My NP231 transfer case has been real noisy and the fluid in it was shot. Doing a little research, it seems that some folks had good success using 75w-90 gear oil rather than the recommended ATF.

I filled it with the gear oil and it is way quieter and smoother running.

Should I expect any drawbacks to using the heavier weight lube? (I am OK with losing a tiny bit of gas mileage...)

 

The rear seal on the case is leaking too, and the thin ATF just oozes out of it. My hopes are that the heavier gear oil won't ooze out of the seal so quickly...

 

Why not just fix the seal?

 

People have had success using *engine oil*. Gear oil is WAY too thick! The oil pump cannot move nearly as much of it versus the factory specified ATF. I see you're in Prescott - have you seen how thick gear oil gets at 30°F? The planetary gears are ALWAYS turning and the oiling passages in the main shaft aren't exactly generous.

Why not replace the output seal? The rear seal is cheap and easy and increasing internal wear is not a good trade-off.

On second thought... if your transfer case had a noticeable reduction in noise when you changed to a thicker oil it needs a rebuild anyway...

 

I agree. Sound like you need another t case to find. Cause since it needed rebuilding but now that you starved the other parts of t case with thicker oil having a hard time going through the oil passages.

 

Yup you took one easy problem to fix and went to a complete rebuild needed now or at least replace with another used one.

When there is one type of fluid stated to use by the car company for an application its for a reason...........You just found out why

 

I think I will drop the gear oil out and put some 10w30 in it. I am gonna look into rebuilding it, but I gotta come up with cash for it. I am gonna replace that seal in a few days...

 

10w30 is still too thick when it's cold. Synthetic 5w30 is what's been tested as good

 

^^ What he said.

I hd and article about using motor oil in the TC but I can't find it now. CRS!

 

Yep. It was published by Rockland Standard Gear in 2003 on how the NP231 has a habit of evaporating ATF. This is why so many wind up running dry and eventually failing. Pages 5 & 6 of this PDF: http://www.rsgear.com/articles/2003_10.pdf

 

I had to change the output shaft seal on mine (twice) because the vent was clogged. be sure to check this when you do yours. it took the new seal leaking for me to investigate further. what sucked is that it leaked out the back and got thrown onto the cat. talk about a terrible smell. it had gear oil in it when I first addressed it, most likely because it was leaking due to the vent. gear oil burning on the cat is a horrid smell. I put synthetic atf in mine. I had to add about half a quart after 35k miles for unknown reasons, it is not leaking. perhaps there is some solidarity to the evaporation claim.

 

Yeah, it's nasty, and that reminds me of a funny story. I few years ago I was driving back to DC from a track event at Watkins Glen. My friend's old Audi 4000 track car started blowing a center diff seal a few miles on the road and was dumping hot oil on the exhaust. We pulled off the road to climb underneath to see where the nasty smell and leak were coming from. It was only after we lay down in the dirt and stuck our heads under the car what we noticed the dead and rotting deer carcass lying a few feet away in the grass. We found out the hard way that one of the only things worst than burning gear oil is burning gear oil and rotting deer.

 

Good article.

The one I had/have was different but also proclaimed the use of motor oil in TC's was the way to go.

 

That vent is a crazy design (pic). Mine was clogged as well so I got rid of it and used a front differential vent fitting in its place and moved it up the firewall. Moved the AW4 vent up as well.

Pics.

 

I needed to find this article about a month ago when I did my sye. I know one person that has motor oil in there tcase but she bought it like that. And she even asked me why. I looked all over the place to see if it worth it or not, or which would be better of the two.