - it takes a while to heat up enough to do anything. When looking at it, I can see areas where the grid lines are thinner - years of cleaning the inside of the glass, wearing it down, maybe? I dunno... hoping to try out the grid repair kit when the waether warms up again so it will hopefully work better next winter.

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I noticed some of mine appeared thinner near the center too. Hard to tell if it's deliberate or not to focus more heat towards the center. Might want to try and add a supplemental ground like I did and see if it heats better or not as that at least got it working again. I'm thinking the ground broke out internally in the casing, which would be possible if the original wire was inadvertently an incorrect gauge. It didn't feel overly pliable like a normal wire should. The very bottom of my window, which is coated with a wide swathe of that reactive material, gets very warm to the touch. I imagine yours should do the same if it's working correctly. Might need to remove panel to touch that area of the glass to verify. If that section isn't warm to the touch after it's been on for 15 minutes or so, add an extra ground (even temporary) and see if it gets hotter. If it does, you just fixed it ;)

 

Wish I'd even thought about this being an issue as the rotted out original hatch had a perfect defroster. For $50 or so, it would have been worth trying to have the glass swapped.

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adding a second ground wire might be a worthwhile experiment, but even that is gonna have to wait for slightly warmer weather right now - supposed to be quite cold all this week - single digits to very low teens during the day, below zero at night - and I dont have an indoor spot to mess with anything...it sucks when you have a garage, but cant get a car in it! LOL

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I was lucky it was in the mid 30's yesterday. Hovering around 10 today and had dog out briefly. Have around 8-10" of snow and top 1" is frozen, so he was hopping allover it and seemed to like breaking the surface snow up LOL. But yeah, way too cold when it's under 25 degrees.

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I've always been disappointed with the rear defoggers on my Legacys. They just don't seem to get very warm. Mine are sedans and a royal pain to get to the tabs, (have to pull out the rear seat and pull the side trim off) so I haven't messed with any wiring upgrades, especially since the only time I get mad enough to want to mess with them is when its cold out!

I've also had the tabs repeatedly break off in both my GFs and my 95s. Last time mine did it I soldered the tab to the braided wire strip that's on the glass, and that seems to be holding, but I'm kind of afraid to mess with it anymore.

 

If you check voltage on all of the grid lines near the middle of the window that will tell you which ones work and which don't. In the middle of the window you should get somewhere around 6-7v. If any are still at 12v, or at 0v, there is a break in that grid line.

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The previous glass with the rotted hatch heated up really nice. Usually anything on there from sitting overnight would be rolling off in 5-8 minutes.

 

An easy visual to see if they are heating is when light amount of snow is on the glass. It'll be apparent by what's melting.

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a heavy frost shows it, too - which i had this morning... and low single digit temps

 

went out and started the car, adjusted the heat settings and turned on rear defroster - went back in the house, fixed a coffee to go, put the dog in her crate, turned on the radio for her, etc - started out the door - remembered I needed something, back in, grab that last item, and out the door finally - probably 10 mins... rear window was just barely showing signs of defrosting <_< to be fair, the windshield was barely starting to defrost, too... sidewindows? yeah, right...LOL

 

have I ever mentioned how much I dislike winter? -_-

Edited by heartless
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