Hi,
I need some help on a starting problem on my truck i could use some help with. It a 1997 F250 HD 7.3L with 140k only mods are bigger airbox and new exhaust downspout. About 2 months ago it started acting like one of my batteries was going bad, slow crank and then start. After driving it would start fine with normal crank. The problem then went away and i forgot about it. About a week ago it started doing the same thing, then yesteday it would not start at all, it would slow crank and not start then stoped cranking just like a dead battery. Tried to jump didnt work. Had the batteries load tested both are perfect. Thought it might be a bad starter as now it will not even slow crank, i took it off and had it tested at autoparts store and they said it tested good. Checked all power and ground leads and they are good (relays sound like they are switching) and volt meter shows power at starter when swithed.
Can the starter be bad under load but test good at the autoparts store?
What else could it be or should i check?

 

Yeah the starter can be bad and not show a problem unloaded.

You said you checked all the power and ground leads... ALL of them, at both ends, for corrosion and looseness?

What I would do is put a voltmeter on each of the batteries, and watch the voltage while you have someone else engage the starter. Then do the same at the starter, using the engine block as ground. Should be almost the same voltage in each of those places, if not then you've got a problem with one or more cables/connections.

If you DO have good power at the starter, then it's off for a rebuild. Stay away from AutoZone/Advance/discount auto chain store starters, even the new ones. I went thru 3 of them on my old truck before I finally got one that stayed together.