Cat 3116 tachometer signal, GMC topkick

[apm]

I posted this earlier in the trucks forum, but didn't find the answer. I've got a 1998 GMC Topkick with the Cat 3116. The tachometer doesn't work. The engine serial number is 9GK64306. Can anyone tell me where the tach signal comes from? Some seem to think it's from the alternator, some think a sensor on the flywheel. Any idea where to start troubleshooting?

Thanks,

Greg

 

[apm]

I posted this earlier in the trucks forum, but didn't find the answer. I've got a 1998 GMC Topkick with the Cat 3116. The tachometer doesn't work. The engine serial number is 9GK64306. Can anyone tell me where the tach signal comes from? Some seem to think it's from the alternator, some think a sensor on the flywheel. Any idea where to start troubleshooting?

Thanks,

Greg

I finally got the service manuals I had ordered. According to the manual the signal for the tach comes from the generator. Purple wire, in case anyone else is looking for this information.

Greg

 

[f.sava]

Thanks for talking the time to reply to your own question. I had the same problem.

 

[Coaldust]

I have ran into the situation where the alternator gets replaced at some point with a one that doesn’t have an R-terminal for the Tach. They are available both ways. Cat and GM have both used the R-terminal for tach inputs.