Cat 420D 4x4 is not locking. I have changed the switch,the surpressor that’s inline with the switch, the solenoid, valve, fuse and relays. There is 12v going to harness that plugs into the solenoid. I have also checked the grounds. Does anyone know what engages the 4x4 I see there’s a copper tube that goes from the top of the transmission to the bottom of the transfer case. Don’t know what else to check besides going into the transmission
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