"Will not start" can mean a number of symptoms. Do you mean the starter would not crank the engine, it cranked too slowly to start, or it cranked normally but the engine would not run?
If the starter was cranking slower and slower over a period of days or weeks, that suggests the battery was not being recharged while you were driving. That's a charging system problem, not a starter problem. Charge the battery at a slow rate for an hour with a portable charger, then have the charging system tested. You can start the test yourself with an inexpensive digital voltmeter. Measure the battery voltage with the engine running. It must be between 13.75 and 14.75 volts. If it is low, suspect the generator or voltage regulator. If it is okay, that only means it is okay to perform the rest of the tests, but that requires a professional load tester. Your mechanic will measure the full-load output current and "ripple" voltage. There are six diodes in every AC generator. If one of them fails, you will lose exactly two thirds of the generator''s capacity. 30 amps from a common 90 amp generator is not enough to meet the demands of the electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks.
All AC generators produce three-phase output. One phase will be missing when a diode has failed. That causes output voltage to drop quite a bit during that missing current pulse. That is where ripple voltage is a clue. That is the variation between the highest and the lowest voltages the generator is developing. With a failed diode, ripple voltage will be very high. Some testers actually measure the amount of ripple voltage, but knowing that is of little use. Most testers just show it on a relative scale. "Low is good; high is not good".
Wednesday, July 27th, 2016 AT 7:07 PM