Jumping Tach

[4 R LABS]

I have a Cummins 300 ISB in a 2005 Monaco Caymen. I just had a new alternator and belt installed (2nd in 13 months). The problem was when driving at speed 60-65 the alternator and ABS lights would come on plus the temp would rise and the tach would jump all over the place. Also this last time the trans light came on and I had to down shift to pull into the shop.
The shop told me that the computer told them the alternator spiked at 17 several times. Now with the replacement all is back to normal except the tach is still jumping. Any ideas? I am very grateful for any suggestions.

[Skip426]

Your dash gauge package , was probably supplied by the chassis manufacturer, and to check it's operation you need to know where the tach is getting it's signal .
Possibilities are the Cummins ECM or the Allison ECM .
JMHO. Contact the chassis manufacturer for testing info.