Under the Hood: Your Car's Alternator

Well, that was an odd sound. I had gone out to my truck and instead of a nice crank...I got a lousy chattering sound from under the hood. Yep...Something wrong with the charging system.

I popped the hood and considered the problem for a few minutes. The ignition switch works because the starter tried to engage. The starter worked when I turned the ignition switch so it must be okay too. That leaves the battery and the charging system as the likely culprits.

I brought out the battery charger and clipped the leads onto the battery posts and switched the charger to on. I watched the ammeter on the charger to see how high it would go under load and it went all the way to ten. A ten amp draw indicated a very drained battery. Well, it had been a few days since I had tried to start the old truck. Could be there was a bad connection and a slow drain on the battery from somewhere.

A few hours later the draw was down to a two and I unclipped the leads from the battery and the engine started right up. Well, this actually fixed nothing as I had charged the battery from the charger. I still needed to determine why the battery had gone dead. Taking a volt meter I selected the DC setting and placed the leads from the meter across the terminals of the battery. They showed twelve volts with the engine running. Twelve volts was not the reading I was hoping to get. A healthy charging system should have showed thirteen to fourteen volts. Something in the charging system was not working well. And that something, was probably the alternator.

The auto parts store wasn't all that far away and I took the chance to drive over there. They offer a free charging system analysis and that test would tell me for certain what was actually wrong.

The technician at the parts store hooked up his test leads and had me start the engine again. He flipped this switch and that, checked a few connections and informed me with certainty that the alternator was bad..and must be replaced. So, eighty dollars later I was under the hood pulling connections and turning wrenches. Alternator replacement is dead easy on most cars. It's right up front where you can get to it. All you do is release the serpentine belt, pull off the electrical connections and unbolt the unit. Installation is the exact reverse of disassembly.

Years ago cars used a device called a generator to charge the battery but a generator just can't keep up with the heavy electrical demands that todays cars need. An alternator can produce a full 100 to 300 amperes and do it at engine idle speed. Your car actually runs it's power demands from the alternator...and not the battery. If it were the battery only having to heat the rear windshield, heat your seats, run the wipers, and the heater blower fan and the headlights and radio and fog lights, plus keep the engine running..it would soon exhaust itself. But the alternator has a built in rectifier and so it is able to convert the ac current it makes, right into dc current, which your car wants. And no battery on Earth can be charged fast enough to replace the power that is consumed by your cars electrical system. That energy has to come from someplace else. The alternator.

I felt good about the repair because I had gone that one extra step before leaving the auto parts store. I had the guy load test the battery. This will tell if the battery is in good condition and not getting so old it will need to be replaced as well. This was once done using a device called a hydrometer. This was a glass tube with a floating gauge inside of it and a squeeze bulb on the end. You inserted the end of the hydrometer into the battery water and by depressing the squeeze bulb and slowly releasing you could suck up a measure of the water/acid mixture in the battery. All liquids have what is called a specific gravity. For a battery this specific gravity is measured using the floating gauge inside the tube. The water level will rise around that floating gauge, once the meter is full enough to cause that gauge to float, you look inside and see where the water level is relative to a scale on the side of the gauge. The reading you get will tell you the health of the battery to a good degree. A good strong battery should give you a twelve hundred to thirteen hundred reading.

So, all was well with the battery and the charging system. Maybe in a few days I will check around and see if there really is a slow drain on the battery coming from somewhere.