How to Tell the Difference in Semi Transmissions

Hi, my name is Collin Helma. I am a current Diesel Tech student and have experience Eaton Fuller transmissions and others. I am also an experienced truck driver. Have you ever gotten into a semi and wondered what speed transmission it has? Well today I will show you 6 easy steps on how to tell what speed of transmission you have. Knowing how to tell what transmission a truck has will help not wreck the transmission and will help prolong the transmissions life and drivetrain.

Semis have all sorts of different speeds as well as different brands of manual transmissions. It can be hard to tell what transmission is in a truck due to age or if the transmission has been replaced.  

What you will need is just yourself and a semi. Or a couple of different trucks for more practice. 

1-First Its good to get familiar with the transmission as well, so put it into gears and mess with the splitter and the high-low switches. With the truck off of course.

2-Then the next step is to look at the shift pattern, usually on the dash of semis. Most semis have the speed and brand of transmission on a tag with the shift pattern. But what that shows is not always the case. The transmission could have been replaced especially in older trucks.

3- After that, then look at the shift knob. There are many different types of knobs, aftermarket etc. but the air switches, splitter and high-low, always stay the same. 

4-The High-Low (bigger switch on front side of shift knob) is always in that place and is usually black. That switches your transmission from low gear set to high gear set. So that indicates that it has more than 5 forward gears. 9- speed and up.

5-Then look for the splitter switch, usually on the side of the shift knob and is colored. If it doesn’t have one then the transmission is a 10-speed. The main colors are red, grey, and blue. The color of that switch is very important to tell what speed of transmission it has. Red is always a 13-speed transmission and can only shift that switch when the High-Low is in high. Blue is always a 15-speed and the splitter only works when the High-Low is in low. And 15 speeds can have a double reduction which means the splitter down shifts not upshifts. Then Grey is an 18-speed which means the splitter can be used in both high and low gears.

6-Then lastly, it would be good to go out and drive the truck and get used to shifting it. Shift up, shift down, use the splitter and the high-low and just get comfortable with shifting. With or without a trailer on and preferably not on a major highway.