I have noticed that my 2000 superduty with the 7.3 liter diesel has been starting hard in the morning. I checked the voltage voltage right after shutting the truck off when I got to work and had around 13.5 volts. When I got off work 8.5 hrs later I only had about 10 volts. I went home and tested all the cells of both batteries which are 3 years old with a hygrometer and they all tested good. I cleaned all of the battery terminals and checked the voltage and had about 13.5 volts on each battery disconnected from the truck. I hooked both batteries up and checked again and had about 12 volts. Then on a hunch I removed the power terminal from the output of the alternator and got the 13.5 that I had before. This to me means that there is something shorted out in the alternator such as a diode. I does however seem to charge ok. I removed the alternator and took it to the place I bought it from a year ago. They tested it and claim it is good. They say if the diodes are bad they won't charge. I would agree if they are burnt out but in my mind if they are shorted it would still charge it just wouldn't be a true DC output and drain the batteries as this would be a short to ground. I also understand that the PCM and other devices draw current when the truck is off but I wouldn't think that you would see a voltage drop of 1.5 volts (alot of load) with the truck off. I plan to hook up one battery up with an ammeter in series to check the drain tonight with out the alternator connected. Is the auto parts store full of it or do I need to look else where for a short? If everything else checks out fine and it is the alternator are there any ideas on getting the store to replace it under the warranty when they claim it is good?

Thanks,
Eli

 

Welcome to FTE Eli. I've seen several bad alternators pass the test at the auto store. It may take some persistence to get them to retest it, including at higher rpm's to get them to find something bad.

Sounds like you're on the right track so far. Do you have a way to check and see if you have some amperage draw draining the batteries down? There's also a possibility one of your batteries is starting to go. You might consider getting them load tested to be sure. The batteries may show a full charge immediately after driving, but are unable to hold a charge over the 8.5 hr time period.

 

I had a NEW Duralast from Autozone that decided to put out 15.5 to 16 volts and flicker at idle after about 1 year. I took it back and it passed all their tests. I explained what it was doing. They retested it. The manager then came over and asked what was happening. After explanation he said he's seen it before. His tester especially, wont test for high voltage. So... I now have another new duralast.

 

The simplest self test would to be at the end of the day when you're finished using the truck, unhook the big lead to the alternator and then see if it starts in the morning.