I am using a Cat 920, about 1968 version. Wiring has been messed with in several places.  They were previously jump starting the solenoid from the ground,  with the solenoid wire disconnected. I have a question on the start and glow plug switch. SHOULD THE STARTER CRANK WHEN THE SWITCH IS TURNED TO GLOW?  It cranks when I turn to start as it should. But I was expecting the starter NOT to turn in glow mode, as in old John Deere , Kubota, IH etc.   There. is no glow plug indicator coil on the dash either. Maybe it it correct. 

Seems there are two types of start/glow switches. Those that have separate glow and crank and those that have combined glow and crank in the start position. I don't know of any that crank in the glow position.

if you could get a wiring diagram it would be easier to rewire it then try and get the old wiring to work.

this old girl would be fairly basic electrically.

Old beek.   That to me would be simple just to wire sorta in my head. I personally would want my glow plugs on momentarily before I cranked on it. So I would keep the glow plug circuit completely isolated from all else even if it’s on the same physical switch.  Even if the glow plugs are energized while cranking the starter would rob the voltage and they would not help anything.  And by the way all the new equipment I have dealt with lately have glow plugs on before start, on a timer. Then when the timer times out the light goes out and you know to crank. Sir I hope I gave you an idea and best of luck. Mike Hudson 

even if the starter draws the voltage down to half you would still get half of the glow plug output

not really the glow plugs draw the same wattage no matter the voltage what will happen is you blow a fuse or burn them out due to lack of power when voltage drops current increases for the same given wattage.

and i would do as mike suggests wire them in with a glow timer you can get aftermarket ones quite cheaply. a separate switch to control the timer controlling a relay power the glow plugs. with a pilot light to show glow status.