As with all diesels the tach works off a Tach generator, the Alternator W terminal or inductive pick up. I'm sure you have seen the chart attached and it does not list your specific motor. So you need to first find out what is supplying the tach signal or the 3 listed.
If it is the alternator W terminal then need to find out the gear ratio and number of teeth in the ALT. The gear ratio is obtained by measuring the diameter of the crankshaft pulley and ALT pulley, then divided the crankshaft by the ALT.
To determine the number of teeth of the ALT would need to know that part number or gain access to the ALT gear inside.
You can also just connect the tach and take a guess at which code based on what method is driving it. Then get a hand held photo tach and read the rpms with motor running. Take the reading from the photo tach and compare with the new one.
If take is to high or low, look at the chart and change the code, repeat until successful (turning off motor between changes)
You can also contact VDO via email or phone call during business hours, but they may also need the same info about your motor to determine
https://www.vdo-gauges.com/contact-us/
