86 Euro 230E Tach Inoperative

The tachometer on an 86 230E (124 body) I just bought does not work. All the other gauges and clock are working fine. How should I diagnose if the tach itself is bad or if it's not getting a signal. I have a basic wiring diagram which traces the signal wire from the tach back to the fuel pump relay and then branches off to both the KE injection system unit and the diagnostic plug contact #1. How do I check for a signal at the gauge connector (or at the diagnostic plug for that matter), do I measure for a voltage or a resistance as the engine speed is raised with a DVM, or can I hook up a dwell/tach tester? Can I remove the tach and connect it directly to the diagnostic plug and ground to see if it works?