control box

i posted this in the 5 ton section also. can anyone explain what the control box located on the left side of the fire wall in the engine compartment on the 800 series does ? i am having an intermittant short and im trying to isolate it. also i see talk here of a wiring diagram for 800 series trucks. cant find it on this site. does anyone have it on cd or in print that they would sell me?

It's called a protective control box. It controls the starter based on the oil pressure (it uses the oil pressure idiot light sensor). This prevents the starter from being engaged while the engine is running. The electronic TM TM 9-2320-260-20 has the wiring diagram that shows the wiring diagram. The wiring diagram is in the back, page 1328 in the PDF.

My TM 9 2320-260-20 shows the wiring several ways as follows (images).
Among other things (?), it protects the electric loads and alternator from a reverse polarity condition with a diode connecting one relay coil to ground. A reverse polarity here would keep the relay from being energized.

thank you men. i knew it had to do somthing just judging from the number of wires in and out. my short is not good enough to trip a breaker or smoke a wire. makes it diffacult to locate.

mv mag about 15 mounths ago had a rebuild write up on this box, went into great detail on the inside parts and wireing, hope this helps

i will dig back in the stack. thanks Ron.

The author sent me a copy a year ago. I don't have it now, I think Dave and Joe have a copy. I'll see if I can find the address of the chap that wrote the article.