Ohm's Law Applied

[Superman]

I've rewired the defrost circuit to the rear window of my 911SC. Long story. Anyhoo, the circuit is 1.8 Ohms. The fuse is 25A and the wires are fairly robust. With this current, the wire did not change temperature.

For 14 VDC into 1.8 Ohms I am calculating 2.79 amps. Am I correct?

[fanaudical]

V=IR (or I=V/R).

Should be ~7.8A

What part of the circuit did you measure to get resistance?

[Superman]

This is the rear window defrost circuit. You know....those thin lines in the rear glass. Schematics show a single circuit but that's wrong. There are two circuits. One feeding 9 lines and the other feeding ten. I've reconfigured so that one wire feeds 7 and the other feeds 12.

To answer your question, I measured the resistance of the circuit feeding 12 of those thin defroster lines. 1.8 Ohms. I measured from the ground lug wire running to the "-" connections in the glass to the "+" wire coming from the circuit in question. Everything is removed from the car. This is really 1.8 Ohms. Fluke.

The wire may be 14 ga. Can a 14 ga wire handle 1.8 Ohms of resistance in an automotive application (12-14V)?

[GH85Carrera]

Porsche is renowned for their engineering......

BUT they had some really bad electrical engineers and they were expecting the 911 SC to the be the last of the 911 line at the time. Ernst Fuhrmann hated the 911, and wanted it dead and forbid the engineering department to spend any money on the 911. Only when Peter Schutz took over was the 911 line saved. That is why there is no fuse at all on the instrument cluster lighting. And the delay wipers at a little knob on the dash next to the tach, and many other electrical oddities. And, why the AC system was so pitiful.

[Mike Billings]

There's no current in the wire. If there was it would have to change temperature. You can measure current if you want.

Measure the voltage right on the glass. If there is 9V or more, the glass is problem. Less than 9, then power source is the problem.

[dad911]

Originally Posted by Superman

This is the rear window defrost circuit. You know....those thin lines in the rear glass. Schematics show a single circuit but that's wrong. There are two circuits. One feeding 9 lines and the other feeding ten. I've reconfigured so that one wire feeds 7 and the other feeds 12.

To answer your question, I measured the resistance of the circuit feeding 12 of those thin defroster lines. 1.8 Ohms. I measured from the ground lug wire running to the "-" connections in the glass to the "+" wire coming from the circuit in question. Everything is removed from the car. This is really 1.8 Ohms. Fluke.

The wire may be 14 ga. Can a 14 ga wire handle 1.8 Ohms of resistance in an automotive application (12-14V)?

gauge 'handles' current. 14 awg should be fine for 7 amps.

Measure voltage across the load (defrost glass) should be close to 12 volts. If your meter can handle it measure the current, or if you have a test light put it in series with the defroster and it should light up.

[Superman]

Originally Posted by dad911

gauge 'handles' current. 14 awg should be fine for 7 amps.

This is what I wanted. Thank you.

[asphaltgambler]

Also, you shouldn't be seeing 14 volts with the engine running, it should be @13.5 at best - which alters the equation. If you're seeing 14 volts constantly, the alternator is over charging.

[T77911S]

how Is the ground side of the wiring.
no ground = no current.

[T77911S]

get a test light.
check it at the battery. that is what B+ looks like. I will use 12v for this.
ground it. check the + side of 12v at the glass.
connect the clip of the TL to 12v then check the ground side of the glass. (with glass removed from circuit).

[john70t]

1. You are getting 13.5v to the window connector, and this changes from 0v to 13.5v when the dash switch is closed?
(If so, it is upstream so the fuse and circuit up to there is good.)

2. The heating element lines on the window have full ohms/resistance, and thus continuity.
(this will change with higher loads. a wire with one strand left will show continuity.)

3. But the element circuit is not heating up.
(amps not flowing through it)

I would suspect the return ground to the fuse bus, or a ground wire got broken somewhere hidden, or maybe there is a relay on the ground leg.

[vash]

ohm law is the only thing i found in EE that was easy. the very next lecture was a big "?"

i still remember that Wheatstone bridge lecture...hahha.the look of pure confusion on my face was permanent.