rear window defog

[gserve]

How do you test a rear window defogger? There is 14.5 volts at the terminals when the button is pressed. There is no broken grids. HELP!

[danielsatur]

Wait until the morng, see if it works!
Isolate the grid by disconnect at rear window, and measure the resistance of the grid.

[gserve]

What is the resistance supposed to be? I have already repaired one tab and it worked for a few days.

[danielsatur]

How many amp fuse is assigned for the rear defroster?
12volts / ? = little

R=E/I

[gserve]

It has a 30 amp fuse to it. I also have heated mirrors on the same circuit as the defogger. The heated mirrors work.

[danielsatur]

Here's a good fix, see http://www.popularmechanics.com/auto...o/4221212.html

[old_master]

If you show battery voltage at both terminals on the glass, there is no ground being supplied. Check the ground wire connection at both ends of the wire.

[gserve]

old master, Are you saying that I shouldn't have 12 volts when putting my tester at both terminals when the switch is on? Wouldn't one wire be + and one be - and you get 12 volts when the switch is turned on? Please clarify. Thanks

[J-Ri]

Measuring across the grid, you should have battery voltage.

I, and presumably he, thought you meant that with the ground probe on ground you measured voltage on both sides.

Sounds like the rest of the circuit is good then, so the problem has to be with the grid. I'd double check the terminal you repaired. Not sure on the resistance, I'd expect up to 10 ohms... definitely not "OL"

[old_master]

J-Ri Exactly.

With the ground probe of the DVOM connected to chassis ground: If both terminals of the grid show battery voltage, that means the grid is good but not getting a ground.

[danielsatur]

Hey!
I found some videos, see http://www.youtube.com search ''Auto rear defrost repair''

[91goldbomb]

simalar to the headlight problem i had on our cavalier the sockets have constant power but when the switch is turned on it completes the ground kinda reverse to what youd nomally think youd think theres constant ground and switch completes the + circut not sure why they did it this way or if theres any reason behind it but def check all your grounds and make sure they are all clean and tight and no breaks inbetween

[old_master]

Originally Posted by 91goldbomb

.... not sure why they did it this way or if theres any reason behind it

Ground side switching is used in certain circuits to reduce or eliminate corrosion.

[91goldbomb]

Originally Posted by old_master

Ground side switching is used in certain circuits to reduce or eliminate corrosion.

ah i see interesting well it sounds like agood idea but not sure it works 100% as in our cavys case it did not but who knows prolly just got one of them cars that needs the constant attention