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There have been many issues with the 4x4 solenoid on the firewall causing many different 4x4 issues. One of them being the 4x4 light may not come on. To properly diagnois, the vehicle should be scanned for trouble codes in the Generic Electroincs Module or GEM. C1979 and C1980 are the codes that would be in the module.
The problem lies with water that runs off the cowl panel and gets injested into the solenoid which causes the problems.
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Different expert here. I skimmed over all of this and may have missed this response. I need to verify, when in 4 high does the dash light show its engaged, when in 4 low do you get a reduced gear ratio as well as a 4 low light?
You asked about the wire diagram is that all you want here?
OK without doing any diag so far it looks like multiple issues. Even if the front hubs are not locking in the light on the dash will still work. The front hub lock in components have nothing to do with the light on the dash. The encoder plate inside the shift motor on the transfer case turns on the lights. So its looking like a faulty shift motor. Now if you also think its locking in but its not locking i n the front axle then odds are you have a faulty integrated wheel end IWE solenoid mounted on firewall next to the battery. However you need to check the IWE hubs and the hoses as well for restrictions. On this model vacuum is applied to the hubs to keep them unlocked. Below is some Ford info that can help out.
Here below is how to diagnose the light issue.
Below is the wire diagram associated with the tests.
Are you sure you went through the test correct?
When the light is blinking is it actually kicking out of 4wd high and not working or it is just the bulb blinking from a possible bad connection in the cluster?
So it only flickers the first time you go into 4wd high then it just stays off but you do have 4wd and drive to the front wheels?
Thanks for the update, you taught me something as well, I never knew the IWE solenoid had any effect on the light, I have seen way to many solenoids bad and the lights still worked. The only thing I can assume was a internal issue and it was throwing back feedback messing up the module, any who great job and I am glad you figured it out.