Ask the Mechanic

Sneaky Voltage Drain

(Steve Thompson)

How is it possible for a good battery in a vehicle to slowly drain down in a week with the negative battery cable disconnected?

I had this problem with the batteries in my old grain-hauling semitruck. The truck had a slow drain, so I put a master switch on the negative post; but after buying three sets of batteries, it would still drain the batteries in 10 days.

Then, one day, I was in the cab, and I noticed the fuel gauge moved a little when I turned on the key -- with the negative post disconnected. I went back to the batteries, and from the positive post ran a small, single wire that was made on the positive cable (see photo accompanying this article).

I don't know where it went, but it moved the fuel gauge. I put a separate switch on that wire, and it fixed the problem. I assumed it was finding a ground through a controller or something. The tops of the batteries were clean, so there was no drain there. Does anyone out there have a better answer?


-- Write Steve Thompson at Ask The Mechanic, 2204 Lakeshore Dr., Suite 415, Birmingham, AL 35209, or email, and be sure to include your phone number.


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