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Author Topic: Rear Defrost Issue 12v Showing On the Glass But No Heat  (Read 970 times)

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Offline Beaverseagle

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Rear Defrost Issue 12v Showing On the Glass But No Heat
« on: January 11, 2016, 09:28:52 AM »
Anyone have this issue?  I'm getting 12v on the hatch and all the painted lines.  I don't see any brakes, but still no heat.


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82 SX4 258 w/ 2 inch lift 235 tires and TBI kit.

Offline rollguy

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Re: Rear Defrost Issue 12v Showing On the Glass But No Heat
« Reply #1 on: January 11, 2016, 10:46:24 AM »
Are you testing for power at both connections to the glass?  You can also test it with a light bulb by disconnecting the - side connection and wiring a bulb between the wire going to the glass and ground.  If the bulb lights up, you are golden. You can also remove both wires and use a meter to check continuity.  Also, you may not be able to feel any heat for a while, even if it is working properly.
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Offline Beaverseagle

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Re: Rear Defrost Issue 12v Showing On the Glass But No Heat
« Reply #2 on: January 11, 2016, 11:23:20 AM »
I didn't disconnect anything.  I turned the switch on at the front and took a multimeter and touched the positive lead to all relevant connections on the glass (connections at each end of the painted lines and the painted lines) and grounded the black lead to the body.  I was getting 12v every where.  When I turned off the switch, I got no reading.

It was very cold this weekend and I drove around with it on for long periods of time and nothing melted.  The timer on the defrost switch did go off but only after 10 minutes.  Is my testing incorrect? 

My logic is if I get 12vs there should be heat!  But I guess not because I'm not getting anything.

As you can tell, electronics are my thing.  :banghead:
82 SX4 258 w/ 2 inch lift 235 tires and TBI kit.

Offline AMC of Houston

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Re: Rear Defrost Issue 12v Showing On the Glass But No Heat
« Reply #3 on: January 11, 2016, 01:03:16 PM »
Those tests won't tell you if the ground is good.   My waggie had a broken ground wire (broken inside of the insulation from flexing with the opening/closing of the hatch).
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Offline macdude443

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Re: Rear Defrost Issue 12v Showing On the Glass But No Heat
« Reply #4 on: January 11, 2016, 03:09:39 PM »
Those tests won't tell you if the ground is good.   My waggie had a broken ground wire (broken inside of the insulation from flexing with the opening/closing of the hatch).

My thoughts exactly.  The only really good ground reference for that test is after the lead terminates at the hatch.  My rear defrost didn't work either and I found the braided lead had broken inside the rubber insulation near the left hatch hinge.  I also don't think the switch will engage without a complete circuit.  I know mine will not, but I have the later style all-in-one defrost switch, not the early one with the push button.
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Offline rommel1940

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Re: Rear Defrost Issue 12v Showing On the Glass But No Heat
« Reply #5 on: January 11, 2016, 03:24:33 PM »
Mine isn't working ethier, my 84 limited has a lighted rocker switch, witch lights up when on, but after a long drive , it doesn't seem to be heating/ no affect.
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Offline rommel1940

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Re: Rear Defrost Issue 12v Showing On the Glass But No Heat
« Reply #6 on: January 11, 2016, 03:49:36 PM »
Well for giggles,  I checked and have no voltage at rear glass, no broken wires . I'm guessing its my defrost switch, because it seems funky, it will only come on if you really push on it it, I'll swap one out of a parts car soon, when I do my radio pod switch
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Offline JayRamb

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Re: Rear Defrost Issue 12v Showing On the Glass But No Heat
« Reply #7 on: January 12, 2016, 03:26:25 PM »
Both of my wires on each side of the hatch were broken off from opening and closing
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Offline Beaverseagle

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Re: Rear Defrost Issue 12v Showing On the Glass But No Heat
« Reply #8 on: January 22, 2016, 02:42:24 PM »
Bingo. That was the issue. One of the wires broke inside the black wire casing.  Working great now!
82 SX4 258 w/ 2 inch lift 235 tires and TBI kit.

Online vangremlin

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Re: Rear Defrost Issue 12v Showing On the Glass But No Heat
« Reply #9 on: January 22, 2016, 04:53:39 PM »
Bingo. That was the issue. One of the wires broke inside the black wire casing.  Working great now!

Good to hear!
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