Depending on if your Softgloss has clutches, or not, the ultrasonics are used to profile the vehicle as either a sedan, tall vehicle or pickup. This is used relative to the clutch logic (on/off), top brush interrupt and rear arm oscillation methods. It's not used at all to prevent overtravel at the rear.
As the machine travels toward the rear, it is looking for two blocks, and unblocks, of the wheel scrub photoeye (the one in the middle of the frame on both sides). It should detect the front tire, then the program ignores where the guide rail should be, then it should detect the rear tire. If it sees the two blocks, then as it detects that second tire, it will continue travelling reverse for a brief time, but it should slow down to the trunk wash speed. If it doesn't decelerate arount that area, you have a photoeye issue.
If the machine is decelerating in the trunk area, it starts looking for the rear arm proxes to transition on, then off. When a rear arm is in to the center (closed), the prox input will turn off. This indicates to the PLC that we have found the rear of the car, and it will go into it's dwell cycle, followed by the rear arm oscillation if it is setup for this (a sedan will only oscillate one arm if it is setup for this).
If a rear arm prox is not detected on, then off, it will continue to travel to the rear endstops.
If you are only seeing this on a small vehicle, I suspect you have a photoeye issue that could be causing this. If the rear wheels are real close to the guide rail, it's possible it's not seeing an unblocked condition between the wheel and guide rail. You would know this if the machine isn't decelerating around the trunk.
