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I've used this test to troubleshoot the GP relay more than once. It is a common failure point as it carries over 100 amperes and cycles at least 3 times on every cold start.

Measuring over the Red to Orange wires should be ~24 volts except with glow plugs cycling. Red.to orange is zero when cycling. Red is 24 volts to ground except when glow p!ugs are cycling, the voltage should be ~12 then. With GPs on, the orange should be ~12 volts to ground.

The firewall resistor is a ballast resistor which divides the 24 volt output of the two batteries to 12 volts if all glow plugs are functional. Voltage output will rise when glow plugs fail. Voltage over the solenoid input (red wire) and output (orange) will be ~24 volts when engine is off...