Alternators, love 'em or hate 'em, we can't live without 'em. Or, at least our cars can't. You have a battery in your car to start it up, but what keeps all the lights on, engine running, the car's computer running (if so equipped)? It isn't the battery, it's the alternator, a miniature power plant under your hood converting your engine's rotational energy into electric energy. Here's a pretty good, simple explanation of what an alternator is and does.

Pretty cool, eh? Not when things go wrong. Alternators can go bad in a number of ways and you, Dr. Electric Troubleshooter Person, are going to diagnose the patient and, perhaps, operate to fix the problem.

Voltmeters

So, what can go wrong and what are the symptoms? Let's talk about voltmeter and ammeter gauges, your "auto medical instruments." Voltmeters measure the voltage put out by the alternator/battery. Voltage is basically like water pressure: sort of a measure of available energy (I'm simplifying). Normally your voltage should read around 13.8V with the engine running, meaning the alternator is functioning and all is well. It may increase with RPM, decrease with load, but generally it should be well above 12V. We'll talk more about diagnosing alternator component failures in a moment.

Ammeters

Ammeters measure the current going to or from the battery. Normally, alternators are supposed to be sending current to the rest of your vehicle electrical system, as well as some left over current to the battery--the trickle charge which keeps the battery topped off. If the electrical load in the vehicle exceeds the capacity of the alternator, no current will be left over for the battery. Ammeters have a needle that swings either to the + or - side of the gauge to indicate how much current is going into or coming out of the battery. The + side means current is going to the battery. The - side means for whatever reason current is being drawn from the battery.

When you first get in and put the key in "run" the needle should be slightly to the left indicating battery discharge because the motor isn't running (and the alternator isn't doing anything). After starting the vehicle, the needle will likely swing far to the + side. The alternator is replacing all the energy used to start the motor. The needle will swing back towards the center (0) after a few moments and eventually settle just to the right of the 0 on the + side, meaning a little bit of current is going to the battery to keep it topped off. If you add too much load, or if the alternator is not putting out enough voltage (see below for why) the needle will swing negative.

Diagnosis

So you know how to read the instruments, now time to diagnose, doc. It is very common for a single component to crap out on alternators, such as the

• diode assembly, manifesting itself as headlights or other incandescent lights flickering at a stable frequency when idling (may be hard to see this problem on the gauges), or
• regulator, resulting in under voltage which you can read on a voltmeter (12V or less) or ammeter (negative swing of needle) if you have one, or over voltage (15V or more on the voltmeter, harder to read on ammeter) and could result in several electrical component failures, or
• brushes, resulting in under voltage which means your battery isn't charging and the vehicle lights grow dimmer as you drive and eventually you stall, or, you end up with a dead battery after you park and go to start up again (however a dead battery after parking can be also caused by other problems)
• front bearing, resulting in bad bearing/bushing noises including grinding and eventually the alternator seizes and your alternator belts slip, break, or melt off. Tip: if a persistent noise suddenly stops, something bad probably just happened.

Second Opinion

I've had every single one of these things go wrong at least once! If you have these symptoms, you can take your vehicle to a local chain auto store to have the alternator tested to get a "consult" (that's doctor talk, at least according to the doctor shows I watch on TV).

Cure

If you've verified the alternator has crapped out, you can either fork over a lot of money to get a new one and install yourself (lots cheaper than having a shop install it!), or you can save quite a bit of dough, spend some time, and rebuild it yourself.

Here's how you rebuild a Delco-Remy Alternator.