[Don Plumley]

From the search button:

Mine does this too. I go thru this sequence. Turn the key on so the dash lights up, press the gas pedal to the floor and leave it pressed there for about 15 seconds then turn the key off and release the gas pedal. I then start the car. Works every time so far. I think it has something to do with the Wide open throttle switch or idle switch getting confused.

From RennlistIf your tach stays at 0 and doesn't bounce, its likely your speed sensor and not your reference sensor. The reference sensor wil only send a single pulse to the DME per every revolution of the flywheel - every time the flywheel goes back to TDC, the reference sensor sends a signal to the DME. The speed sensor sends a constant signal to the DME based on how fast the flywheel is spinning. If you've seen your flywheel, you've seen the multiple set screws positioned around the flywheel - these are the reference marks the speed sensor uses to output its signal to the DME.

A bad sensor is probably your problem as the DME must see at least 200 rpms before it will allow the coil to fire. If it is not reporting the correct data, the car won't run. The best way to test this is with an oscilloscope on pins 8 and 27 of your DME harness, but if you don't have an oscilloscope, a multimeter on the same pins will tell you the nominal impedance. It needs to be between 600 and 1600 ohms.
An o-scope is the better test as it will allow you to see the waveform. The best test is an oscilloscope capable of overlaying dual inputs onscreen. Put one input on pins 8 and 27, and the other on 25 and 26, then crank the motor. You should see a wave with peak to peak voltage of greater than 2.5 volts on input one and a regular single pulse of greater than 2V to correspond with TDC. If either are outside of their impedance tolerance or if either do not display the correct waveform they need to be replaced.

Replacing the sensors is pretty easy. If you remove the speed sensor take care to put the shim back in (if so equipped) as you can ruin an improperly gapped sensor pretty easily.

With a 964 you need a Bosch Hammer to read the OBDI codes - if you are going to do serious DIY on this generation 911, pony up the $1K and be master of your domain.