The Ford Barn

drolston · 12-12-2021, 08:20 PM

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?

MGG · 12-12-2021, 09:12 PM

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.

fordor41 · 12-12-2021, 10:48 PM

have an alternator?

Frank Miller · 12-13-2021, 07:50 AM

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.

JayChicago · 12-13-2021, 12:50 PM

Quote:

Originally Posted by drolston

my Ohmmeter showed about 500 Ohms from the battery to ground.

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.

drolston · 12-13-2021, 02:34 PM

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Quote:

Originally Posted by fordor41

have an alternator?

Yes, a little one stuffed inside of a generator look-alike housing. I will try disconnecting it to see if the current drain goes away. Good thought!

drolston · 12-13-2021, 02:37 PM

Quote:

Originally Posted by JayChicago

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.

Of course the battery was disconnected at the time of measurement. The 500 Ohms was from the disconnected battery cable to ground.

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.

Frank Miller · 12-14-2021, 08:59 AM

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.

Jack E/NJ · 12-14-2021, 01:49 PM

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.

12-12-2021, 08:20 PM	#1
drolston Senior Member Join Date: Jun 2014 Location: Williamsburg, VA Posts: 1,627	Current Leak? 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?

12-12-2021, 09:12 PM	#2
MGG Senior Member Join Date: Apr 2015 Location: Oregon City, OR Posts: 285	Re: Current Leak? 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.

12-12-2021, 10:48 PM	#3
fordor41 Senior Member Join Date: Oct 2011 Location: elmira,ny Posts: 1,518	Re: Current Leak? have an alternator?

12-13-2021, 07:50 AM	#4
Frank Miller Senior Member Join Date: May 2010 Location: Auburn, MA Posts: 2,106	Re: Current Leak? 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. __________________ “The technique of infamy is to start two lies at once and get people arguing heatedly over which is true.” ~ Ezra Pound

12-14-2021, 08:59 AM	#8
Frank Miller Senior Member Join Date: May 2010 Location: Auburn, MA Posts: 2,106	Re: Current Leak? 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. __________________ “The technique of infamy is to start two lies at once and get people arguing heatedly over which is true.” ~ Ezra Pound

12-14-2021, 01:49 PM	#9
Jack E/NJ Senior Member Join Date: May 2010 Posts: 2,173	Re: Current Leak? 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.