Diagnosing nonworking rear window defroster lines.

[Kestas]

My 2001 Sable has nine random grid lines that don't work on my rear window defroster. I can't see any breaks, and chasing this down with my voltmeter shows voltage all the way down each nonworking line. How can this be?

 

[redbone3]

Maybe no ground??? Turn off the defrosters and check for continuity.

 

[Jetronic]

My 2001 Sable has nine random grid lines that don't work on my rear window defroster. I can't see any breaks, and chasing this down with my voltmeter shows voltage all the way down each nonworking line. How can this be?

the voltage should drop the further down the line you go, otherwise it's a bad ground as redbone said

 

[Kestas]

Bad ground? But eight of the lines do work.

 

[Jetronic]

well, if you measured correctly anyway. but the break(s) can be anywhere on the ground side. The voltage drops gradually over the lines if there's current flowing. Won't drop with no current flowing.

 

[JHZR2]

What about measuring resistance between various points on the working and non working lines?

 

[Jetronic]

can't as they are all connected together, you will measure around multiple wires

 

[Trav]

My 2001 Sable has nine random grid lines that don't work on my rear window defroster. I can't see any breaks, and chasing this down with my voltmeter shows voltage all the way down each nonworking line. How can this be?

I use a simple test light or DVOM, connect the ground and gently touch the grid until you find a point the light will not illuminate, you can pinpoint it there.
You can put the DVOM on diode and check it with no voltage applied but you will get no beep when the open is found. Connect one cable to the grid tab and use the other to touch the wires.

 

[wtd]

The defroster is like that on my 92 Cavalier. Part of the lines work and the rest don't. I never really tried that hard to see what was wrong. There was a spot that looked burned and I used the repair stuff on the line but it did not fix it.

I may try Trav's suggestion and see if I can pinpoint where the power or ground is gone just for the heck of it.

 

[eljefino]

Your voltmeter doesn't put much load on the line so "phantom" voltage can still get through with hardly any current to go with it. Use the test like as Trav suggested.