Diesel engines use a signal from the alternator to synthesize a tachometer signal. When you change your tachometer or your tachometer pulley, you need to re-calibrate your tachometer.
Here is the Perkins-Sabre document on how to set the electronic tachometer on your brand-new engine. Read it if you want. It may be the worst tech writing to ever be thrust upon unsuspecting customers: PerkinsSabre Tach Programming.
It took me several years, but I finally figured out what they’re saying. Here’s the process.
Figure our your drive ratio. With an 8″ OD crankshaft drive pulley and a 2.625 OD alternator driven pulley, my ratio is 8/2.625=3. Multiply that times the number of poles on the alternator (usually 6). 3 x 6 = 18 alternator pulses per engine revolution.
This is a picture of the back of the control panel. There is a greenish/blue wire hanging out of the back of the tach. That is the programming wire (called pin 11 in the documentation). Right next to it, there are a bunch of black wires connected to the circuit board. These are all ground wires. Clip a test lead to the programming wire and a different test lead to the ground pins.
Get where you can see the front of the panel and hold both test leads at the same time. This would be a good time to find your reading glasses, too. There is a decimal point on the display that is approximately the same size as no decimal point.
Ground the programming wire (PW).
Turn the panel ON (do not start the motor). The beeper will beep. Ignore it.
Unground PW. The display will change. Pulse-ground PW to change the digit, hold-ground PW to move to the next digit. Beware of the tiny decimal point. Once you’ve got the ratio set correctly (18.0, in my case), ground PW and turn the switch off. This will save the setting. If you screw up or get confused, turning the switch OFF with PW un-grounded will not save your changes.
Clearly you can screw things up if you do this wrong. Also, you’re on your own. If you do something because somebody on the internet said to and it doesn’t work, it’s your fault. Feel free to contact Perkins/Sabre and tell them to fix their documentation. Tell them you know about this guy that can rite gud and is remarkably affordable.