Almost identical problem on my 740. If no indication of RPM during cranking, it is a dead giveaway to the hall sensor or the wiring. If you take off the distributor (entire thing), and hook up 12 volts to the terminal labelled (+), then ground to the (-), and a meter to the center pin, you should see fluctuating voltage as you turn the shaft. Now, the FAQ for 740's says that my voltage output should be 5 volts on that sensor output. I was getting .5 to 2.2, and mine works great. The biggest thing is to get a fluctuation in voltage as the vanes and openings pass across the sensor.

Good luck. Don't kill her. They give it all back in an accident. My 82 245 DL gave it's life for mine. But they are twitchy at times.

Also, if you pull the distributor, you might as well replace your seals while you're in there. If the sensor is bad, then I would just get a rebuilt distributor (new are 300 or more). The process for replacing the sensor can be difficult without all of the right tools.

Also, I have a spare distributor here if you like. I got it for 35 and have not tested it, but I'd be willing to pass it on to you for the same + shipping. I have NOT tested it. The guy I bought it from claims it worked when he pulled it from his old car. I have no idea how long ago that was.