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#1 |
Senior Member
Join Date: Jun 2014
Location: Williamsburg, VA
Posts: 1,811
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In trying to find my intermittent headlight short circuit (blown fuses), I came upon a mystery. With every switch in the car off, my Ohmmeter showed about 500 Ohms from the battery to ground. On my 12v car, that would result in a current leak of 24 milliamps, with the car turned off. That drainage would be pretty typical in a modern car with a lot of "keep alive" computer stuff, but for an old Ford, it should be completely open circuit, or zero drainage.
The only hot circuit that is not on a switch is the horn. Maybe the leakage is through that foam ring that insulates the horn ring from the column. Any ideas? |
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#2 |
Senior Member
Join Date: Apr 2015
Location: Oregon City, OR
Posts: 285
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In my 1934 truck, I had the same type of short. When I would activate the system via the battery cut-off switch, I noticed a very slight jump in the ammeter in the discharge direction. The tail light would come on dimly. Down at the lower left of the engine was a bundle of wires which included the always-hot horn wire and the wire which went back to the tail light. In the bundle, both of the wires had a joint next to each other. The joints were not pushed together completely, and in the darkness I could see the current jumping from the horn wire to the tail light wire through these joints.
Simple fix - I separated the joints and thoroughly insulated those and all of the rest of the wire joints. Problem solved. |
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#3 |
Senior Member
Join Date: Oct 2011
Location: elmira,ny
Posts: 1,568
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have an alternator?
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#4 |
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Join Date: May 2010
Location: Auburn, MA
Posts: 2,106
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Even if your light switch is off it still has power coming to it and that could be the problem. You have some sort of a partial short and the best way to find it is to systemically remove one circuit at a time to see which one makes a difference.
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“The technique of infamy is to start two lies at once and get people arguing heatedly over which is true.” ~ Ezra Pound |
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#5 |
Senior Member
Join Date: Jan 2019
Location: Chicago
Posts: 927
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The ohmmeter is not measuring a current leak. It is measuring the resistance (conductivity) from one meter probe to the other. I would think your ohmmeter should have measured zero resistance. Infinite conductivity.
To look for a current leak, use the ammeter function of your multi-meter. Disconnect either battery cable, one probe on the battery post, other probe on the cable. Measures current running to/from the battery. |
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#6 |
Senior Member
Join Date: Jun 2014
Location: Williamsburg, VA
Posts: 1,811
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#7 | |
Senior Member
Join Date: Jun 2014
Location: Williamsburg, VA
Posts: 1,811
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I will put the meter in DC current mode and connect from disconnected battery cable to the battery terminal to see what the actual leakage current is. |
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#8 |
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Join Date: May 2010
Location: Auburn, MA
Posts: 2,106
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The ohm meter should read 0 ohms unless there is something like a clock with modern guts running. A short which will lead to a fire or blown fuse will give you infinity but here we have a partial short or draw the owner is not aware of.
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“The technique of infamy is to start two lies at once and get people arguing heatedly over which is true.” ~ Ezra Pound |
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#9 |
Senior Member
Join Date: May 2010
Posts: 2,260
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fordor41>>>have an alternator?>>>>
Yes, the sensor in a 1-wire can leak as much as a 100mA. The sensor on a 3-wire can also leak as much unless it's switched. |
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