Firm Ride system
The ARC (Adjustable Ride Control) will come on in four conditions.
1. Speed-If the vehicle speed exceed 90-95 mph, it will switch to firm ride.
2. If the vehicle is steered quickly enough (sensor located under steering column), it will switch to firm ride
3. If the brakes are applied with enough force to raise the brake system pressure above a certain pressure (not sure of the pressure) it will switch to firm ride.
4. When the throttle position reaches a certain angle (determined by the TPS and the ARC computer), it will switch to firm ride.
The last one is of course if you hit the switch on your center console.
If you are hardly applying the brakes and the vehicle is switching to firm, it could be a malfunction in the pressure switch, or whatever/wherever activates the firm ride via the system pressure.
Hope this helps,
Phil