Auxiliary transmission for heavy haul?

Can you even get a manual shift anymore? All the new ones I’ve seen are air shift.

 

You never want the main and aux box in neutral at the same time.

 

Spicers it’s just the top plate on the transmissions. If you had a 4+4 it had two sticks. If there was an air-shift tower on the auxiliary then you had one shift bar with a 5 position valve on it and it was sold as a 16spd. Same pair of transmissions other than the air tower on the auxiliary.

 

Yeah theyre all air shift, a manual would be like a twin stick I guess

 

With a spicer 4 speed you can go up by being in the 18th gear Let off the gas pedal shift the auxially up and wait a second till it engages and then you can down shift to the appropriate gear

 

the Spicer 16 speed was a married set of boxes, there was no drive shaft or u-joint between the boxes. It used the same air shift top plate if I remember correctly and I know the shifter knob was used on both. I sourced a shifter knob for a buddies married box in his 67 k100 in about '82, he was broke down in Moosomin SK and called me to pick one up in Edmonton.

 

How many miles do you get on your 4spd’s on average before you consider rebuilding/replace
Have 543K on mine now.
No issues with it yet, temp runs about 190 under load. Just Wondering if I should swap it out before Murphy’s law catches up to me.

 

Can someone explain why you shouldn’t run your 4spd in 4th. Keep hearing this again and again.
I’ve had same truck 10yrs this year.
18sp, spicer 4spd, 4:30rears.
IMO, a huge benefit is using 4th as overdrive to drop RPMs when empty, huge fuel savings.
I’ve always shifted up to 4th while grossed out w 13axle, 200k plus, last 8yrs 160k w 8axle freight. No issues whatsoever.
Just curious , what am I missing here.