FU5 (15amp) is what powers the key switch and is what powers the DC to DC converter. If you are getting 3.9v before the key switch then it's possibly an issue with FU5. Otherwise when the key switch is in the on position the K1A relay. You can test this relay by testing the input at CA201-9 and output at CA201-5 on the distribution board.
For traction the motors go through FU1 (120amp). The traction scr card gets it's power from TB403-4.

These lifts are wired very differently from newer lifts. This lift need to see all the brake switches, presence petals and an input to work. If the brake switches won't close then the next switch will not get powered.
This lift also is a contactor based lift so most likely if it does move a bit then shuts off, one or more of the contactor tips is pitted or has too much resistance due to wear to allow enough voltage through.

Hopefully that is a good starting point, let me know if you want me to go more in depth about something.