VR's are pretty tricky to work with. They use magnetic inductance...basically, they put out a sinusoidal voltage, with frequency that is related to engine speed (each time a tooth passes, a new pulse is generated). As the speed increases, so does the amplitude though.
The crank sensor is harder to work with because it has a 58 tooth ("60" teeth, with 2 missing as a TDC marker) wheel, so you get 58 pulses + a 2-pulse gap per revolution. The cam sensor is easier and you get 1 pulse per 2 revolutions of the engine. Because the VR sensor's output voltage can hit pretty high peaks (up to 50V), you need a special conditioning circuit to convert each positive zero-crossing of the output into something nice & clean, like a brief 5V pulse that most microelectronics can read. If you read up on the LM1815 chip on the Megasquirt sites, you can find some good info on setting something up.
Actually getting at the signals is a pain too. You do not want to go chopping up the shielded wires on the VR sensors. The only spots to get at the signal lines are inside the black cable box under the intake (and even there, you only get about 1" of exposed wire), or by soldering a wire onto the input pin in the Motronic.
Topic: Fluke 88 RPM option (Read 2141 times)