308 Tach Flywheel Sensor

There are three crank sensors around the flywheel that generate electrical pulses from the teeth on the flywheel. These three sensors feed a pair of ECUs that control the spark, one for each bank. The tach is driven by one of the ignition ECUs. There isn't a specific sensor for the tach. One crank sensor feeds one ECU, the other sensor feeds the other ECU, and one sensor feeds both. Take a look at the schematic and you will see what I mean.

The fact that your tach is freaking out as the problem appears is a dead giveaway that the crank sensor that is bad is one of the ones feeding the ECU that drives the tach, which makes it pretty easy to trace using the schematic.

The sensor could be toast, or the connection from the sensor to the ECU could be dirty. I would start by disconnecting the sensors, cleaning all the contacts with electrical contact cleaner and put them back.

Also, check the grounds on the ECUs as these are also known to cause the problem.

If that doesn't fix it, you probably have a bad crank sensor. (They are no big deal. It's a coil of wire with a lead on it. They are about $100 from Ferrari UK, which is about 3 times what they should be, but not terrible.) They are accessible through the left rear wheel opening. They are not hard to remove, but they each have a two pin connector on a short lead that is very easy to reach and disconnect. Very easy to trouble shoot. Just disconnect the connector and measure the DC resistance across the pins with a ohm meter. They will read about 600-800 ohms if they are good, and very high to infinite resistance if one is bad. The good thing is they will tend to read bad when they are cold and may get flaky and intermittant as they get warm and they wire in them expands, so measure cold.

Good luck.

Birdman

 

ahh so.. birdman say crank sensor too.. very curious indeed. great minds think alike. you're getting good at this. don't you still have one of those in your tool box? the world awaits the outcome. good luck laddie.