1996 F250 power stroke diesel 7.3 2wd auto
2 batteries, I put in cheap temp batts just to get it sold (with appropriate notification) so I have to keep charging them time to time as it just sits for weeks
I put on my auto trickle chargers and after a day they're reading green so theoretically charged
still won't start
I test with meter both batts 12.6v
so I have volts but no amps?????
I don't have someone to hit switch while I test to see what happens to volts when key is turned
I don't know how to test amps instead of volts
Is it possible to charge a batt to volts but have no oomph ? or is 12.6 not even enough for that engine

 

Is it no crank no start or crank no start?
Trickle charger in theory is not really a charger, it's just a maintainer to avoid the SOC (state of charge) to drop. If parasitic draw beats your trickle charger your SOC is still going to drop.
You have 12.6V which looks good, but if your cranking voltage is below 9.6V you still could have difficulty starting. You need to find a helper so you can capture the cranking voltage while the helper is cranking and you are measuring the battery voltage.
Another thing to check is the cables you should also inspect and clean. Also put it on real battery charger like noco or ctek and slow charge it once a week.

 

You are on the trail and here is why I say that
We all have a pretty good idea of what would happen if we tried to start an engine using a piece of wire that will operate a bulb and we know it would never be enough power to start an engine.and that can be pretty easily seen. When we try that where we can't see that, it gets a little confusing but the result is the same.
Either the conductor (wire) is too small or puts high resistance on a wire or connection the power just can't get through. It might be available at the source, but because of the resistance to flow either we get nothing or poor performance.

A defective battery may show 14 volts and yet cannot deliver the power required. By the way that flow is called AMPERES (amps) Even a good fully charged battery can't provide the amps required if there is high resistance in the path. That resistance can be insufficient diameter of the wire,corrosion, or poor connection (s).
Anyway, I hope some of the above will help when confronted by a problem that may not be easily seen.

Most ordinary test equip cannot handle more than 10 amps going through it and if tried will blow the fuse inside the meter . Those fuses are special and now we have a meter we can't use until we replace that special fuse.
One thing that will help is, voltage is like the pressure pumping water through a pipe. The amount of water going through is the flow (amps)
Enough for this long winded story, hope it will help understand those things we cannot always see..

 

thanks,
no crank...i'll put a charger on it and clean cables

 

Cranking voltage simulates a load test on the batteries. While it is better to charge and load test them individually, monitoring the cranking voltage is an easy first step.

Dealing with this as we speak on my sons truck. Voltage looked great ...... took a charge great. Put it on a tester and cranking amps were below 100 and state of health was almost zero (even though state of charge was 100%).

 

You said no crank look at the diagram below and locate the "starter relay" on the left of the diagram:







When it's no crank no start again, above diagram locate circuit 32 (R/LB - Red/Light Blue Wire) at the relay terminal and connect to the red lead of your voltmeter, the black lead of your voltmeter connects to the engine block with good ground.
Find a helper and crank, what voltage you get at your voltmeter?

 

A little tip with something to consider.
A lot of folks take a voltage reading when starting the engine at the battery POST. Everything could look great there, but the better place to do that check is at the battery cable terminal not the battery post.
If there is any unseen corrosion between the battery post and terminal , the reading will not be accurate .


By the way the diagram previous to this post is inaccurate.
It says the starter relay is closed when it is not closed in the diagram. ..

 

my update is that I put 2 new batts in and same thing, click - no crank
I'm thinking to look at starter relay next then starter solenoid
or maybe starter just magically stopped on a dead armature lead

 

Heiko !
I just saw your diagrams and notes, thanks very much - awesome diagram
that's where I'm headed

Ok with key on and wait to start light off, I jumpered starter relay posts and get click but no crank
That, I believe, shows that it's not the relay that's bad...is that correct??

unfortunately i have no second set of hands to crank while I read voltage...I'll have to get some long leads to bring vm into cab when i crank
but when I have key on and read voltage at the input lead to the starter relay I only get 4 +/- volts which seems weird...
bad ground somewhere?

whats' weird is it just happened out of a clear blue, it always started fine as long as the batts were charged, even the old ones that I just replaced
it started a few weeks ago when I moved it from one side of driveway to the other, then next time i try, nothing...:-(

 

You are not supposed to test the relay yet, you are testing just the trigger wire to the relay, the very first step. Just testing the R/LB wire for now while cranking.
If you get 12v while cranking in START position, what voltage you get?
If no voltage at all continue to step (2) below:

2) Check fuse no. 5 at engine compartment fuse box it's 15A as shown below:




Is the 15A fuse blown or not?