2002 GMC Durmax

[Buddaorf]

I have a 2002 Durmax that I bought used with an aftermarket high Idle switch, that im trying to figure out what brand it is so i can up the RPMs, been told that its running too low ( 900RPM's been told that it should be about 1200RPM's). Anyone able to explain this and where to locate the Moduale to find out what brand it is.

 

Thanks