I don't drive my truck much, i try to run it at least once a week, the other day i go out to start it , it turns over for about 10 seconds & then "clunk" the starter locks up tried it a few more times that day same results. i think that the starter was engaged but could not turn over engine, so i ckeck the batteries & they are both low. i charge the batteries ( both are NAPA about a year old ) and the truck starts, so i'm thinking maybe the alternator is weak, had it checked it is putting out 15 volts. Could something be draining the batteries
If the truck is going to sit allot I would put a battery power switch on it. Like equipment has. It doesn't take much to drain a battery. The reserve capacity isn't much on these batteries.
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79/83/86/95 F-??? 6.9 4speed
94 350 4x4 SRW 7.3 factory turbo
1930 Model A Original
My wife has three of her own.
15V but how many amps? I'm sure whoever checked the alternator checked this too and it was OK but I'd make sure, you never know.
My Cherokee was draining the battery every couple of days if I didn't drive it everyday. Turns out my glovebox light was on all the time because the mounting braket came apart. I never did track it down I just got lucky one night and noticed the light through the windshield from the outside. If I hadn't seen that I might still be trying to figure it out.
I'm sure that's not your problem but wierd little things like that can cause the problem.
When you say you try to run it once a week, does that mean you are starting the truck and let it run for while, then shut off?
If so, it takes more time than that (especially at idle) to put a charge on the batteries. A few weeks of that and the batteries will do just what you describe. In colder weather it takes more juice to start than would be replaced by a 30 min idle.
There is a fuel relay that will cause a battery drain if it sticks open. it happened twice on mine, I replaced it for a GM fuel relay and solved the problem.
The relay is on the driver side in the engine compartiment. If the relay gets wet, it eventually short and it will kill your battery.
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Black and chrome beauty, Off Road F250 roarring diesel, 1991, 4x4 and still going strong.
Don't count of someone at a parts store knowing to check the amps.
I was having starting problems and had the charging system checked.
When they load test the batteries, they have to be checked seperately with the battery cable removed from one of the batteries.
If your batteries are low, the alternator should be putting out at least 80% of what ever your alternator is rated at.
On my test, the person doing the test says, "Your batteries are low, but the alternator is charging 14.5 volts and 5 amps."
Excuse me, that is a 150 amp alternator, it should be putting out more than 5 amps if the batteries are low.
They insisted that the alternator was fine, I needed to charge the batteries.
I installed a new alternator and everything was fine.
Depending on what a previous owner has installed, you can get alternators rated anywhere between 56 amps and 150 amps for the diesel application.
Fuel relay on a 7.3 IDI?
Never seen one.
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86 F250 HD 6.9 IDI ATS turbo "not exactly" stock 4x4 T19 BW1345 3.55LS both ends D60 front, 10.25" Sterling rear, ram air, dual stacks.
I would guess that when you run it your not running it long enough, and not generating all the power back you used by starting it. My truck will be dead if I let it sit for over a month or so, when I travel, You don't have to have a switch if your not doing driving it much, just take a cable off each battery, its about as easy as installing the switch.
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1997 F250 Crew Cab 4x4
1988 F350 7.3 Crew Cab 4x4 Dually C6 w/ GearVendors Overdrive currently in a deep sleep until motor overhaul is finished. 1984 F250 2 wheel drive 6.9 Extended cab 4 spd manual. (3) 1976 F250 Reg Cab 4x4 All 390, two C6 and one 4 spd.
I agree.....? please elaborate a little Kidoo I had 8 tired Ford Pick Up's (IDI's) stripped to their bolts in 2007. Mainly for donations to the IDI cause my brother in law and I are still driving. Don't recall that one.
something draining batteries
1997 F250 Crew Cab 4x4 
until motor overhaul is finished. 1984 F250 2 wheel drive 6.9 Extended cab 4 spd manual. (3) 1976 F250 Reg Cab 4x4 All 390, two C6 and one 4 spd. 
