Does anyone know what the resistance across the defogger grid on a 140 series rear window should be?

Or is there another way of testing the grid aside from just turning it on and watching the fog dissapate? The window is out of the car now so it would have to be tested "on the bench".
--
'60 PV544, '68 220, '70 145, '86 745T

The electric formulas posted will tell you watts. Bench test?

You could take party ice or snow and lay it out on some fluffy towels.

Then lay the window on the cold stuff and wait for moisture to condense from some humid air. A steam iron could bring on the fog faster. A bench test setup!

Al;

You certainly could use the power-up technique...as far as measuring resistance goes, I expect something under 10 Ohms or so...after all, it's just multiple tiny micro-gauge wires in parallel...at 12Ohms, 1 Amp would pass and dissapate only 12Watts total (I don't think that would defrost a window too effectively)...at 1.2Ohms, 10 Amps would pass and result in 100Watts (that's a bit toasty!), so I expect something between the two, but this is strictly my off the cuff estimate... I hope it helps a little.

Cheers

Ron,
I see from my "green book" that the output is listed as 150W.
The fuse is 15A and controls the defogger and horn.

So how does the math work?
Do we assume that it draws 10 amps (is that a reasonable figure) and therefore the resistance is 1.5 Ohms?

Thanks

Al;

V = I x R

...and P (Watts) = V2(squared) / R

...solving for R: R = V2 / P

...entering values: R = 144 / 150

...so R calculates out to just under 1 Ohm.

...putting that back into the first equasion will give current (and is a good double check on the math):

12 = I x 1

...so current is about 12A!

Note: To run that substantial amount of current, it is especially important to make SURE all connections are clean and tight! Use ACZP on fuse ends!

She blinded me with SCIENCE!

Let the testing begin!
Thanks Ron, that's the kind of info. I was looking for.
--
'60 PV544, '68 220, '70 145, '86 745T

Might do to connect it to 12v and feel the grids to see which (if any) are warmer than others.
Seems to work with warm weather fog also, so you should be able to feel the heat.
--
George Downs, Bartlesville, Heart of the USA!

Hi George,

This is something I actually tried once. Not on a 140 Volvo, but a mini van.

The metal film on my old one was not so much scratched away as "eaten" by some chemical process - I could see it. Only about 3 lines heated up. But I could not feel the difference. I figured it was probably some protective coating on the film got cleaned off the inside of the window. It was beyond metallic paint repair kits.

I wanted to check out the junkyard window before buying it. I had no idea what the resistance should be, and was aware that any low current measurements I could make with an ohmmeter would not reflect the warm resistance presented in operation.

So, I took an old bagphone battery in my toolbox and a magnetic compass after testing the idea on the old 3-line window. The little battery wasn't nearly up to the current, but for the twenty seconds or so it took me to scan the lines with the compass I was able to buy the window with confidence.

Sort of a fast-fading memory, owning a minivan.
--
Art Benstein near Baltimore

It may be that your sole purpose in life is simply be kind to others.