Vehicle:

1986 F100 4x4 Lwb cab chassis with tray.

Rear diff 9" with Detroit Tru trac
Front diff Dana 44 with ARB airlocker. airlocker currently not working as no air available on truck so effectively an open diff.

NP208 transfer case

ZF5-42 gearbox

7.3idi diesel

Now the question.

Wet grassy area very slight uphill no issue when dry in 2wd. In 4wd hi and slowly let out the clutch with no accelerator. Rear wheels start to spin so tru trac is doing it's job as both rear wheels are spinning. Vehicle just sits there and no spinning or movement of the front wheels. The same happens in 2wd so almost as if 4wd not engaged.

I've checked the following.

Front hubs locked, transfercase in neutral, gearbox in neutral, wheels on the ground can not turn the front drive shaft.

Unlock the hubs, driveshaft turns freely. So hubs are locking.

Put transfercase in 4hi, can not turn the driveshaft and can feel some movement in the rear drive shaft when trying to turn the front shaft. However the front shaft rotates about 10deg but the rear is just a slight movement which is only detectable when you put your hand on it.

Same in 4lo.

My impression with the NP208 is that the front and rear shafts are locked together by the chain and sprockets when in 4wd and with the front diff open 3 wheels would have to spin to have no forward movement. Only the rear wheels are spinning and no movement of the front wheels.

Any ideas what could be going on.

 

Yes the NP208 is chain driven. Sounds like a t-case issue to me. I'd drain it and see if you get any pieces. Dont think that case was ever offered behind a diesel, so there is a chance it took a dump.

 

Thanks.

It's the first time it's been used in 4wd after complete rebuild so I'll see.

Been raining here for weeks now and the ground is sodden and I need 4wd to get from the shed to the road.

 

Well I checked the fluid in the transfer case no bits of metal etc.

I then disconnected the rear drive shaft so I could test a couple of things on the front.

Put in 4hi and hubs locked, no movement, 4lo the same.

Get the video camera out and set it up so it's looking at the front driveshaft.

Try again, and can see from the video it's turning in 4hi and 4lo just like it should.

Connect up an air line to the ARB locker in the front and try again. It moves in both 4hi and 4lo forward and reverse.

Disconnect the air and no movement again.

So obviously something has happened to the diff.

I'll pull it out later in the week and check it.

In the mean time any ideas on why it works when locked but not when open? It's an ARB Airlocker, in a Dana 44 diff.

 

I got a break today so started looking into this.

Got one of the other centres I have ready to install.

Put the front end up on jack stands.

Removed the left hub, caliper and rotor.

Removed the right hub and caliper and then I'm looking at the inner axle shaft on the right hand side. It does not seem right.

Compare with the other centre and it's obvious the inner axle is not all the way into the diff.

So push on it and it moves in another 50mm (2in).

So put the hubs back on and check the movement, all as it should be.

Reassemble rotors and callipers and off the jacks to try it out. 4hi and 4lo all works correctly.

When I've put this together I've obviously not pushed the shaft all the way in but it still leaves me with a quandary. What's to stop the inner shaft from moving out again. On an open diff the c-clip on the inner end holds the inner shaft in to the diff.

Finally found an installation manual for the air locker here why arb does not have them on their site I don't know.

Now I just need to find a suitable spring, I'm sure I have some old valve springs somewhere.

The spring is inserted in the female spline of the inner axle shaft and when the outer axle shaft is inserted this applies pressure on the inner axle to hold it in the diff.

If your looking for an ARB locker for your Dana 44 IFS then it's an RD-117 for 3.73 and down gears.