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.

 

When shifting any auxiliary transmission, remember ( only shift the auxiliary transmission when the main transmission is in gear). If you don't, you will in many cases and times, coming to a stop to get back into gear and starting over.. I experience is, I started driving in 1972 dump truck building logging roads, from that to interstate to Lowboy Heavy Haul, Alaska to every state in the lower 48, except Hawaii. I have driven just about every combination emaginable, One old B model Mack with the quadraplex transmission ( 5 and 4 speed, same transmission box ) with a 4 speed auxiliary box. Hope this helps. For Heavy Haul, I personally think that the 18 speed,, with at least 500 HP. REMEMBER more HP more drive tires being replaced.

 

Just go with the 18speed and forget about the auxiliary transmission. The time the auxiliary transmission helps is when you don't have enough HP. Been trucking since 1972.

 

My neighbor drives for a heavy haul company and they use W-900s with the 600 HP and 18 speed , with their 13 axle rigs . They regularly carry the same cargo
That weighs 130k lbs on the trailer.

 

From my ancient memory a large reason for the "never pull an overdrive under load" is its configuration in the gear chain.
You get a chance to see inside any transmission with overdrive, take a peek. Getting in OD is slipping a shallow toothed gear over the end of a similar shallow toothed shoulder area of another gear. It makes for a "short internal connection", not the wide thick teeth of the other mating gears. Hard to explain in words, clear as glass when you examine it inside a transmission. The phrase "Never pull an overdrive under load" is quite ambiguous. Only cruise in OD would be better. Same pulled weight; in rolling country, with a "driver", will run your eyes out. Same setup in different terrain, with "not a driver", not last long.
From what you have posted; you unknowingly why, are well aware of how to drive your truck and not hurt your transmission.

 

I currently run a 2 speed Eaton rear end behind an 18. Those you can shift while moving but not downhill. Only uphill or flat ground.

High range 4.1 rear, low 5 something rear.

Honestly it's not going to come in much use if you're 150,000 or less.

But I do find that if you are unfortunate enough such as I to run a mechanical trailer with decent weight on it it makes picking up the thing a lot easier.

The few times I really have used it generally I just take off with the low engaged get up to the high gear or at least 7th and flip it back into higher range

And also, that goes from about 75 at 1800 to about 55 at 1800 roughly.