If the 84 appears during Self Test (Key Up) make sure the brake is adjusted properly. During self test the control will pulse the drive motor and if the drive motor encoder reads rotation, 84 will appear. Check coupler between drive motor armature shaft and encoder for loose set screws, etc. If 84 appears during normal driving situations the brake master cylinder or slave cylinder may be bleeding off causing deadman switch to de-energize. Check deadman brake switch (S2) adjustment. Or Broken wire from control to the switch. These wires run from control to the switch routed with the drive motor cables. They constantly twist back & forth and they do intermittently break.