All the electrical system is isolated from the truck chassis.
As carbon brushes wear down the dust collects inside the motor and over time begins conducting to the chassis causing all sorts of weird things. The wave has dual drive motors and if the one for turning left is "dirty" it could cause this.
If you are not familiar with forklifts it might be best to hire someone. No offence, I have tried assisting people with no repair background before with limited success.