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Rear Defrost not working.....any suggestions

30K views 21 replies 11 participants last post by  vball61 
#1 ·
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:undecided
 
#2 ·
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.
 
#3 ·
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.......:undecided
 
#7 ·
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.
 
#8 ·
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. :hail:
 
#9 ·
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.

 
#11 · (Edited)
Hmmm... thinking about it, my rear defroster hasnt had anything but 1 line working since I bought it in 2004 :hissy: Mine has always worked like this (the red line being the only line that works):


The connectors to the defroster grid are on, but I cannot see any other reason for just 1 line of the grid working besides the grid just being bad. I mean, if the wires were loose, bad ground, etc.. wouldnt it not work at all?

Anyways, I called a local junkyard and was prepared to buy a back glass out of another t360. Has anybody ever replaced the rear glass?
 
#12 ·
I have one grid line that is not working. It appears that the problem is where the line attaches to the 12v vertical supply line. I cannot get to the connection as it is covered with a black coating (paint??). Would it be possible to simply connect this bad grid line to one of the adjacant lines using a repair kit? Would this pull too much current through the "good" grid line? Ideas? Thanks
 
#13 ·
You can try scraping the paint off the vertical distribution bus. For the short part where you have two lines being fed by one narrow line, that section will have double the current and could overheat and die, yes. Good to worry about. Then you'd have two dead lines, potentially worse then today.
 
#15 ·
A couple months ago I too started having a problem where my mirrors would heat but my rear window barely did or not at all. Last night I metered the connections and they were registering anywhere between 0 and 3 volts. I took the door panel off, started wiggling wires and my meter jumped to 13v...yea! Everything seemed firmly seated so I traced the defrost wires to figure out which harness was the problem and on closer inspection, I found that the power contact had turned blue and the socket was slightly melted! Hmmmm....a Ford in Chevy's clothing?
 
#18 ·
Now I've just got to find me a new connector.
 
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