……… from my reading I understand that the whole resistor deal was to facilitate slave starting. I fail to understand how it helps for that ………………
The 12v glow plugs seem to be sensitive to high voltage as evidenced by the cascading burnout effect.
It is possible for all of the glow plugs to see more (or less) than 12 volts while slave starting with resistor bypassed.
The following might be the reason for the dropping resistor:
Batteries charging in series drop voltage just the same as resistors in series.
One can consider two extremes while slave starting from a 24v source.
If the rear battery is COMPLTELY shorted: No voltage will be dropped, at the rear battery, resulting in the front battery and the entire 12v truck circuit seeing 24v.
If the rear battery is COMPLTELY open or missing: All voltage will be dropped, at the rear battery, and the front battery and 12v load will see nothing.
Anything in between these two extremes is possible while slave starting.
In a situation where the rear battery’s charge state (resistance) is lower than the front battery, it (the rear battery) will drop less of the 24v and the front battery will see the remainder. With the resistor bypassed the glow plugs will see a higher than normal 12v and possibly be damaged. The reverse is true if the front battery is lower than the rear.
This battery imbalanced will not affect the glow plug voltage while the resistor is in series with the 24v slaved source, assuming all glow plugs are good and connected. In a complete and well maintained circuit the glow plugs should not see more than 12v as the resistor should drop the other 12v.
I’m not saying the bypass is a bad thing, actually it’s is a good thing -- just be very careful when slave starting the dual voltage CUCV -- follow the TM
The TM -10 clearly states:
"If one battery is missing, DO NOT attempt to slave start."
"Wait 3 to 5 minutes after hooking up slave cable to "dead" truck, before attempting to start it. Damage to the truck’s electrical system may result if the truck is started sooner."
See: TM 9-2320-289-10 Section VI 2-86 Page 125 in the pdf
This wait time is an attempt to allow the batteries to receive charging current form the source/slave and become somewhat balanced allowing the 12v circuit to see a voltage close to the designed 12 volts.
Be very careful when slave starting the dual voltage CUCV -- follow the TM.