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I've been wanting to add a tachometer to my conversion for some time now and I wanted to use the original instrument cluster in the PT Cruiser. After doing some research I discovered that the crankshaft position sensor signal was input to the PCM/TCM and the processed RPM signal was sent to the instrument cluster over the data bus. The crankshaft position sensor is a 3-wire hall-effect device with 5V input, ground, and sensor output. The output is a pulse train, with 8 pulses per revolution (this was determined by inputting different frequencies and plotting the RPM).

For the sensor I decided to try something different. I used an OMRON photomicrosensor PN: EE-SG3. http://www.omron.com/ecb/products/pdf/en-ee_sg3_sg3_b.pdf

The sensor circuit is very simple with only two resistors required: one to limit current through the LED, and another to develop the output pulse. I chose 330 ohms for both which limits the input and output currents to about 15mA. The cable is from a "re-purposed" 9-pin serial cable and is shielded.

The mounting bracket is constructed from scrap aluminum. I had to cut off one of the mounting feet of the sensor to get a low-profile mount.

The encoder is constructed from a mini compact disc that is epoxied to a 7/8" shaft collar. I used a dremel to carve out the 8 "teeth" on the encoder. Not pretty but it works. I will probably fabricate a better one as I see how this one holds up.

The whole thing is mounted on the back shaft of the Warp 9 motor and wired into the existing wiring harness that feeds the PCM/TCM.

The only thing I had to buy was the shaft collar @ $6. I had several of the EE-SG3s already so I don't know what they cost. The sensor works great and has confirmed my shift points of 2nd to 3rd @ 35mph, and 3rd to 4th @ 50 mph with the RPM @ 4,000.

Hope someone finds this useful.


Ralph
 

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