Tachometer

I've decided to get the original Tachometer in the dashboard working. I've saved the blue (BLU) wire on the wiring harness, that ran from the distributor to the junction connector C415, to connect to a hall-effect sensor and 2 magnets on the motor auxiliary shaft. Thanks to Brian's work at S2KEV the tach signal should need 2 12v pulses per revolution of the motor. This coincides with 2 spark plug ignitions per revolution for a 4 cylinder car.

The gauge goes to 8000 RPM so the FB1-4001A should register on the gauge well. I've got 2 magnets to JB weld to a 3/4" shaft collar for the auxilliary end of the motor, a bolt to mount the Melexis sensor, 2 nuts to adjust the sensor and an angle bracket to mount the bolt on the motor mount instead of on the motor directly. They told me I was reinventing the wheel at Ace. We'll see.



I used an old 4 wire plug from an old computer and only used 3 of the sockets. The parallax site has recommended cables and magnets to go with the sensor. Another option for the hall-effect sensor is this Honeywell SR13C-A1 which is pre-mounted and wired in a plastic snap case. I ended up using this one with a metal strap bent into position.

Well, my first try didn't work. There was almost no voltage on the output when a magnet passed by the sensor. But after I dug a little bit into it the wiring diagram they recommended sensor circuit have a pull-up resistor between the Supply (12v) and Output (tach wire). I tried out the resistor with my son's Circuit Jr. kit and it worked so after putting in a 4k ohm resistor the tach works. With the car on jacks and in 1st gear I was getting about 2000 RPMs for 10 MPH.

Here is the sensor mounted to an L bracket on the motor mount on the auxilliary end of the motor. I've re-crimped the ends with the resistor and wrapped it in heat shrink to keep it clean.