Aternator or Battery

2003 FORD F-250
136,000 MILES • V8
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ERIC B
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Trying to trouble shoot a battery/alternator issue. I have a F250 Diesel. I charged both batteries and they held the charge for three weeks without significant draining (not a daily driving vehicle). Drove the truck around town yesterday and the batteries were pretty much dead by the end of the day... Does this sound like an alternator issue?

Thanks for the help, Eric
Aug 7, 2011 at 4:25 PM
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CARADIODOC
  • ✓ASE Certified Mechanic
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  • ASE Certified Mechanic
Yup. Measure the battery voltage while the engine is running. It must be between `13.75 and 14.75 volts. If it's low, measure the voltages on the small wires plugged into the back of the generator. If one of them doesn't have 12 volts, look for a blown 15 amp fuse.

If the battery voltage is okay, the generator could still have a defective diode. That would limit its output to one third of its rated capacity. That's barely enough to run the necessities with nothing left over to recharge the batteries. A load test will confirm that, and most professional testers will measure ripple which will be excessive.
Aug 7, 2011 at 7:11 PM
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