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Re: late model 4020 wiring
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Accessory circuit gets it's power from the "A" terminal on the keyed ignition switch. Looks like it passes through the the upper of 3 circuit breakers, then to the lighter and fuel gauge.
There's a wiring harness connector involved in that circuit.
The light switch uses two separate battery feeds, through the lower 2 circuit breakers to "BAT A" and "BAT B" on the light switch. One of those terminals not being powered would explain half of the lights and the gauge illumination lights not working.
With keyed ignition switch "ON", check for power at it's "A" terminal, then at the group of 3 circuit breakers.
The upper breaker (if no one has switched them around over the years) should have "ACC" power at both sides, the other two breakers should have battery (unswitched) power to both sides to feed the headlight switch and both the "BAT A" and "BAT B" terminals on the light switch should have power.
If you need more help, post back.
Also, there's some serial number breaks in the "late 4020" years as to the wiring. It would help to know the serial number.
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