Foretravel Owners' Forum For owners and Fans of Foretravel Motorcoaches September 10, 2025 ...

I have heard about transmissions "locking up" and don't know what that means. I only have air brakes in my coach, and when descending grades I will usually shift to 3rd gear and use the air brakes. I believe that 2800 RPM is the red line for a Cummins 5.9 so I start braking as the tack approaches 2800 RPM and it takes a lot of braking to keep at a safe speed on a long and steep grade.
My questions are: What causes a transmission to lock up? What happens when it does lock up?
* In contrast, my old 1981 Foretravel with a 3208 Cat and an Allison 4 speed automatic had so much torque that it would crawl down almost any grade without even needing to brake.

You have a smaller engine with the 5.9 liter displacement so it offers less engine braking than larger displacement engines. Lockup means the torque converter is totally out of the loop when the transmission is in lockup. It's just like you have a manual transmission. As you get toward max RPM, use the service brakes until the speed and RPM has dropped about 500 RPM, let off the brakes and let the speed slowly build and repeat. You should be in a lower gear if you need to do this often. Larger engines won't have to do it as frequently. Be conservative and use lower gears rather than heating the brakes. The parking/"emergency" brake won't do anything if the brakes are overheated.

Your 3208 was a far larger engine so had much better compression braking going downhill. Transmission is programmed to lockup at certain speeds and RPM. This is a good thing as it creates less heat, gives better fuel mileage and provides a little better engine braking going downhill.

Pierce

Having said that, the difference between a 3208 Cat and a 5.9 Cummins in being able to slow a vehicle's speed going downhill is huge.