Black Wire Syndrome

How about this for a puzzle. The rear window de-mister was not working on my Toyota Supra MK3, so I checked the voltage at the terminals and got reading of 12volts. So I suspected that the element had gone , checked it out and all was o/k. Next I thought, right, I will put a test lamp on the terminals and it did not light. This was a real puzzle, how can you have 12volts and no power to light a very small bulb ?. I next checked the wiring by stripping a small part of the insulation off, and it was black. The only conclusion that I can come to is that the black wire has set up a very high resistance and cannot pass any current but still gives a reading of voltage. Unless anybody has any experience of this and knows better?. Your suggestions will be of great assistance. As an aside I bypassed the positive wiring with a new wire temporarily to test this theory and the heater worked and also lit up the test lamp using the old negative wire and the new positive.Interesting eh. Peter

Reply to
Peter Keating
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No its the classic case of a voltmeter not drawing (hardly)any current, you have just proved this yourself. Any highish current circuit is best tested with a 20w bulb across the meter probes. Somewhere on the original feed wire is a bad connection.

Reply to
Mark

The wire has gone black due to oxidisation (copper oxide has two forms, one green, one black). Can be due to water ingress, water can enter the insulation at the end through capillary action, or heating, which is more likely in this case. The oxidation on the outside will create extra resistance to a test meter. As someone else has said, somewhere there is a high-resistance connection. Best option is to replace the wire.

Reply to
Chris Bolus

Reply to
Peter Keating

Easy. Voltmeters have a very high resistance - typically a thousand ohms or do. So when you connected the voltmeter, you were drawing practically no current, hence no voltage drop over the dodgy wiring. As soon as you try to draw current, the wiring drops the voltage and the bulb doesn't light.

Ian

Reply to
Ian Johnston

The fuse should blow first.

Reply to
Dave Plowman

Even a pretty old decent analogue voltmeter would have a resistance of

10,000 ohms per volt when measuring DC. A modern decent DVM should have an imput impedance of 10,000,000 ohms.

So you really need to measure the circuit with a load applied - open circuit voltage measurements are pretty meaningless.

Reply to
Dave Plowman

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