Auxiliary transmission for heavy haul?

Short answer is that it’s a small gear and spinning that fast with that much load on it will cause the heat buildup and cook the bearings.

 

I imagine you’ll never have problems if you put it in 4th last and drop out of it first under a pull.

 

@Dirk Diggler Thanks for the response.
Only 4spd experience was 5&4 way back when. ‘Ol driver helped spec this out.
I’ve always swapped parts before they break, but this trans seems to be a freak of nature according to all I’ve read.
Temp has always stayed around 190, btw it’s water cooled, maybe that helps.
Any incline I’ll start in 2nd or 3rd, grab a couple in the main, shift auxiliary up drop one in the main and do that again til about 35mph then hit 4th in auxiliary and stay there all day.
I’ve heard several times about 4th being smaller and weaker, makes sense, that’s why curious about its life expectancy.
I’m sure a bunch are shaking their heads but hey that’s worked for over 540k.
IDK…

 

How do you figure a huge fuel savings dropping rpm some. You'd be lucky to gain even a tenth from reduced parasitic drag, but then add in the extra drag from the auxiliary and you are back where you started.

And just a guess but maybe no issues running in 4th because you aren't grossing very much to even need an auxiliary. I've pulled 200k all over west coast. With factory tune DD15 18spd no auxiliary with 3.90 gears no problem.