There is a large driving gear system with a lot of mechanical switches. Each switch is a condition that must be met for the cycle to move onto the next thing. It maybe as simple as a switch contactor not correct. It is pretty simple inside.
Mine had a problem with the main disc not spinning freely. It caused it to bind and the machine would not work. I believe over time something had warped. I filed, sanded, and polished stuff down until I got a smooth, freewheeling surface and the car wash has worked fine ever since.
