I have a 2005 F-150 that the 4x4 is not engaging. I will click in when you switch it to 4 hi and 4 lo, you can hear the transfer cases engaging and the lights come on but it does not feel like it is locked in. I have read that a lot of the issue is possibly from the IEW switch under the hood, is there a way to check to see if that is working first.

 

There is no way that I know of to check it other than replacing it. They usually run $25 to $30 at any local auto parts store.

 

I jus did mine YouTube checking ford 4x4 there is a ton of info on how to do it.

 

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Use the Search button - LOTS of great information. Here is a thread from less than 2 weeks ago, worth looking at. But there's many more threads as well about all things 4x4:

https://www.f150forum.com/f4/4wd-not...advice-167768/

As a quick note, the vacuum solenoid on the firewall that you mention does not seem to be the problem based on your symptoms. Sounds more like fried axle hubs (which is not good news) i.e. IWE hubs. But I could be wrong.

 

IWE Solenoid is what you are looking for.. and if you disconnect the vacuum line .. this will allow the wheel ends to engage and stop making noise.