Choosing a bus with the right transmission is critical! Doing an upgrade after the purchase can be extremely expensive, and often exceeds the purchase price of the bus. Unfortunately, the most popular transmission found in a bus is also the least desirable, the Allison AT545. My rule of thumb is that any diesel bus engine in the years we typically recommend (early 1990s to 2003) is going to be OK, but the choice of transmission and rear axle ratio are what matter when you’re actually on the road.
It’s common when you see a bus for sale that they just list “Allison transmission” or even “Allison automatic transmission”, which essentially is the same as saying “Yes, it has a transmission”. Allison only makes automatics, and in the US all “real” (aka not based on a GM/Ford van) school buses with automatics will have an Allison automatic. Occasionally you’ll see a bus with a Spicer manual transmission but those are pretty rare.
Of course, the way to know for sure which transmission you’re looking at is to climb under the bus and wipe off the manufacturer’s tag where it is clearly printed, as the transmissions that use a mechanical cable (everything but the MD3000 series) are more or less interchangeable.. But that’s uncommon. Most of the time, you can tell at a glance.
The other big caveat is that the AT545 and MT643 use the same lever, despite being very different. While they’re both non-overdrive 4 speed units, the MT643 is a MUCH more stout unit with features that make it a lot better on the road. The primary factors in this are a much higher (445 lb/ft max on the AT545, 640 lb/ft on the MT643) and torque converter lock-up in 3rd and 4th gears. In brief, a lock-up converter creates a mechanical connection between the input (engine) and output (driveshaft) rather than a fluid (impeller/turbine/stator) that “slips” , which turns input power into heat rather than propulsion.
This image should help you determine quickly in person or from a seller’s photo which transmission you’re dealing with. Good luck!