Try first confirming the voltage across the batteries themselves. Should be 24v.
Cab switch should have battery voltage I think.
EDIT: You may not have noticed this ........ in Grid A-3 of the schmmatic is a table of Colour Codes. This table relates to the colours of the wires "as drawn" on the schematic, not the actual colour of a specific wire. The Code refers to what the wire does, or in some cases what voltage it carries. Pay particular attantion to the wires coloured RED that are live with Batt+ when the master switch is ON, and BLUE that are live with Batt+ when the key switch is ON.
The power wire for the Hi/Lo switch is C742-OR located at D-8 on the schematic. If you follow that wire back to Grid A-6 it connects to wire 308-YL at a jumper (CONN 20). Because 308-YL is coloured BLUE on the schematic it is supposed to have Batt+ when the key switch is ON. Ergo you should have 24v on C742-OR at the speed selector switch.