i did many stuff to go in car. You MUST use a serie resistor (let's say 10Ohms ) an a big capacitor (let's say 1000-2200) + 0.1 uF ceramic + 10-22uF tantalum at the input of the voltage regulator for a better noise regulation/supression. At the voltage regulator output, you MUSt have 0.1uF + 10-22uF tantalum. The PCB MUST have many ground plane + all unused pins of the PIC must be tie to ground OR VCC. AND for sure 0.1uF as close as you can to the PIC.
Most remote starter or car alarm use the above configuration.
Your PIC pin for the pulse count, must have a pull down resistor (4.7K-22K) to avoid "weird reading" In My Tach reading application, i've use the appended circuit without any kind of problem.
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