It depends if it is a control fault or power circuit fault. Start with raising drive tyre off of the ground or removing one of the drive motor cables from the terminal block on the transmission casing. Swap door sw wire to the N/C terminal and then pull down tiller and test either Fwd or Rev, then 2nd speed. If middle contactor doesn't close, then check input voltage at 2nd speed sw, then output voltage from sw. If OK, then check voltage at the 2nd speed contactor coil (middle contactor). If contactor is closing, check power circuit voltage in and out of contactor and then in and out of power resistor that is hidden inside the door behind the steer post. I'll take a stab in the dark and say the 2nd speed micro sw in the handle isn't closing or o/c

Now that you've explained that to me, I honestly had no idea it had 3 speeds. Now it makes sense why there's so many contactors on the left hand side.
I should clarify then: I do not have 2nd or 3rd speed. So am I right to not suspect the contactors now and move along to the travel control handle switches and wiring since the fuses are good? Or, if the contactor for 2nd speed drops out, does it effect the 3rd and 1a?

The middle contact assembly is 2nd speed. The 3rd speed (1A) is the next one to the left. The one on the the far left is the interlock which puts all four batteries in series. Begin with testing 2nd speed operation then proceed with testing 3rd speed. Most traction faults are caused by high resistance in control circuit - switch, broken wire, bad connection. I've worked on this model for almost 30yrs and have replaced contact tips hundreds of times, but would have replaced less than 10 contact assemblies. The resistor has nothing to do with top speed. 1st speed goes through the resistor, 2nd speed goes through 1/2 of the resistor, 3rd speed bypasses the resistor and changes to 24v. The best tip that I can give when a 3rd speed fault presents, is test each individual battery and then do voltage drop tests on the power circuit and control circuit (fuse, door sw, key sw, brake sw, SAS sw, F/R sw, 2nd speed sw, 3rd speed sw, rab/turt sw, mast sw, interlock sw)

Thank you so very much! I appreciate it.

The 1A contactor is what allows for high speed travel. In the line of 5 contactors it is the center one. It is possible the contactor went bad, the resistor across the contactor winding is bad, or the contact tips are too worn to make contact.
If the contactor does not try to close when requesting full speed travel, it is one of the first 2 options. If you order a new contactor it come with a new resistor.
Typically I just replace the full contactor as they do go bad over time. PN: 090304