ISX Slow Start

Good morning y’all, I’ve been dealing with this slow start/sometimes no start problem for few years now. I got a ‘11 367 with an ISX 15. When I bought this truck I replaced the starter and had found that the main disconnect under the driver side cab was broke and the contacts were just laying on each other. So I replaced the cables from the battery to the starter without the disconnect and had all 3 batteries tested. Making sure there was no loss of power. No luck. Did some more digging and had found the the ignition had been replaced and the starter interrupt relay had been replaced with a jumper no relay to be found. It’s been okay over the years I’ve just been careful about shutting it down on the job seeing once it got warmed up it had a hard time cranking and sometimes would crank at all. Here lately in the colder months seems likes it’s getting worse. I pretty much have to jump the truck if it gets below freezing. I’m at a loss and I’d like to solve this problem once and for all before blacktop season.

 

Only one that I know of is the one from the starter ground post to the frame. Should there be another?

 

Replaced 2 of the 3 batteries; had AZ test them. Not sure how much I trust those testers. But anyway. I added a master switch rated for 500 amps. It works but I’m going to replace it with a heavier rated one when I find one. My uncle saw sparks flying from the starter like some one was welding when I would start it. So I tightened everything down on the starter. The wire from the solenoid to the starter was hot to the touch, wasn’t terribly loose but was able to get a turn or two on that post. All that seemed to help quite a bit. It doesn’t stop after one rotation of the engine. It’ll actually gain some speed and start. Still feels sluggish compared to our other truck, granted it’s also a decade newer.