starter staying engaged 98volvo wg64 w/m11 cummings, on 6th starter

Could be something as simple as a bad alternator, if it doesn’t get proper charge it can make it stick, have had that happen, as crazy as it sounds.
If it’s only throwing 12.5 not 13.5-14.
It was on a C15 cat engine, but it can happen

 

When was the last time the ignition switch was replaced?

 

Solenoid

 

One thing that could be possible is if the starter came with a spacer plate and someone left it off. You could check with Volvo and see if it came with one originally and if it has one now. Cummins sometimes has this plate and without it the starter drive may be going in to far and getting hung.

 

Some of those trucks had a starter control relay in the wiring harness on the left side of the engine.It was toward the rear and about have way up the block as beast I recall. Looks more like a big fuse holder.
Power went from the starter button to this relay. Then this relay connected power from a larger supply to the starter relay. They were bad to stick and smoke starters. You can just replace it as original but I would up grade it to a forklift type solenoid or something. Mount it on the firewall and run a little bit of wire.

 

Low voltage, bad ring gear, ignition? Can’t imagine it being the starter every time, you said it’s the 6th one, right?

 

Did the past starter failures end up with the starter staying engaged?