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.

I have been looking at this for a long time, and no one seem to find anything to fix this error code. I have your same model forklift and i am going to change the throttle position sensor. I think is that. I will advise. In the mid time, have you found anything on it?

Did you ever get a response to your question or did you figure out what the problem was? I have the same code and symptoms.

Helloooooooo,,, some body there ???