On my 2002 7.3 liter for the last about 1 week it has been getting harder and harder to start. Up until yesterday when it will not start at all. What it does is this. A week ago I noticed it cranked slightly weaker but still ran perfect once started. This has progressed slowly but definitely. It has never gotten better, only worse. Even with jumper cables hooked up it does not improve at all. The starter is a Carquest POS but only has 50 thousand miles on it. I have driven hundreds of miles with this problem in the last week and the truck has never skipped a beat aside from the starting issue. I took the batteries in to no less than 3 places and they all said the batteries are good. However when I put my multimeter on them when it is being cranked the volts drop from 12.6is down to about 8 volts. When I did get it started the other day and put meter on it the batteries were charging up to around 13.7 or so. When there is no load on batteries they show good but have been unable to get a solid answer if any of these places are load testing them: I am convinced they are not. I checked the resistance of the cables and they all have none. The grounds are all good. So I pulled the starter and took it in as a last result. They say the starter is good also! The connections were all tight and I cleaned them all very well. I am at my wits end!!

 

Sounds like the battery is junk. You have to load test a battery to see if its good. A volt meter is only good for voltage but you need to read the amp draw to see if the battery has enough amps to spin the starter. It will also tell you if the starter is pulling to many amps. If your battery is old than i would just replace it anyway.

 

At the first sign of cranking trouble always put the battery on an outboard charger till the source of trouble has been remedied. A battery that has trouble cranking is nearly dead and may be ruined if not recharged asap. The alternator will not do this.

Check for loose or corroded cables and replace periodically. Ensure clean, tight grounds at block, frame, and firewall. In the case of a dual battery system it's important that they are closely matched, electrically it is a single large battery. They are always replaced in pairs.

Battery cables cannot be tested for resistance in the manner I think you described. A voltage drop in the circuit can only be measured by testing the cable or connection under load, in this case while cranking the starter. A positive voltage exceeding a tenth of a volt or so is considered excessive.

http://www.aa1car.com/electrical.htm

 

Grounds,Grounds,Grounds! Start there. They are your friend in a neg ground system. First thing Id do is to clean all of them even the little ones bolted to the inner fenders. You want all connections clean bright and tight. Then spray battery terminal spray over them once tight to keep the connection good. I spray ignition sealer on the fender bolts and others that dont move often. Dont use the sealer on the battery terms, only protectant.
Clean and tighten all the secondary (large battery cables) cables too. Ohm the cables. A cable can visually inspect ok but have high resistance. Pull the starter and bench test it for high draw. Ill bet your problem is the starter and the cables. If the cables are questionable, new batteries will mask bad cables until extreme conditions require max amp draw. Same applies to installing new batteries with a weak starter. Bad cables will accelerate deterioration of the batteries and starter.
Did you verify the solenoids OK? Thats another problem spot.
I just "redid" all the cable ends with hi-press crimp on connectors and shrink tubing on my friends 99 350 dually. We also installed a new starter, as the big box chineasium junk that was on there which was 4 months old was locked up. Truck starts like a new tuck. One rev it fires. Its a 7.3.
Make sure if you truck had/has any ground straps from the block to the fire wall that its present and clean too.

 

I'd have your batteries checked at an auto parts store. I had a hard/no start problem for awhile. 12V engine off, 14V engine on. Finally gave up on troubleshooting and took them to O'Reilly Auto Parts and had them tested. 11 of 12 cells were dead. I had the potential of 12V but no CCA to turn the starter.


Just my .02

 

Seems like a lot of trouble hauling batteries around. A simple voltmeter will do the same thing. Here's the "poor man's" load tester.

Disable the engine safely, so it won't start. If the battery is bad, it probably hasn't been starting anyway, right? Anyway measure voltage while cranking engine for 10 to 15 seconds, no more. A healthy battery will not dip below roughly 9.6 volts, with any number of bad cells it will choke. Badly.

Midtronics (and others) make midlevel or entry level battery analyzers using the AC conductance method for not too much money. A little fancier than the carbon pile method. These will weed out defective batts quickly, without the need to charge/recharge before and after testing or torture the starter and battery. They are slick, and in fact this may be what NAPA or other firms are using for testing or warranty service and such.

What's nice about battery technology is that it was fully matured about a hundred years ago and so the science is well known and all hashed out. All anybody really needs though, is an accurate voltmeter and a temperature compensated battery chart and yer all set.