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rear defroster stopped working


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saharatj98
Novice

Dec 31, 2007, 10:33 PM

Post #1 of 9 (1424 views)
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Rear defrost Issues!

Something that should be really easy has got me stumped...

This is on a 1985 Porsche 944. Power comes in on the driver's side of liftgate glass. 3" jumper connects to the lift strut. Likewise another jumper connects the glass side of the strut to the defrost. Ground is the same on the passenger side through the strut.

I have 12v at the terminal and through the jumper through the strut and across all but 2 defrost wires and to the ground side when ground is disconnected. WHen ground is connected I lose 12v and have nothing. I disconnect 12v jumper and have 12v again from the switch. I checked my ground side and it is fine. Defrost used to work and slowly has gone less and less effective this last year to totally not working at all. Thought about increasing resistance in the grid as old wiring can deteriorate, but it's been garaged since new and has low miles. I have a few older Volkswagens and defrost works fine in all.
2005 BMW M3
2003 Land Rover Discovery SE 7
1985 Porsche 944
1984 Volkswagen GTI
1983 Volkswagen Rabbit


(This post was edited by saharatj98 on Dec 31, 2007, 11:52 PM)


Tom Greenleaf
Ultimate Carjunky / Moderator


Jan 1, 2008, 4:45 AM

Post #2 of 9 (1419 views)
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Re: rear defroster stopped working [In reply to] Sign In

Note: The grid on ON the glass not in it. Just cleaning it can mess them up. Closing that on cargo or scraping a sticker off will kill them as well.

You can test these with a test light and find the breaks. May be all of them now. With a flashlight you can usually see where the break is and there is a repair nail polish type product that can work but hasn't been perfect in ones I've fixed.

I think you'll find the continuity has been broken,

T
_________________________________________
Long retired now


saharatj98
Novice

Jan 1, 2008, 1:13 PM

Post #3 of 9 (1416 views)
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nope..... got all but 2.... only 2 of the 10 have a break. All the rest have continuity through both sides. Don't haul anything back there to scratch and clean windows with the same amonia free window cleaner I use on all my cars.... THese two broken grids have been broken for years and defrost still worked.

I have voltage, I have ground. WHen I disconnect ground I get voltage through grid. When grounded, voltage drops to 0. When voltage disconnected I have 12v... It's like I have a dead short when plugged in but not enough to pop the fuse or anything. Got current there though, like grid has a dead short, but doesn't short it out enough to pop fuse, but then doesn't get hot either. A short causes heat which is kind of the principal it uses anyway... I think maybe for whatever reason my grid just will not carry current anymore?
2005 BMW M3
2003 Land Rover Discovery SE 7
1985 Porsche 944
1984 Volkswagen GTI
1983 Volkswagen Rabbit


(This post was edited by saharatj98 on Jan 1, 2008, 1:15 PM)


Tom Greenleaf
Ultimate Carjunky / Moderator


Jan 1, 2008, 1:39 PM

Post #4 of 9 (1413 views)
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Ok: I think I see what's going on and am confused like you are. Seems like amps available just doesn't cut it - like using a 9v little battery would stall out trying to run this.

You would think over time the grids would carry less juice thru them - not so much as to stop the show and as you said it doesn't seem to trip anything. I've run these direct from a batter with jumper clips and they work.

These things just must go thru a breaker and a timer that shuts them down or need to be re set to on if you re-start the vehicle. That's where the problem must be. Must be contact points like like used for ignition probably just don't transfer or contact well enough to transfer the current at full amps needed. You said this got increasingly slower. With points back when we used to file them, align them and things to make them work better. I still have a meter to show how well they were letting current thru which is just an ohm reading. For this there really shouldn't be any resistance. Kind of like if you put a test light between power side wire and the grid connector it would light but not enough power thru just the bulb to make the grids warm.

With that I say the breaker/relay works but doesn't let the amps thru it and that would explain it. Now the question is where is that thing? Can you hear it "tick" when you put it on? Otherwise I have no way to locate that for you,

T
_________________________________________
Long retired now


DanD
Veteran / Moderator


Jan 1, 2008, 3:46 PM

Post #5 of 9 (1410 views)
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If you’re loosing power on the left side when everything is connected, you’re going to have to work through the system looking where there’s a bad connection or device (relay?).
What I mean is, whatever is causing this is still partially connected or working. When the ground wire is disconnected; there’s enough connection at the problem that voltage can pass through. Once things are put back so that the window should work, the load on the system from the grid is causing whatever to disconnect (go open).
Find the relay and test whether there’s voltage on the black wire (feed from relay to window) at the relay, when the system is turned on and everything connected.
If there is, start chasing that black wire to the window, looking of a rotted /cut wire. If there’s no power on the black wire, test that there’s power from fuse #13, (30 amp) in the under hood electrical center is getting to the relay; if so the relay is likely at fault.
Remember that German made vehicles use brown as the color making ground wires. I guess they have to be different then the rest of the planet? LOL

Dan.

Canadian "EH"






saharatj98
Novice

Jan 3, 2008, 12:04 AM

Post #6 of 9 (1402 views)
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In Reply To
If you’re loosing power on the left side when everything is connected, you’re going to have to work through the system looking where there’s a bad connection or device (relay?).
What I mean is, whatever is causing this is still partially connected or working. When the ground wire is disconnected; there’s enough connection at the problem that voltage can pass through. Once things are put back so that the window should work, the load on the system from the grid is causing whatever to disconnect (go open).
Find the relay and test whether there’s voltage on the black wire (feed from relay to window) at the relay, when the system is turned on and everything connected.
If there is, start chasing that black wire to the window, looking of a rotted /cut wire. If there’s no power on the black wire, test that there’s power from fuse #13, (30 amp) in the under hood electrical center is getting to the relay; if so the relay is likely at fault.
Remember that German made vehicles use brown as the color making ground wires. I guess they have to be different then the rest of the planet? LOL

Dan.

2005 BMW M3
2003 Land Rover Discovery SE 7
1985 Porsche 944
1984 Volkswagen GTI
1983 Volkswagen Rabbit


DanD
Veteran / Moderator


Jan 3, 2008, 5:08 AM

Post #7 of 9 (1400 views)
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Are you trying to say something by not saying anything, quoting my last post?

Dan.

Canadian "EH"






saharatj98
Novice

Jan 3, 2008, 7:32 PM

Post #8 of 9 (1393 views)
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strange, my reply did not go through...anyway, found fuse that feeds relay old, had continuity but weak. Replaced it but still nothing. Switch has power and sends power to relay to operate it. Just not getting power supply to relay and obviously not back out to defog..... but, i have 12v to defog when switch is on. somewhere somehow i have a cross feed or something. more digging in tonight..... wiring looks totally stock and clean so no butchering under there at all.... it's a 25 amp circuit and ya, all german cars are tan grounds.... used to it:) but thanks.
2005 BMW M3
2003 Land Rover Discovery SE 7
1985 Porsche 944
1984 Volkswagen GTI
1983 Volkswagen Rabbit


saharatj98
Novice

Jan 3, 2008, 8:01 PM

Post #9 of 9 (1392 views)
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I have good power to the relay.... when switched on, it does click but voltage in to the relay drops to .8 and out is .8..... Going to take relay apart and check it out...
2005 BMW M3
2003 Land Rover Discovery SE 7
1985 Porsche 944
1984 Volkswagen GTI
1983 Volkswagen Rabbit




rear defroster stopped working


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