Originally Posted by richlindquist
. . . No glow plugs (I wish!). It looks like it may be part of the Cold Weather Start system - which has been disconnected. This says it is NOT used for 12 volt systems...???
I think we have a winner! The drawing actually says "ONLY 12 VOLT SYSTEMS WITH 12 VOLT GLOWPLUG DO NOT REQUIRE RESISTOR."
If you had lesser voltage glow plugs, the resistor would
be used. If the bus once had the system, there are very likely some parts of it left behind.
I spent some time driving ex-military Chevy CUCV diesel pickup trucks. They had 24-volt starting (and NATO battery boost plugs, one of which got wet and caught fire on the Interstate
) but they had 12-volt chassis systems. The glow plug relay fed 24 volts to a dropping resistor on the firewall, which fed eight 12-volt glow plugs in parallel.
I'm guessing it was part of an emergency escape plan. As the current drawn goes down, the voltage dropped goes down, and the voltage out of the dropping resistor to the remaining plugs goes up:
8 glow plugs ran on 12 volts
7 glow plugs and one dead one ran on 12.8 volts
6 glow plugs and two dead ones ran on 13.7 volts
5 glow plugs and three dead ones ran on 14.8 volts
4 glow plugs and four dead ones ran on 16 volts
3 glow plugs and five dead ones ran on 17.5 volts
2 glow plugs and six dead ones ran on 19.2 volts
1 lonely glow plug runs (for a while) on 23.6 volts
Not replacing a burned-out glow plug obviously shortens the life of the remaining plugs, but when the bogeys come over the hill shooting, something
under the hood gets hot to get your tail out of there, even if the truck is only running on a couple of hot cylinders at first.
If the designer of the Cold Weather Start system had similar ideas, and used any voltage plugs less than 12 volts, there would be a resistor. Using 8-volt or 10-volt glow plugs would not show as severe a voltage rise when others fail as using ones at half battery voltage, but you get the idea.