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73-87 Chevy _ GMC Trucks => 4 Wheel Drives => Topic started by: mechanic58 on September 27, 2014, 12:30:35 PM

Title: NP 203 won't "lock"
Post by: mechanic58 on September 27, 2014, 12:30:35 PM: I recently acquired a '76 model K20 Silverado with a TH-350/NP-203 combo in it. It has been sitting up for a very long time. I recently got it running and driving and have discovered that the transfer case diff will not lock when in Hi-loc or Lo-loc. The shifter is working correctly and all the linkage is hooked up and moves freely. According to all the external indications it is engaging the diff lock - the diff just isn't locking. Is this a common failure with an NP-203? I've had several of them over the years and I have even converted several to part time - with both types of kits that were available. I can't imagine what would be causing this malfunction.

Edit: It's been a couple years since the last time I had one of these apart. I don't recall how the diff lock mechanism functions.
Title: Re: NP 203 won't "lock"
Post by: mechanic58 on October 01, 2014, 11:39:54 AM: Wow, I guess no one ever looks in here.

*crickets*
Title: Re: NP 203 won't "lock"
Post by: Jason S on October 02, 2014, 07:24:59 PM: You might give others a bit more time to respond.

The NP203's I've had were already part time converted, but the biggest issue with either Hi-Loc or Lo-Loc has been with the shifter or incorrect adjustments... You said you've checked those already. Short of adjustment issues, I personally haven't had any problems out of my 203's (untold miles on them).

Will either position lock when you push or pull back on the shifter and hold it at the loc position? Have you disconnected the shifter from the t-case and pushed the t-case lock lever into position?
Title: Re: NP 203 won't "lock"
Post by: Cabcorner on October 03, 2014, 05:45:47 AM: If you are sure the shifter is engaging ,then the next step would be to open up the back of the case and check the shift fork and coupler.
Title: Re: NP 203 won't "lock"
Post by: mechanic58 on October 03, 2014, 06:54:50 AM: I have checked the linkage and pressure washed all the mud out of the shifter lubricated it - it's working as good as any ever worked. None of the linkage appears to have ever been tampered with and appears to be properly aligned, etc. When all 4 wheels are off the ground and I have it in Hi-Loc or Lo-Loc, I can turn the front and rear driveshafts separately from each other - through the center diff. This is a first for me, I reckon I am going to have to take it apart to see what's up. The truck only has 82,000 original miles on it - none of the drivetrain appears to be worn out.
Title: Re: NP 203 won't "lock"
Post by: Cabcorner on October 04, 2014, 05:17:05 AM: You have disconnected the shifter and manually shifted the levers on the case ?
Title: Re: NP 203 won't "lock"
Post by: mechanic58 on October 05, 2014, 05:53:26 AM: No, I have not done that yet. I may get a chance to get a closer look at it in the near future because the transmission is ailing. I may have it on my bench top before it's over with. Thanks for all the input fellas.
Title: Re: NP 203 won't "lock"
Post by: Irish_Alley on November 04, 2014, 11:19:36 PM: ok i have a question have you tested this thing on the road yet? the reason i ask is are you sure its even able to move and the whole thing aint broke. like if you have it in unlock and try to drive will it even turn a drive shaft?
Title: Re: NP 203 won't "lock"
Post by: Irish_Alley on November 04, 2014, 11:20:23 PM: i know this thread is kind of old but i just got back from wife having a baby a couple weeks ago ;-)