Loses all electrical power

There may be a main breaker in the battery box that is failing, or tripping correctly, and then re-setting itself. I am not sure about an IH, but many of the older trucks had one.

 

Follow the positive wire on the back of alternator all the way to the starter junction.there is a cluster of 3 wires that come together and there all the same size guage wire as alternator.they actual come together in the main harness that runs along oil pan on drivers side.one wire comes from alternator+,one comes from starter+and the last wire feeds positive to your fuse panel.all three are probably heavy corroded.this happened on my old 96 ih

 

Ask him if he noticed his volt guage has been charging on the low side lately..

 

If he is losing everything then I suspect a bad connection at either the starter or alternator. The main cab power feed for those is feed from either the starter positive post, or the alternator positive post and then runs into the cab. You could have some arching going on and once it burns out it goes open until it cools down and makes a partial connection again. Right at the fuse/breaker panel behind the passenger side glove box with where the main cable comes in and connects to a brass buss bar. Check all these locations and of course check the main batteries too. The cab grounds at both the left and right side firewall down to the starter and frame, so bad grounds are less likely since there are several paths, but make sure you check them too in case one has been disconnected.

 

Could be the red wire on the battery.