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Posted by Ronzo [24.63.212.229] on Tuesday, July 10, 2012 at 14:15:45 :
In Reply to: sorry o/t- chevy military cucv 24 volt question posted by junkyardog [72.71.209.46] on Sunday, July 08, 2012 at 14:36:56 :
Screwy mixed system is due to requirement to jump or be jumped by other M series vehicles which are 24vDC and run military electronics and equipment which are 24vDC. Some of the dually pick ups were designed as radio shelter vehicles from the start and along with the M1010 ambulance had two 200 amp Leece Neville alternators instead of the two chevy 100 amp alternators used in the M1009 (blazer) or (most) single wheel CUCV pick ups.
Grille mounted jumper plug goes direct to starter and the 24 vDC resistor buss to allow jumping. Resistor bank can be by passed and vehicle can be set up to use only lower (12vDC) alternator. One pia failure point is circuit card in glow plug module failing and most vehicles have had the glow plug card eliminated via a push button switch on dash activating the glow plug relay.
If a glow plug fails in 24 vDC set up the glow plugs will fail sequntially with totatl voltage getting through the resistor going up by approx 1.5 vDC for each failed glow plug since it's load vanishes. So 8 good plugs through resitor see 12vDC. One burns out and now resistor sees current drop and remining 7 plugs see 13.5 vDC, so another burns out and so on and so on.
M1010 and radio duallys used a wet dream called a DuVac System, for Dual Voltage Alternator Control System. which caused the upper alternator to switch between 13.4vDC and 28vDC as required to feed the ambulance body air mask filtration system heaters and the upper 12vDC battery or on the radio dually the 28vDC radio load and upper 12vDC battery.
There are a couple of under dash relays to the right of the ash tray also prone to fail and either not allow the truck to start or fail and cause the starter to not drop out of circuit after engine starts causing starter to run until it burns out.
Fun trucks if you like puzzles.
Oh, btw, only connect 12vDC loads to the (LOW, 12vDC source) battery closest to grille. Battery closest to firewall is the (High, 24vDC) battery and 12 vDC loads across this battery causes the high battery to eventually go dead since the upper alternator will not sense the load properly and recharge that battery. Don't make sense, but trust me....
Your mileage may vary
Have fun with your beast!