Common problem with Allison LC1000 series transmissions, Range switch on trans is in a high water splatter area on unit and water works its way into switch causing wrong range selection input values. Since this transmission relies heavily on computer control to apply the multiple solenoids in unit, when the computer doesn't know what range it's in, it does all kinds of strange things, clunks, no movement. They usually set range switch codes and the old switch is black in color to indicate that it has a problematic designed switch. The only trouble with these (and all similar GM truck range switches) is that the potting material on the switch connector area melts with heat and seals the connectors to the switch so removal is impossible. On the smaller trucks we leave the switch attached to the harness when we remove the transmissions(as long as the switch is fine which it usually is on the 4L60-E trannies). On these units, though switch replacement is the problem so the wires have to be cut and new connectors attached to the harness. This is time consuming; switch replacement takes only minutes, but putting on the new connectors takes at least an hour.

Old bad switch, water sprays onto part where shaft goes through switch

Switch with driveshaft and selector lever removed, old connectors glued to switch

New parts, new design switch is neutral colored, black piece is a water shield

Back of switches; new design has white foam water seal in back also.

Front view, note connectors had to be cut off with old switch

New switch installed, waiting for new connectors

New connectors wired in, new water shield installed

All better!