mk1-forum.net

Just got my OE heated rear window out. It's never been tried - or even wired in - since I restored the car in the 80's. So I don't even know if it works.

TESTING it. Is the only test to put the two terminals across a 12v source and huff on the filaments across the screen to make sure that they clear the steam? Steam from the kettle?

Obviously, when we were in our 20's there was always a lot of steamy windows in the back of our mini's

I'm not clever mate, but yes, you are thinking the right way here Pete, they only had 12V on them. Might be an idea to check the current too. I maybe off the mark, it's been a very long time since I had a rear window that could be heated, but I seem to recall they drew around 5 - 6 amps. Might also be an idea to check the heat along each trip too, so you can be sure there's no hot spots - the glass doesn't take too kindly to them.

Not sure if steam from a kettle is the most accurate way to check it for it's demisting ability, need to get the glass temp below the dew point, you may well have to fog it up the old fashioned way

I suppose the simplest test is simply to test for continuity with a multimeter. Then like Chris suggests checking for hot spots. I should definitely do all this with my heated screen for the Inno. It was OE at the time, good excuse to dig out the 12V LPS. A lower voltage initially could maybe help detect any hotspots before they become really hot and a problem for the glass...

Continuity alone won't tell you much, just that at least one of the strips works. Each of the horizontal lines are parallel to one-another (both literally and electrically) so it only requires one of them to be intact in order for current to flow from one side to the other. Theoretically you would need to measure the resistance, but the problem with that is that it will be only a few ohms, and most multimeters are too insensitve to be worthwhile.

5-6 Amps at 12.7V works out to 63.5-76.2 Watts, so that's probably about right, and Ohms law (12.7V/5A) tells us the resistance would be around 2.54 Ohms which is very small. At the lowest ohms range on a multimeter the display would be jumping around all over the place just from the changes in resistance of the probes as you press them against the contacts on the windscreen.

Unless you have access to a thermal imaging cameras, probably the only way to test it is empirically. On my old Ford the element is hopeless, scratched off in several places. In the few strips that do work I can feel a slight warmth, but only on a cool day.

Tim