isx overheating

i have a 2005 9900i with the isx engine. i am having a slight overheating problem. i put it in the shop a month ago and they fixed everything except the main problem, which is the reason i brought it in. everything else i could have fixed myself. it is a vacuum truck with a lot of idle time. during this time the temp starts to rise to 190 and sometimes right at 200. it does break 200 on occasion. if i turn on the heater in the cab it goes down. i did notice that the fan seemed to be spinning slow. i checked one of my other trucks (identical truck) and the fan is obviously spinning faster. i disconnected the airline that ran to the fan clutch from this electrical air dump thing by the steering shaft on the firewall. when disconnected the fan spins much faster and the temp drops. my question is, is there a sensor or something that tells that air dump how fast to spin the fan or maybe it is the air dump itself? i also checked out thermostat, it is good.

thanks in advance for the help.