Shifting a Mack 12 speed

Forget about gear numbers at a particular speed. An Eaton 13-speed on a set of 3.90 rears is going to shift differently than one on 3.73, 3.55 or 3.28. Even tire size will effect shift points.

First, figure out how much RPM drop there is between gear changes. Then figure out where the motor likes to shift at. Then go up and down through the gears. The thing about this line of work is that there is no "fool proof" way to do things. If you can't adapt to a new situation, you won't succeed.

Now your boss gives you this:




Good luck!

 

I absolutely agree. FORGET about drive by numbers (shift by speed). If you want to pay attention to numbers, then pay attention to the tachometer, not the speedometer.

Bear in mind even the tachometer won't tell the whole story. Some engines like it when you rev them up to 1800 rpm's. Other engines call the cops and file a complaint you are abusing them at 1800 rpm's.

Get a feel for the particular truck.

I typically drive a few dozen different trucks each month for various customers. Some don't have any indication whether they are a 9, 10, 13, or 18 speed on the gear shift. I haven't yet had the pleasure to encounter a twin stick. Some want to wind up over 1800 rpm's to shift and are governed in top gear at 1850. Others want to shift at 1600 rpm's and cruise at governed speed at 1300 rpm's.

Every truck is different. Get a feel for it.

 

Lol.. a silver 92 loves the rpms at 2100

 

Direct and go fast. Over drive

 

Back when diesel trucks had more in common with race cars.

 

On every Mack 12 speed I've driven, you have 4 positions on the air shift lever. High, low, neutral, and reverse. You then have either a second stick or another air switch on the dash for deep reduction.

For normal driving, you just shift the main stick and split between height and low with the stick air shifter, much like driving a super 10.

To go backwards, you flip the switch to reverse and put the main stick in any of the 5 gears. You have 5 reverse speeds.

To go to deep reduction, you filp the dash switch or second stick ( depending on how the truck is set up) and put the main stick in 1st. You can only use deep reduction in 1st on the main, plus high and low on the splitter.

If you have a PTO, you must put the splitter in neutral and the main stick in any of the 5 gears. You also have 5 PTO speeds.