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Old 05-03-2021, 01:51 PM   #1
Mini-Skoolie
 
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Join Date: Feb 2020
Location: Way Out West
Posts: 38
Year: 1990
Coachwork: Century
Chassis: C5500, Bus not a School Bus
Engine: 3116 9YN, CAT, 250HP
Rated Cap: 30k
GMC Topkick Chassis 3116 Tach Wire

I have seen several posts online with it being noted the purple wire on the back of the alternator is the tach signal for the 90s GMC topkick diesel.

I have a 1990 model with the 3116 and the factory tach but I have no purple wire. I have nothing attached to the terminal on the alternator where it should be. Looking through the 1990 GMC topkick factory wiring book it only shows the model for the gas engines and the coil hookup for the cluster which is a white wire. The 1990 diesel supplement book has nothing in regards to the wiring of the tach for the 3116.

I looked on the bellhousing just in case something was weird with the setup and it was trigger reading from the flywheel but nothing there either.

I pulled back the wiring to the "Y" where the alternator wiring comes off to make sure it just had not been broken off but no luck with that either.

Does anyone have a 1991 GMC medium duty wiring book that can get a picture of the tach wiring diagram so I can chase this down?

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Old 05-04-2021, 08:37 AM   #2
Mini-Skoolie
 
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Join Date: Feb 2020
Location: Way Out West
Posts: 38
Year: 1990
Coachwork: Century
Chassis: C5500, Bus not a School Bus
Engine: 3116 9YN, CAT, 250HP
Rated Cap: 30k
Problem is resolved!

Turns out the color people have been listing for the tachometer wire is not purple on the earlier models, it is yellow and has a 5 amp fuse inline with it.
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