Mitsubishi Repair: Galant not starting, reply thanks, alternator

Question
Hey Anthony,
A few days back, I was driving my car and went into a mall for about 45 mins. When I got back my car wouldn't start anymore. A jump start would start the car but doesnot keep the engine running for more than 2-3 minutes. The mechanic first thought it was the alternator and then said it's only the battery. The battery was reading 3-4V with the engine turned off and the mechanic took another measurement with one end at the alternator and the other end in some other place, still with the engine off and it was reading slightly higher voltage. After taking the second one he told me the alternator was fine and said the battery had to be replaced. But, if just the battery was bad why would it shut the engine off after it was jump started ? Isn't the battery just used for starting the engine and supplying power when the alternator is bad or is overloaded ? Also, I noticed that the day my car broke down my ignition key wasn't turning very freely. My check engine light also did NOT come on.

I'd really appreciate if you could give me a reply. Thanks.

Steve

Answer
Steve,
Typically once the car is started, the alternator can keep the car running, without much need for the battery.  But the battery must be able to conduct current through itself so other components can receive adequate voltage (like the ignition coil that fires the spark plugs).  If the alternator has to work too hard to push current through the battery, it will not be able to provide enough current to the ignition coil and fire the plugs.

The alternators Mitsubishi put in their cars usually last over 10 years (at least from my experience) and they give you a lot of warning before they go bad.  Batteries, however, give you almost no warning before they go bad, and typically need to be replaced every 5-7 years, depending on the quality of the battery and the climate you live in.  And when you alternator does start to go, usually all your dashboard gauge lights start to flicker; including your check engine light.

Just have the mechanic replace the battery.  After he does so, ask him check the alternator and confirm that it's putting out 75+ amps like it should be.  Your alternator should also be putting out 14.5 volts while the car is running.  The actual performance of the alternator really can't be tested unless the car is running.  If the amperage and voltage is within spec when the car is running, then your alternator is fine and replacing just your battery should be ok.
Good luck!