Does the battery go dead while you're driving? If so, the charging system isn't working. If you start with a fully-charged battery you'll only be able to drive about a half hour if the charging system is totally dead. The alternator could also have one bad diode of the six. That will reduce its maximum output to exactly one third of its rated value. That's not enough to meet the demand under all conditions and the battery will have to make up the difference until it slowly gets drained.
Start by measuring the battery voltage with the engine off. You'll find 12.6 volts if it's fully charged. If it's around 12.2 volts it's okay but discharged. At closer to 11 volts it has a shorted cell and must be replaced.
Next, with the engine running you must find between 13.75 and 14.75 volts. If it is low we will need to do some tests first but suspect the alternator. If the voltage is okay, have the charging system professionally load-tested to see what the full-load output current is.
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Friday, May 24th, 2013 AT 4:53 PM