Heated rear window wires measures 1K5. Could the wires be corroded? - Page 1
Forum section: electronics / general

author: king.oslo
author_rank: Frequent Contributor
timestamp: August 26, 2012, 11:06:04 pm
content: Hello there,I have a problem with the heated rear window in my 71 VW beetle. The problem is that all the resistive wires in parallel measure 1K5 ohms. Therefore, they dont heat up from 12V.I couldn't find anything on google, apart from paint-on-conductor repair kits for boken traces. But I could not find a break in the wires.I get the feeling the conductivity of the trace-material has declined with age. Is this possible? What are these traces made of? What test can I carry out to learn whether I recondition these traces?Thank you for your time.Kind regards,Marius

author: Mint.
author_rank: Frequent Contributor
timestamp: August 27, 2012, 12:08:08 am
content: I may be wrong, however if the wires are acting as heaters aren't they supposed to have a resistance so they dissipate more heat???

author: elliott
author_rank: Regular Contributor
timestamp: August 27, 2012, 12:14:05 am
content: Quote from: Mint. on August 27, 2012, 12:08:08 amThe higher the resistance, the lower the current, the lower the wattage. They should be less than 10 ohms on most cars, anything more than that and they are under 14 watts and not very useful.

author: Mint.
author_rank: Frequent Contributor
timestamp: August 27, 2012, 12:40:34 am
content: Quote from: elliott on August 27, 2012, 12:14:05 amHmm true that. Ohms law...

author: SeanB
author_rank: Super Contributor
timestamp: August 27, 2012, 04:42:17 am
content: Old method is to turn it on, then take a small piece of copper strip and use it as a trace to go along each strip to find the breaks. You attach ther strip tp the positive lead of the multimeter and the negative to a good chassis earth, then slowly wipe along each strip. A good strip will start at 12V and gradually change to 0 as you move along. A bad one will be 12V until you hit the break, then drop to 0V. Have a dry wipe marker to mark each spot, then get the repair kit and do each break.A long job, but the only alternative is a new window glass, or strip it off and a aftermarket demister strip panel.I only wish the previous owner of my car had done that, there are 5 breaks, and they occurred before the inner solar film was installed. Remove that and I need a new rear window.

author: Fraser
author_rank: Super Contributor
timestamp: August 27, 2012, 08:33:35 am
content: As already stated, this will be either very fine scratches across the wires causing a physical break or poor connections at the lucar connector to wire interface. The resolution, when the breaks are found, is the silver loaded repair paint that I used to apply with the tip if a pin. Worked great. A magnifying glass and carefull visual inspection may find the breaks as well. A common cause of failure was 'helpful' female rear passengers wiping the condensation off the rear window with the back of their hand to help the driver, and catching their engagement ring diamond on the demister strips, cutting them cleanly ! Corrosion is not that big an issue IMHO.Good luck.Fraser

author: AndyC_772
author_rank: Super Contributor
timestamp: August 27, 2012, 08:44:08 am
content: 1k5 is very high; are you sure you're measuring the resistance of the strips themselves, or could there be a bad connection between the strips and your meter?It should be easy enough to tell the difference. Turn the heater on and measure the voltage between the middle of each strip and ground. If you get about 6V then there's current flowing through the strip. If it's 0V or 12V then there's a break at one end or the other.

author: amyk
author_rank: Super Contributor
timestamp: August 27, 2012, 08:57:23 am
content: QuoteThe same silver-based conductive paint the repair kits use.

author: notsob
author_rank: Frequent Contributor
timestamp: August 27, 2012, 08:58:51 am
content: Agree with Aurora, someone wiping condensate (mist) from the window with their hand ( that had a ring with gems on it), is a common cause of minute scratches/cuts in the window heater elements. Try tracking them down with the tracer method already mentioned & then mark the scratch beside the heater trace. You should be able to buy, automotive window heater repair compound/kit at you local auto parts store, it is a conductive paste that you paint over the break/scratch and then let it dry. I've also read of people using the same repair compound to make their own thru hole plating on pcbs !

author: AndyC_772
author_rank: Super Contributor
timestamp: August 27, 2012, 11:07:37 am
content: I fixed the rear screen heater on my car with a silver loaded pen.There's a design flaw on my particular car; the rear window glass can be opened separately from the rest of the tailgate, and the rubber seal actually wears through the heater element over time. First one wire broke, then another, then all the rest went at once. (I even used "why did this happen?" as an interview question!)I cleaned and exposed the metal traces using a fibreglass pencil, then applied about 3 coats of silver paint. Worked like a charmDo be careful to ensure your repair is conductive enough. I've heard stories of people who have repaired their screens this way, then had them shatter when they heat up. A resistive connection will overheat and could certainly cause local thermal stresses in the glass. Also be aware that some people erroneously blame the silver paint for being "too conductive" and therefore "generating too much heat", which is completely backwards.