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When activating the 4WD dash switch, I could hear the transfer case actuator working and the forward drive shaft locks, But no 4WD.

In my trouble shooting:
I tried everything I found online: Backing while turning, tapping on both actuators while trying to put it in 4WD, etc.
Took the cover off the Transfer case actuator and all looked clean.
The A.D.D. motor sounded pretty weak when apply voltage to it, and would sometimes not even make any noise.
Both Transfer switches tested OK (continuity test).

After a lot of troubleshooting, I was able to get the truck into FWD by: 1) unplugging the A.D.D. actuator plug, 2) With the key on, I pressed the 4WD button, 3) Applied 12V to A.D.D. Terminal 2 (+) and 6 (-), the truck went into 4WD and the 4WD Hi light on the dash stopped flashing and stayed on, I then plugged the A.D.D. actuator plug back in.

Excitedly, I took her out for a test run. Everything worked as it should, wheels all spun on gravel, chirped when turning tight on pavement. I was able to go into 4WD Lo without a problem, then back into 4WD Hi without issue. I then took it out of 4WD and into 2WD without any problems. When I tried to put the truck back into 4WD, the problem was back (flashing dash light and no 4WD).

Thanks for any ideas where my problem might be?
 

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If all the switches and indicators are good sounds like the motor may be the problem. I haven't had the pleasure of working on one of these systems though.
I know you are supposed to exercise the system to prevent issues. Is there anything serviceable that could be bound up and causing the actuator to strain and not be able to engage the front. 2 to 4 is harder than 4 to 4 in my experience. Dana 20 and dana 44s with warn lockomatics. Keeping the system clean and well lubed/ exercised prevented most problems.
 
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