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I have a 2000 f350 and the 4wd won’t engage. I flip the switch and the light turns on but it isn’t actually engaged then the defrost always kicks on in the front. I remember there was a step by step diagnostic online somewhere before that said if defrost actuated that a specific part of the vacuum system was probably the issue but I can’t remember what.

Or should I just upgrade to some warn hubs and skip the whole vacuum system game lol
 

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I'm assuming by not engaging your just talking about the hubs, not the t-case.

If you have the funds I would just switch them out to manual hubs and not have to worry about it again

If you do, cap the vacuum lines on each side of the axle and at the little pump on the passenger side.
 

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Did you try manually locking the hubs?
 

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If the light comes on that means the transfer case has shifted, so the issue is with the hubs. The hubs are vacuum actuated so when it goes into defrost that means you have a vacuum leak. Manualy locking the hubs overrides the vacuum system, but if it's still not working after that then one or both of the hubs aren't engaged. You can try pulling the hubs and cleaning them, and if that doesn't work, replace them with a good set of aftermarket hubs, and unplug the vacuum actuator.
 
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