”This master switch issue has cropped up” what exactly does that mean? In The OPs post, his wouldn't shut off, I am getting the impression that yours won’t turn on? The A0 uses 12v batt power thru Circuit Breaker 70 and the ignition switch to energize K2. K2 provides 24V ignition power to a bank of fused circuits to operate the truck. so I would start with looking for battery 12v at CB70. Remember voltage is not good untill it is tested good under load. if CB70 shows battery voltage, measure it again after you turn on the main switch. If CB70 voltage falls off when you turn on the switch, you have a bad connection somewhere back along the path to the batteries. The most likely spot is the polarity protection box between spare tire and air filter as it is out in the weather.
Take along an assortment of automotive blade fuses. Make sure they are the type with exposed metal spots on their top edge. These will plug in where the Circuit breakers fit and the little metal test points on top allow you to easily sample the circuit voltage with the fuse installed.
if CB70 voltage stays good with switch on, K2 should go click when you turn on the switch. If not look for 12v from the ignition sw at pin 86 in K2s socket. If it is good there, K2 may have a bad coil. Remember it is a 12v coil relay.
Below is a simplified/condensed A0 schematic that I drew of the ignition and start circuits for the A0.
When you get it home, scrap the 8D batteries and install a pair of group 31’s, a small equalizer and a float charger(solar or plug-in).. Like the four 6T batteries they originally came with, a pair of 8Ds is way too much battery for the 100A alt to support. Cat specced a pair of group 31s for the equipment that uses these engines, that is all it really needs… The scrappers are charging unobtanium prices for the 100A dual volt alts($2500), and oversized batteries can place that alternator at high load and high risk… the A0 also has a 12v vampire load thru CB22 to the transmission controller. That + oversized batteries + setting more than they run = failed batts and alts and a good chunk of the electrical issues these trucks experience.
Good luck…
