Peterbilt 379 why would you have to flip the fan on to make jake brake , tachometer and volt gagework?

jake brake,tachometer,volt gage, and speedometer will not work, but if you flip the fan on they will work...its a 1996 379 peterbilt ...|||Sounds like a short to me, Billy. The fan circuit must close the shorted circuit for the non working gauges and jakebrake. Just a guess on my part but I am so used to dealing with shorted out electrical systems. Good luck, Susan Shea