Ask The Mechanic

Just Charge It

Image by Steve Thompson

I have a 1974 Ford 5200 diesel tractor--a 12-volt system with a generator. The generator and starter were shot, so I had them rebuilt. I also replaced the voltage regulator. My question is how can I tell if the generator is charging the battery when running?

STEVE: It’s easy to see if the generator is charging. Simply put a voltmeter--a cheap one at an auto store will work--on the battery posts. If the battery is charging, the voltmeter will show around 13.8 volts with the engine running fast. Put the red lead on the positive post and the black lead on the negative post. Make sure your meter is set on DC volts. If you put the leads on backward, your meter will read around minus 13.8 volts, but the good news is that you have not blown your meter.

Is there a noncorrosive alternative to the use of calcium chloride solution for ballast in tractor tires? If so, how does it compare in effectiveness and cost?

STEVE: Whenever I need weight in rear tractor tires, I fill with water up to the level of the bottom of the top of the wheel. This allows room for the liquid to roll around the wheel as you travel. It also keeps your tractor from looking like a Harlem Globetrotter’s basketball rolling across the floor when you get on the road. I also put 4 gallons of antifreeze in each tire, but I’m in Texas, not Minnesota.

I have a gas tractor with a 12-volt system. Is it necessary to polarize the generator? If so, which terminals do I connect together? Also, how can I check my generator to make sure it’s working? If it’s working, I am not sure that I am not getting any benefit from it. With the engine running, I disconnected one cable from my battery, and the engine died instantly.

STEVE: Sounds like you have a generator or regulator problem. In order to check your generator, find the wire leading from your generator to your battery. It could be marked A. Next, ground the other post. When you ground this wire, you should hear the generator load (especially if you can turn on the lights). If the generator is good, it will charge full amps when fully grounded. If it does, then your generator is OK, and you probably need to replace the regulator and polarize the generator. Each time the battery is replaced or disconnected from the tractor, the generator is replaced or repaired, or the regulator is replaced, polarization is usually required for the generator to charge. Do not run your engine too long with the wire grounded to check your generator, or you will overcharge your battery. Before you start the engine, polarize a negative ground system (6 or 12 volts) by using a jumper wire (16 gauge works great) with small clips on each end. Clip one end of the wire on the terminal on the generator marked with an A, and touch (briefly, you’ll usually see a spark) the other end of the wire’s clip to the positive post on the battery. If the system is positive ground, then go from terminal A on the generator to the negative post, and briefly touch it. Start the engine, and the generator should charge. If the amp gauge is reading backward, simply reverse the two wires on the back of the amp gauge.

Safety Tip of the Month:

Be careful with wires hot from excessive electron flow. When the amps going through a wire overload the capacity of the wire, the insulation melts and likes to stick on skin; and the bare wires can cause an electrical fire. If your battery cables are getting hot from cranking on an engine, then the battery cables are too small to handle the high amps the starter is drawing and can short and get red hot. If you have a starter that is “dragging,” battery cables that are too small can get hot very quickly. Hot wires (battery cables) also slow down the movement of electrons through a wire significantly reducing their ability to move electrons. Hot wires can hurt you and your pocketbook when they damage sensitive electronic equipment on today’s “customer-convenient” machines because of a short. Electricity is a great thing, but it’s very hard to control. It is always looking for a ground--anywhere it can find it.

Write Steve Thompson at Ask The Mechanic, 2204 Lakeshore Dr., Suite 415, Birmingham, AL 35209, or email


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