Rear Defroster Woes
11-23-2014, 12:33 PM
#1
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Rear Defroster Woes
I suspect this may be the only ever post on a 997 board about a rear window defroster on a sports car board, but here goes!
My rear window defogger is FUBAR'ed, but in an odd way I've not seen before. There are 18 (maybe 19) "lines" on a 997's rear grid, and half of them (9) don't work. It turns out that the prior owner of my car had at one time put up a tint on the window, which was later removed. This caused some kind of damage -but to what I'm not sure. I have looked at the grid (with a flashlight on the other side) and have not found any breaks. I have also used a multimeter to check for continuity, and again (while this is hard to do exactly), haven't found any breaks on the non-functioning lines.
However, I noticed that the "tab" (electrical connection to the car) on one side looks like it is somehow disconnected from the car (but this is only based on a visual).
My understanding on a window defogger is that one tab was power, and the other was ground - but I'm no electrician.
Given that half of my lines are out, is it possible that this is related to the fact that one of the tabs appears to be disconnected? Or is that just not how the grid works? Any input appreciated!
My rear window defogger is FUBAR'ed, but in an odd way I've not seen before. There are 18 (maybe 19) "lines" on a 997's rear grid, and half of them (9) don't work. It turns out that the prior owner of my car had at one time put up a tint on the window, which was later removed. This caused some kind of damage -but to what I'm not sure. I have looked at the grid (with a flashlight on the other side) and have not found any breaks. I have also used a multimeter to check for continuity, and again (while this is hard to do exactly), haven't found any breaks on the non-functioning lines.
However, I noticed that the "tab" (electrical connection to the car) on one side looks like it is somehow disconnected from the car (but this is only based on a visual).
My understanding on a window defogger is that one tab was power, and the other was ground - but I'm no electrician.
Given that half of my lines are out, is it possible that this is related to the fact that one of the tabs appears to be disconnected? Or is that just not how the grid works? Any input appreciated!
11-24-2014, 09:08 AM
#2
A rear window defogger is like a bunch of resistors in parallel across the window glass. Current flows through them heating them up which defogs the window. One side effectively connects to power, the other ground. If the circuit is incomplete due to breaks in the horizontal lines on the glass, breaks in the vertical bus on the glass, or a poor connector, no current flows and you have a failure.
You say about half of yours are not working. Are the failing lines random horizontal ones? All at the bottom or at the top? If random then each individual horizontal line has a break in it somewhere. All at the bottom or top means a vertical bus on the glass is bad. Horizontal lines the fail intermittently or change from time to time is often a connector problem. A vertical bus failure or a single horizontal line or two failure can be repaired. Same thing with a connector issues, usually. But multiple horizontal random failures is not worth repairing. Replace the rear windshield. Consider used if you are looking to same some money.
By the way, when checking for breaks with a multimeter it's easier to check voltage with the defroster turned on than to measure resistance with it off. Each line is in parallel. Resistance measurements measure the whole grid because each line is connected. Instead turn the defroster on then measure volts at each connector (should be 12V). Then move the positive lead in across each line until you see a sharp decrease in voltage. The break is right at that point. Here's a pretty good site for reference: http://www.frostfighter.com/defroste...x-it-guide.htm
.
You say about half of yours are not working. Are the failing lines random horizontal ones? All at the bottom or at the top? If random then each individual horizontal line has a break in it somewhere. All at the bottom or top means a vertical bus on the glass is bad. Horizontal lines the fail intermittently or change from time to time is often a connector problem. A vertical bus failure or a single horizontal line or two failure can be repaired. Same thing with a connector issues, usually. But multiple horizontal random failures is not worth repairing. Replace the rear windshield. Consider used if you are looking to same some money.
By the way, when checking for breaks with a multimeter it's easier to check voltage with the defroster turned on than to measure resistance with it off. Each line is in parallel. Resistance measurements measure the whole grid because each line is connected. Instead turn the defroster on then measure volts at each connector (should be 12V). Then move the positive lead in across each line until you see a sharp decrease in voltage. The break is right at that point. Here's a pretty good site for reference: http://www.frostfighter.com/defroste...x-it-guide.htm
.
Last edited by semicycler; 11-24-2014 at 09:37 AM.
11-24-2014, 06:22 PM
#3
Poseur
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It sounds like the PO screwed you over. The dark film that is added to windows conducts electricity and the normal solution if you must add such a film is to pull the fuse on the rear window defroster when you add it so that it won't be a problem. Unfortunately, the decision to tint has long reaching implications (that you are discovering) and it should not be done without giving it considerable thought.
As you attempt to restore the capability you will have to do a series of continuity checks on everything. Every line, and everything leading to the window. I recall that the demand is so massive for power that it may be broken into two separate circuits--they come on together, but they are wired separately. Keep that in mind as you work the problem. You can sort of repair these breaks but it's not that easy. I would highly recommend a professional who does this sort of thing routinely. There are people who can restore it.
As you attempt to restore the capability you will have to do a series of continuity checks on everything. Every line, and everything leading to the window. I recall that the demand is so massive for power that it may be broken into two separate circuits--they come on together, but they are wired separately. Keep that in mind as you work the problem. You can sort of repair these breaks but it's not that easy. I would highly recommend a professional who does this sort of thing routinely. There are people who can restore it.
11-24-2014, 08:38 PM
#4
Just a thought. Since it is every other horizontal that is not working you may have a blown fuse. I don't have a wiring diagram for the 997 (yet), but as was mentioned there is a lot of power used in this circuit so sometimes it is split into two. That could mean two fuses. The symmetry of your problem just seems to invite this analysis. One good fuse, one bad fuse, ...
Is window tinting really conductive? If so, I don't want it anywhere near my car!
Is window tinting really conductive? If so, I don't want it anywhere near my car!
11-25-2014, 10:07 AM
#5
OP says about half don't work. No mention of every other line, or any type of line grouping of the failures. Likewise there is only one wire on each side of the glass on the 997.1, so its a single circuit. Not sure on a 997.2 though.
11-26-2014, 12:42 PM
#6
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Just a thought. Since it is every other horizontal that is not working you may have a blown fuse. I don't have a wiring diagram for the 997 (yet), but as was mentioned there is a lot of power used in this circuit so sometimes it is split into two. That could mean two fuses. The symmetry of your problem just seems to invite this analysis. One good fuse, one bad fuse, ...
Is window tinting really conductive? If so, I don't want it anywhere near my car!
Is window tinting really conductive? If so, I don't want it anywhere near my car!
More often, the adhesive removes part of the defroster "paint" ,if you will, when tint is removed. This assumes the defroster is not molded into the window. There are repair kits with stencils that allow one to essentially reconnect broken lines with this type of conductive paint.
Finally, continuity checks are not conclusive when it come to these types of systems. You may get continuity, but there's not enough material or strands to conduct the amount of current (amps) required to properly heat the window. A battery cable and the smallest of wires will both read the same on a continuity check, but only one will start your car. The other will melt.
11-26-2014, 01:47 PM
#7
Good points.Thanks.
I didn't realize there was metal in the window film. I park my car outside at the office. It gets some sun. I was thinking I'd tint the rear before next summer. That sounds like a bad idea now.
2 AWG wire reads the same resistance as wire wrap ... I deal with people measuring motors in the fields and mostly their measurements are meaningless.
I used to use copper conductive paint 50 years ago to build custom HO racetracks. It was crude back then, but that could be said for a lot of things! Nice to know they have it for auto repair.
I didn't realize there was metal in the window film. I park my car outside at the office. It gets some sun. I was thinking I'd tint the rear before next summer. That sounds like a bad idea now.
2 AWG wire reads the same resistance as wire wrap ... I deal with people measuring motors in the fields and mostly their measurements are meaningless.
I used to use copper conductive paint 50 years ago to build custom HO racetracks. It was crude back then, but that could be said for a lot of things! Nice to know they have it for auto repair.