Can you tell me what causes the high-Low splitter in my transmission to go bad?
There are two “splitters” in any 13- or 18-speed transmissions, and a single splitter in the 8-, 9-, 10- and 15-speed gearboxes. When they go bad, it often means the mechanism is unable to make the shift from low to high range or from direct to overdrive. The splitter mechanism is air driven, and the cause of a faulty splitter is usually contamination in the air lines supplying the mechanism, or air-borne gunk causing the mechanism to bind and shift more slowly than it was designed to do.
There are mechanical failures as well, but they too are often caused by contamination.
The best solution is to disassemble the mechanism and clean it throughly. Rebuild kits are available from the manufacturer for specific models, including rubber 0- rings, gaskets and the activator pistons. It’s not a difficult job, but since the problem may be internal, it’s sometimes best left to qualified technicians. While you’re at it, replace the air lines that supply the splitter mechanism. They might be full of crud as well.
But don’t stop with the splitter. Chances are your entire air system is comtaminated, and should be purged of oily sludge. Start with the air dryer and work back, but check the compressor too, as that’s where the oil comes from in the first place. Who knows, your brakes might be the next component to let you down.