Vehicle Speed Input Trouble

I am using the included circuit to buffer the vehicle speed sensor output from my 1996 dodge ram.

I am also using an Uno R3 along with the frequency counter library for reading that output.

I have two issues: One is that I am shorting the signal to the other components on the vehicle (Gauge cluster and PCM)

The other is that I am seeing errant frequencies in the same range that I am trying to monitor.

I have had the suggestion to employ an op amp to prevent driving the signal circuit low. I am not opposed to this, I just haven’t had time to try.

The other, the interfering frequency, is being produce by my alternator. I know this because I have checked the R3 against a fluke 88V and an excell meter, all 3 are spot on, and I have also used a PWM motor control to check for accuracy.

I don’t have a problem until the alternator is spinning, and I unhooked it for testing, which removed the symptom completely. I have since performed a diode ripple test and system checks which say the electrical system is fine.

I would like to be able to produce what I have done for use on other vehicles, on which I can;t modify the charging system, So I need a solution that I can incorporate on my end of the circuit.

I had a prototype shield working well which mistakenly used an A1505 PNP transistor salvaged from a camera. That shield had no trouble with noise from the signal circuit.

I built another shield using a seeedstudio proto shield and a 2n3904, and pandemonium has ensued.

I am a hobbyist and will gladly read anything you provide a link to. I am learning so go easy on me please.