My first post on this site... I just purchased an '85 4x4 longbed with 177K and seems to be in good running shape... just going through routine maintenance on it....
I just changed my transfer case fluid with GL-4 80w90 per my haynes manual... Now I can no longer shift to 4hi on the fly, whereas I could before the fluid change. Should I change it to GL-5? Also, when I changed the front diff fluid it had the consistency of clam chowder. Is it possible it hasn't been changed for years or could it be contaminated with something? The knuckles aren't leaking fluid, so I don't think axle juice was mixing with knuckle grease...

 

Im not sure about the gear oil, but if the front diff fluid was a brownish white color that means that it had water in the oil.

 

T-case can use GL-4 or GL-5 oil, makes no difference:
- http://www.4crawler.com/4x4/4R_TechInfo.shtml#Fluids

Shift on the fly only works if the front hubs are locked, if so it should work fine with either grade of oil. If your hubs are not locked, then you can only shift into 4H while stopped, again regardless of the gear oil grade.

Sounds like water in the front diff, add a raised breather tube:
- http://www.4crawler.com/4x4/CheapTri...html#Breathers

 

the front hubs are unlocked... so that would explain being unable to shift on the fly... I figured shifting to 4hi was still possible, front driveshaft and axles would be pulling but just not engaged? This is my first manual 4wd truck... thanks

 

The t-case IS NOT synchromesh (at least the gear driven cases) and as such, the only way to make the gears (front and rear drive) engage is for them to be going the same speed. Two cases make that so:

One is at a dead stop (both front and rear drive gears are stopped as well).

The other is if the front hubs are locked and that in turn makes the front axle turn and that makes the front ring gear turn and that makes the front pinion gear turn and that makes the front driveshaft turn and that makes the front wheel drive gears turn and do so at exactly the same speed as the rear drive gears and thus shift on the fly works.

But you can use 4H (and even 4L) with the hubs unlocked, using 4L in that situation gives you essentially a 2WD-Low range. I use that on my '85 all the time. Using 4H w/ hubs unlocked really has no benefit but can be done.