Taurus Car Club of America : Ford Taurus Forum - View Single Post - Getting less than 12v to power seats?
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post #7 of (permalink) Old 09-13-2016, 10:39 AM
Jeff K
Technical Advisor
Join Date: Feb 2007
Location: Milwaukee, WI
Chapter: Midwest
Year: 2005
Model: Ford Taurus
Engine: 3.0L 24v Duratec V6
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Originally Posted by snowkilts View Post
That's what I was afraid of. But wouldn't an open circuit show 0 volts?

I've never heard that about DMMs. I'll have to get a test light.
Yes, a true open circuit will read zero volts, but..............

Google the voltage divider rule, which come from Kirchoffs voltage and current laws. The DMM looks like a 11,000,000 ohm resistor. If a power circuit has 1 ohm resistance (WAY high for a circuit that must supply a number of amps), one eleven millionth of the voltage will drop across the 1 ohm resistance, and the rest across the DMM, so the DMM will effectively read the correct voltage. If the power circuit has 11,000,000 ohm resistance (such a circuit can only supply microamps at 12 V), the voltage will be divided equally between the circuit and the DMM, and the DMM will read 6 V with a 12V supply to the circuit. This is why a test light should always be used to test power circuits. A test light, which can draw around an amp, will load the circuit sufficiently so that even a few ohms resistance in the power circuit will cause the light to glow dim or not at all.

05 Taurus SEL Duratec - my daily driver
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