This is a bit of an academic posting since the problem went away, but since it may occur again, I think it's worth troubleshooting. Here are the details:

Starting my S5 Turbo this morning I apparently held the key in the start position, and the tach signal dropped to zero. [Car is a hard-starter due to over-rich condition at startup, so the tendency is to hold the key in the start position longer than necessary.] Car starts and is idling just fine. Check engine light comes on and I sit there completely puzzled. No rpm readings on the gauge or on the SAFC. So I sit there trying to decide how next to proceed. Check the error codes? Good idea. Check the fuses? Why not. Next thought is to try the simplest approach--shut the engine off and restart the car. Bingo! Tach is now working and car is running just fine.

I suspect there is a circuit breaker or a relay or some "smarts" in the ECU that tripped that circuit, but I can't seem to find it in the wiring diagram. Comments and ideas appreciated.

 

If voltage drops too low I believe it will kill the trailing coil and it is this coil which provides the signal for the tach. Your issue could always be caused by something else though.

 

I had this happen a couple of times before I rebuilt my engine. Engine was original with 160k on it and it was really hard to start at times when at operating temperature.

This would only happen when I have just about nuked the battery and the engine would decide to fire up. This was a very rare occurrence as I usually nuked the battery first when it was that hard to start.

 

This could be since the engine will probably run with just the leading coil activated. Both coils are connected to battery power through the main relay, so maybe that is where the "problem" happened?

 

Could be, but I took the car for a long run yesterday and the battery should have had a good charge. And I didn't crank the engine very long before this happened. Seemed like the problem had to do with continuing to activate the starter circuit even after the engine had started running. My guess is that there is a protection device somewhere in the starting circuit to prevent burning up the starter motor by the engine spinning the starter motor. Maybe it cuts the power to the coils momentarily. I'll check out the battery in any case.

 

bad ground?