Current Leak?

drolston

drolston

Well-known member
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?
 
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.
 
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.
 
drolston said:
my Ohmmeter showed about 500 Ohms from the battery to ground.
Click to expand...
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.
 
JayChicago said:
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.
Click to expand...

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.
 
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.
 
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.
 
You must log in or register to reply here.
Back
Top