My 98.5 does not have High Idle

Tonight I went to the dealership here in town and I am friends with the head mechanic. He has over 25 years of experience working on Dodge Diesel trucks when he plugged the computer in to enable high idle, in the options it said enable high idle. He went through the steps and it said failed to communicate with the PCM/ECM. He then checked my vin and it is one of the the first production models of the 24 valve engine, he also checked some other parts under the hood and determined my engine computers were the earliest 24v computers and that other parts under my hood were 12 valve parts. So in short it can never be enabled on my truck.