Ok, here's my info, question to follow directly.

'03 F250 Superduty 4-door, non-ambulance, auto, with a 7.3 Powerstroke.

For a couple months now, if truck sits for 3 days or so without being driven, batteries go low and I have to charge to start. This week, it sat 2 days, and batteries died. Family on vacation, so I started playing with truck. I found the serpentine belt probably needed replacing a year ago. It was 4 separate belts side-by-side. Alternator at idle, A/C and headlights on, was charging at 12.2->11.9volts. Rev it to 1300 or so, and it comes up to 13.95v and stays firm. This is after GPR relay shutoff time, etc.

Replaced belt because it needed it no matter what. Same thing. No difference in voltages whatsoever. Am I correct in assuming the alternator should meet those basic demands of battery charge+headlights+A/C? And in that case needs replacing? Testing voltages above at batteries. Going to alternator increases voltage by .1 or so. Seems right to me.

I'll add I'm in Florida, and 100% of the time I drive the truck the A/C and headlights are on. Always. So I'm thinking if alternator is low at idle, I may lose battery voltage when I get home and let truck idle to cool down with lights and air still on. May be my reason for dead or weak batts? I did a parasitic drag test and current flow when parked and off is almost nil except when <acronym title="Global Positioning System">GPS</acronym> plugged in, which it has been for several years now. I will run it through a switch from here on out.

Thanks.
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'03 F250SD 4X4 CrewCab 7.3l psd with bully dog 6 position chip, AEM intake, straight piped, Ranch Hand, 228K Miles

 

I just checked my '99 7.3. This was educational for me. Before start the bat. voltage was 12.8. Then start with headlights and A/C on and voltage went to 12.1. After idling about 45 seconds voltage jumped to 12.8 and then over the next minute it climbed steadily to 14.18. This test was done at idle about 750 RPM. Then I tested without headlights and A/C and after start, voltage was 12.8 until about 45 seconds and it instantly jumped directly to 14.2. <?xml:namespace prefix = o ns = "urn:schemas-microsoft-comfficeffice" /><o></o>
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My opinion: It sounds like your alternator is unable to maintain a charge at idle. There are two routes from here.... take the alternator off and have it tested at the "Zone" parts store. I think they are able to test it under load. Next, a bad battery can create confusing symptoms. The first thing I would do is disconnect the batteries and charge them fully individually, and then put a load tester on them and verify that they are good. The "Zone" has a good tester for batteries. One of those tests should tell you something. I think if your belt was slipping it would make some kind of noise. If the tensioner seemed strong when you installed the belt it should not be slipping. If you have a bad battery I would replace both of them. <o></o>
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My truck sometimes sits 2 weeks but I don't have a problem starting it. Batteries are "Zone's" best (yellow) and 2 yrs old. In summertime (Texas) I never wait for glow plug light to go out. I have a high torque starter that I love. Before I got it I had what I felt was a slow start problem even with GP light cycle. It's my first diesel with 233K and I've had it 2 years. <o></o>

 

Agree. Before making any assumptions about the need to replace components, test the batteries and clean the connections to the batteries. Connections can be sneaky and look good, but be poor enough to offer increased resistance to the alternator. This is always a good starting point because it is free!

My two cents worth

 

Don't know what happened to my post above. I said I agreed with preceding post and suggested testing batteries and cleaning connections before proceeding with anything else.

 

Thanks so much for checking your own numbers! I bit the bullet, bought an O'Reilly lifetime warranty alternator, and dropped it in. Numbers immediately came to 14-14.5V, and stay there at idle with headlights and A/C. Only dip is when I fire up the high beams, as I have the lights and fog lights relayed together so everything lights up together.

I really appreciate y'all taking the time to respond and make it easier for me to drop some cash...


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