topkick with Cat 3116 tach doesn’t work

I've just bought a 1998 C7500 with the Cat 3116. The tach doesn't work. Any ideas on where to start? Does the signal come from the alternator, or is there a tach sensor somewhere? With key off, tach reads about 600 rpm. With key on it goes to zero and stays there all the time it's running. Thanks, Greg