I know the OP posted this a long time ago, but with the recent bump, here goes...

D-75 on these perkins dp50s is usually related to the idle validation switch not switching at the correct time relative to the throttle position ramping up.
I've found its usually either a bad idle validation switch, shorted wiring leading to it, or just mis-adjusted.
The pedal sensor voltage should be about.35vdc at minimum thither and the idle validation switch should be closedat this point, but open with any throttle input. It can take a few tries to find the sweet spot if you don't have the perkins software to view it while adjusting.