Sorting out the Systems,

The main assumption of this article is that the fuel system and the ignition system are in good working order, and the problem is battery or starter related.

The interaction between the battery, the charging system, and the starting system can make troubleshooting a starting problem a bit confusing.  For example, your engine barely turns over or won't start, ...but instead of a bad battery, you might just have a tired alternator that seems okay, but can't keep up with the demands of the electrical system, so the battery runs down over time.  Or perhaps the bearings in your starter are wearing out and causing too much drag as it turns giving the symptom of a low battery.  Here are some tips to help sort things out.

Any electrical problem might be a bit tricky to nail down unless you have a DVM (digital voltmeter) to take some measurements. The voltmeter on the dash will not be of any help,  you need to measure the voltage directly at the battery in order to diagnose what the cause is.  If you have a DVM then first measure the voltage prior to starting (key off), after it has been sitting a while. A healthy battery should read 12.5V or more, a weak one will be closer to 12V or less.

After the engine is running, the voltage should be between 13.5 ~ 14.5V (engine revving, no accessories),  anything less than 13.0V (except @ idle) indicates a weak alternator.  If it is higher than 15 volts, then the regulator (in the alternator) is not working and might be overcharging your battery to the point of boiling out the water.

If the battery and alternator are okay, then suspect either the starter, the cable from the battery to the starter (the cable ends can fail internally), or the Start relay, all three can simulate a weak or dead battery as they fail or become intermittent.  The ignition switch can also become intermittent, but is easier to detect since wiggling it slightly as you try to start the engine will usually yield results.  A dragging starter is hard to detect because they don't always make a suspicious noise.  The best way to check these is with an induction style ammeter clamped on the starter cable to see if it is drawing too much current.  Inexpensive ammeters for testing starters can be purchased at most auto parts stores for around $10~$15 (?), however most mechanics have one of these in their tool box and will check it for you for less than that.

If the engine won't start (or you don't have a DVM), a quick test (best with two people) to help isolate the problem to either the battery/charging system or the starting system, is to turn on you headlights before you try to start the engine. If the lights don't work or are very dim, your battery (alternator?) is weak.  If they are bright, start the engine.  If the engine barely turns (or not at all) and the lights go dim or out, then the battery is weak or not charging.   If the lights stay fairly bright (or don't dim at all) and the engine doesn't turn over, then the problem is in the starting system (ignition switch, starter cable, relay, or starter).
Note: This test doesn't work on some other vehicles that cut power to the lights while starting.

Testing the Start relay is beyond the scope of this writing due to the many variations of the different year models (here's a photo of a pre '92).  However this information should get you on the right path and hopefully avoid replacing something in the charging system that isn't faulty.
 
 

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