Update, I have acquired a brand new Raymond manual for this unit. I performed the nulling procedure for the throttle card per the manual. While performing speed limit adjustments with VR2 and 3 I noticed that altering VR3 (1/4 speed limit) affected the drive speed slightly. This leads me to believe the unit is either in thermal cutback, overload protection, or height cutout mode.
I measured the thermistor resistance (TH1) at 105Ω. I measured shunt resistance (SH1) at 0.3Ω both at its terminals and the ICC card terminals. Voltage at the shunt wires stabilised at 13.3V at both ends of the wires (twisted properly along full length). This truck has no height cutout switches, so I verified that the jumpers on TS3 were still in place. They are. I did not check continuity of the wires back to the card, though.
The truck stays in pulse mode for a much longer period of key-on time now, (was locked in pulse all day today) so I'll recheck measurements of TH1 and SH1 once M-range comes back. TH1 spec is less than 200Ω at ambient, but I could find no spec anywhere in the manual for the shunt. I have been chasing this based on the assumption that since VR3 on the throttle card did successfully affect drive speed that it must be in cutback mode. Should I keep pursuing this or is it possible that there's a bad component elsewhere? 2 and 3 rec test good, have not checked heatsink insulation pad, #2 SCR or main SCR yet.