Ask the Mechanic

What's in Your Battery?

(Steve Thompson)

READER: I have an S185 Bobcat skid steer. It has a problem with the battery going down when it's running and being used. The battery holds a charge when it's shut off. I have had the battery checked by one repair shop, which said it was bad. But, I took it home and charged it, and the battery charger said it was charged. So, I took it to another shop, which said the battery was good. But, after I run the Bobcat for about 40 minutes or so, the battery goes dead.

This Bobcat has no life now, let alone nine. I took the alternator to a reputable alternator repair shop, and after testing it, the tech said the alternator was putting out 17 volts and strongly recommended that I replace it. He sold me a new alternator, and I put it on -- with the help of both neighbors. Wow, what a job; it's way up in there! So, guess what. It still won't charge. What next?

STEVE: I believe you have a wiring problem. In order to check the wiring to the alternator, all you need is a test light. The alternator on your Bobcat has the big wire that goes back to the starter solenoid and is mated to the terminal that goes there from the positive post on the battery.

The other two wires are in the pigtail (see accompanying photo) that fits on the back of the alternator (this is a common setup among many machines). One of these wires is hot, even with the key off. The other wire is hot only with the key on. This wire excites the alternator with voltage that allows it to begin charging.

More than likely, you have a blown fuse in your control panel. This fuse was probably blown when the alternator began overcharging, possibly eliminating a fire by shutting down the alternator. If this fuse is blown, the alternator will not charge because it powers up the wire that is hot when the key is on. Replace the fuse, and I'm confident the new alternator will charge.

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-- Email Steve Thompson at mechanic@progressivefarmer.com, and be sure to include your contact information and phone number.

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