Np 231

[reomack]

New user here. The 4wd goes into 4 high, neutral, and 4 low, but when put into 2wd all I get is a very disturbing loud grinding noise with no vehicle movement at all. This occurs in drive and reverse. If motoring in 4wd then shifting to 2wd, vehicle slows and grinding starts. I have taken the unit out and taken it apart and could see nothing broken or out of the ordinary. Can anyone out there help me? PLEASE

[reomack]

Well, for anyone following this thread, the answer to my problem is that yours truly is not very observant and maybe not very smart either. When swapping parts from my old unit to the new (wrecking yard) unit I neglected to measure the output shaft splines or even to count them. The new shaft is about 1.5 mm less in diameter than the one in the old unit. So what I have to do is take both units apart (again) and swap the output shafts. Then hopefully the problem will be not only identified but corrected. Please keep your fingers crossed for me.

[fredjacksonsan]

Apologies for the slow response but it sounds like you've nailed it. Post back and let us know how it's doing.

[reomack]

Well, I have taken my bitter pill and the 4WD in my truck is back in commission. After disassembly of both units to remove the main shafts, I found that they were different in more ways than just the splines for the driveshaft. The diameter of the shaft under the drive sprocket was different and the new drive sprocket had two caged needle bearings pressed into the sprocket itself while the old one did not. I needed to use the new one because the new chain was the wide one whereas the old chain was the narrow one. The replacement sprocket would not fit the old shaft until I pressed out the needle bearings. I measured both old and new sprockets and without the bearings they were the same. So, long story (somewhat) short I put it all back together and it works. Just appended this for information purposes in case anyone here had to do an NP 231 in the near future. They might look the same on the outside but be prepared for some surprises on the inside.

[ftd carlito]

Nice that you found the issue. I have both a 4 cyl and 6 cyl 231 sitting in my garage and the output shafts are definitely not compatible, you might check the splines on your driveline to be sure its not ruined as well.