Getting less than 12v to power seats? - Taurus Car Club of America : Ford Taurus Forum
 
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post #1 of 7 (permalink) Old 09-12-2016, 01:50 PM Thread Starter
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Getting less than 12v to power seats?

Hi, I'm working on an 05 Taurus. The driver side has 8-way power seats which do not work.

I appear to be getting only about 4.5v in the connector going into the seat (seat is out of the vehicle now). Any ideas as to how this could be happening?

The fuse tests good. There is 12v present at the fuse terminals with the fuse removed. I replaced the fuse with a jumper just to test and still got 4.5v at the seat.

With the seat out of the vehicle, all seat power functions work fine when 12v is applied directly to the seat wiring.

Thanks!
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post #2 of 7 (permalink) Old 09-12-2016, 04:09 PM
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No offense meant but are you sure you are testing the right wiring? 4.5 is @ ref voltage if the you are testing the right wiring then it must be shorted or crossed somewhere.
Wiring Diagrams
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post #3 of 7 (permalink) Old 09-12-2016, 07:13 PM
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Use a test light to test power circuits, not a DMM. The 11 Megohm input impedance of a DMM will lead to false voltage readings in power circuits with high resistance.

Sounds to me like you have an open circuit. Time to trace wires.

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post #4 of 7 (permalink) Old 09-13-2016, 07:37 AM Thread Starter
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Quote:
Originally Posted by jag1959 View Post
No offense meant but are you sure you are testing the right wiring? 4.5 is @ ref voltage if the you are testing the right wiring then it must be shorted or crossed somewhere.
Wiring Diagrams
Sorry for my ignorance, but what do you mean by @ ref voltage?

I'm fairly sure I am testing the correct wires. I was checking between the red and black wires. When I applied 12v to the seat wiring directly from the battery the seat worked fine. I will check again though.

Thanks for the link to the wiring diagrams. I printed the relevant one.
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post #5 of 7 (permalink) Old 09-13-2016, 07:39 AM Thread Starter
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Quote:
Originally Posted by Jeff K View Post
Use a test light to test power circuits, not a DMM. The 11 Megohm input impedance of a DMM will lead to false voltage readings in power circuits with high resistance.

Sounds to me like you have an open circuit. Time to trace wires.
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.
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post #6 of 7 (permalink) Old 09-13-2016, 08:15 AM
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Reference voltage meaning sensor voltage. In this case the thinking being that you may have been reading the seat sensor if you had the wrong wire or if the seat circuit was open the wire may have crossed with a sensor circuit.
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post #7 of 7 (permalink) Old 09-13-2016, 10:39 AM
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Quote:
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.

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