The Grand L's "Cruise Control" works differently depending on whether Auto Throttle is ON or OFF.
When Auto Throttle is OFF, it works as you describe. IOW you can use the Cruise Control lever to set a roughly constant travel speed and take your foot off the HST pedal. Note the engine RPM is controlled by the hand throttle.
But with Auto Throttle ON, the Cruise Control lever acts more like an 'overdrive' lever. IOW you can NOT use the Cruise Control lever to set a constant speed and take your foot off the HST pedal. Because when you take your foot off the HST pedal, the engine speed drops to idle and the tractor slows to a crawl. But you CAN use the Cruise Control to lower the engine RPM needed to reach (or hold) a particular travel speed or increase the travel speed for a particular engine RPM.
For example, the way I have my Auto Throttle cable adjusted, I can be traveling at 4 mph in M at roughly 2,000 RPM. If I hold the HST pedal constant and push the Cruise Control lever all the way forward, the travel speed increases to just under 6 mph at the same RPM. Or I can back off the HST pedal and continue traveling at 4 mph. at roughly 1,500 RPM
Talon Dancer
ps I suspect that the Cruise Control lever actually does the same thing whether Auto Throttle is on or off. BUT the result is different when the HST pedal is controlling BOTH the HST and throttle.