Rear Defrost not working.....any suggestions - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum



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Interior Interior

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  #1  
Old 11-13-2009, 12:33 AM
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Ricky81 Ricky81 is offline
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Rear Defrost not working.....any suggestions

Hello......noticed the issue this morning......rear glass frosted up and turned on the rear defroster.......light stayed on.....but nothing was happening.....checked the side mirror and was warm....but nothing was happening to the rear window......still frozen and showed no signs that it was working.....I used the search but found little info....not to much info on this issue.....on a thread it said something about a bad ground.......Can anyone help me out.....or point me to the right direction on where to start.....thanks....Ricky
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  #2  
Old 11-13-2009, 01:08 PM
vball61 vball61 is offline
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I had the same thing happen to me a couple of days ago. I wiggled the connectors on both sides of the glass, and the next time I tried it, it worked. Go figure.

Does your rear wiper still work? If not, check the 30A fuse 3 (labeled LGM #2) in the rear fuse block.
Touch the defrost grid with a test light connected to a good ground. It should light up when defrost is operating. If it doesn't (make sure you try a few spots) then there's an issue with the power supplied by the liftgate module.
Ground for the defrost grid is shared by the license plate lamps, liftgate module, left taillamp, and chmsl... If those don't work either, check the ring terminal attached to the lower left side of the liftgate opening, under trim.
The wires which pass from the body to the liftgate through the grommet/sleeve at the top of the liftgate opening have been known to break/chafe over time, so that's a good place to look as well.

Good luck.
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Old 11-13-2009, 10:46 PM
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thanks for the response...the day this happend I wiggled the connectors on the rear windsheild .the licence plate lights are on and the wiper blades is working......took the vehicle in at the shop...and checked the wirings.....the mechanic told me it must be a bad ground or a loose wire somewhere.......
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Old 11-15-2009, 10:52 AM
macmic87 macmic87 is offline
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first thing i would do is check for voltage with the system turned on. if there is voltage check continuity of the heater grid with the wires removed. there could be a break in the circuit or a weak mounting tab. that's my 2 cents.
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Old 12-08-2009, 10:02 AM
kickinkz kickinkz is offline
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I've got kind of the same problem...Only I have 3-4 lines working, the others aren't....I'm guessing my grid is toast...there's no visible damage to the grid from what I see....

Phil
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Old 12-08-2009, 11:11 AM
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Quote:
Originally Posted by kickinkz View Post
I've got kind of the same problem...Only I have 3-4 lines working, the others aren't....I'm guessing my grid is toast...there's no visible damage to the grid from what I see....

Phil


I have 2 lines that don't work. The rest work fine
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Old 12-08-2009, 11:46 AM
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It's the kind of problem than be troubleshot (if not cured) in 5 minutes with a meter in the hands of somebody who knows how to use it. Or not solved at all in hours of guessing and experimentation in the absence of a meter. Test lights can do half the job - checking for voltage. But only a meter can test for resistance or continuity with the power turned off.
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Old 12-23-2009, 08:19 PM
CSDenali CSDenali is offline
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Roadie,

Same issue here. Only the top half of the defrost grid works. I have a meter, but am pretty rusty. Will you provide a brief instruction for testing?

Still also battling an intermittent power failure to the driver's door module. (The door switch asm) The power turns on and off at will. I pulled the switch assembly and verified all harness connections were tight. Afterward, the frequency of the "on/off" game slowed up. However, it is still present. Power source? Ground?

Thanks.
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Old 12-24-2009, 01:50 PM
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Think of a defrost wire as a painted-on heater wire just like what's in a toaster, except flat. There are two power tabs, one on each side of the window, and the power spreads out to each wire in parallel. Parallel means all of them have the same voltage (12V) at one end where they come together at the tab, and ground at the other end.

If the painted-on wire is intact, and continuous from side to side, there's 12V at one and and 0V at the other. And 6V in the middle. 3V at the 1/4 way distance. 9V at the 3/4 way distance. The voltage you see with a meter (if you poke through the painted-on insulation with a needle or pin to get to the copper underneath.

A painted-on wire with a break in it will have 12V on one side of the gap, and 1/4" away on the other side of the gap, will have 0V. By inspection and moving the place you look for voltage with your needle, you can discover where the gap is.

I found this drawing in a search, and although the author confuses current and voltage, it shows the idea.

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Old 12-24-2009, 02:46 PM
CSDenali CSDenali is offline
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Thank you. Happy Holidays!
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