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Guest Message by DevFuse
 

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Rear demister/defogger/defrost not working (though power at the grid)

rear demister rear defogger rear defrost

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11 replies to this topic

#1 diem

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Posted 23 February 2014 - 08:32 PM

Hi all,
 
So like many folks I'm suffering a non-operational rear demister on my '07 Outback wagon, but the symptoms don't fit all the threads I've read on the subject (here and elsewhere).
 
The basics; the dash switch is functional, the light comes on when the button's pressed and goes out after the timer period, the relay clicks, both fuses are sound.
 
Originally I was indeed affected by broken wires in the tailgate boots; replaced sections on the broken three wires (including the hot wire to the demister) and all was well - for a few days!
 
The situation now is that I get 0V measured at the +12 side of the grid, however if I unplug the grid at both sides and measure at the plug I've taken off the hot side, I get 13.5V! Plug it in and it sucks down to 0V again.
 
Now I can't see how the grid could possibly be a dead short, as it were - yes there's an electrical path across it but the whole point is that it's somewhat resistant else it wouldn't heat up right? I mean many folks talk about having broken lines across the glass but no-one talks about having ultra-conductive lines right?!
 
I also noted that (with the engine not running) I seemed to measure 10.5V before the weird condensor unit that's attached inside the tailgate, but 13V or so after it (voodoo, like I say). Can it be that some part of the signal path is adequate to pass a bell-test or measure a good voltage but when it's expected to pass a serious current (to operate the grid on the glass) it 'fades' and can't pass enough current?
 
So any suggestions folks? I'll admit to finding electrics to be voodoo - I'm much happier with digital systems where things are just on or off :D
 
 
Thanks,
 
Ian

Edited by diem, 23 February 2014 - 08:57 PM.


#2 gbhrps

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Posted 23 February 2014 - 10:23 PM

diem,

 

Do you get continuity across the tabs from one side to the other of the grid?

 

I know that the tabs are epoxy glued to the grid using a special epoxy that has a very high metal content (glass shops have this, expensive as well). Is it possible that one or more of yours is corroded under neath it? Should it or both be removed, the grid connection point cleaned up, and then reglued?



#3 1 Lucky Texan

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Posted 23 February 2014 - 11:54 PM

what were the other 2 wires?

is it possible some portion of the repair job is allowing some shorting/cross connection?

#4 Fairtax4me

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Posted 24 February 2014 - 12:11 AM

The problem could be that you have a much higher resistance than the grid is, between the power source and the grid.
Or you have a broken wire between the source and grid. Able to pass enough voltage when tested with no load, but not able to carry the current necessary when the load is present.
How did you repair the broken wires? Did you cut out the whole sections and solder in new?

If there is an actual condenser (capacitor) in the tailgate, it would be for filtering out radio interference from the grid.

#5 diem

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Posted 24 February 2014 - 09:15 AM

Many thanks to you all for the rapid replies.

 

I do get continuity across the grid tabs so they seem well connected.

 

I don't think there are any shorts in the repair (I took out complete sections and the new joins are before and after the boots) however I did use inline crimps not solder (don't have a portable iron y'see) - perhaps the crimp can't pass enough current?

 

Cheers,

 

Ian



#6 heartless

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Posted 24 February 2014 - 09:29 PM

get rid of the crimp joints.

 

go to radio shack, or your local home improvement store in the electrical section and get some heat shrink tubing - the 3/16 inch size should be perfect for your needs. you want the rubbery looking kind, not the plastic looking type.

 

Strip about 3/4 inch of the insulation from the ends of the wires, slip a piece of heatshrink over one wire - it needs to be about double the length of the stripped ends.

Twist the bare wires together so they look like this...

 

 

Slide the heatshrink over the twisted wire so that the heatshrink overlaps the insulation on both ends of the splice, then heat the heatshrink with a cigarette lighter, a small butane torch, a heat gun - whatever you have handy. keep the heat moving so you dont burn it.

 

When done it should look like this...

 

 

I have used this method of splicing vehicle wiring for many, many years on all types of vehicles, including under semi trailers, with no problems. when the heatshrink overlaps enough and is heated properly it makes a good enough seal to keep weather out indefinitely.



#7 Fairtax4me

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Posted 24 February 2014 - 10:52 PM

I've had crimps burn out several times.
The rear defog grid is a high amp circuit. Really need really good crimps to handle the current in that circuit. Even then its best to solder the wires.
You can do what heartless shows above, just be sure to use good heat shrink tube.

#8 diem

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Posted 25 February 2014 - 09:22 AM

Brilliant - thanks both! I'll report back once I've replaced the crimps with joins as described by you heartless - thanks so much for taking the time to include the pictures!



#9 heartless

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Posted 25 February 2014 - 06:02 PM

you are quite welcome. :)

 

ideally you should solder that splice before putting the heat shrink over it, but even without the solder, this will give you a much better connection than those crimp connectors will.


Edited by heartless, 25 February 2014 - 06:06 PM.


#10 diem

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Posted 27 February 2014 - 02:15 PM

Borrowed the use of a friend's garage to combat the cold, and bingo!!! :)

 

Opened everything back up, and here's what I found:

 

burnt_demister_crimp.jpg

 

So the crimp's not actually in focus, but you can't miss the fact that it's black and melted instead of blue!

 

Followed heartless' instructions (and even managed to get near a mains socket so that I could solder ;)) and now I get a voltage across the grid - huzzah!!!

 

So then, the moral of this story is don't use crimps where serious current is gonna pass...  ;)

 

Thanks again all,

 

Ian



#11 Fairtax4me

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Posted 28 February 2014 - 04:45 PM

Well there's your problem right there!

Glad you got it sorted out, and thanks for posting the follow-up.

#12 heartless

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Posted 28 February 2014 - 10:20 PM

Good Job!






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