I have checked the 4wd actuator vacuum and lines, they're all working (in good shape, no leaks in lines). They don't seem to be engaging though. My blazer's 4wd selector switch lights are on and neutral is red. I'm not sure if it's a problem with the selector or the encoder. I'm thinking it could also be a problem with the transfer case control module. I'm not entirely sure.
My car has 180000 miles.
My car has an automatic transmission.
The transfer case module is on the computer network and capable of storing diagnostic codes. Your system could have any of the three problems you stated and also wiring problems.
The system would need to be scanned and diagnosed for the codes retrieved. You would save money in the long run getting the system diagnosed instead of throwing parts at it until it starts working again.
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