Vss Buffer box testing question

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Vss Buffer box testing question

Postby stillgrowingup » Wed Sep 17, 2014 7:53 am

After searching countless posts finding no answers. I'm posting my problem, hoping someone has an answer.

I have an 86 Trans AM, TPI, 700R4 and I have an aftermarket dash setup. I finally got a USB to OBD1 cable to do some datalogging on my engine. I noticed that my ECM speed and my actual speed are not the same. So I decided to do some testing.

First I bought a new VSS speed sensor for my 700r4. Same results. Next, I looked in my GM service manual for testing procedure on the buffer box with a voltmeter. The service manual explained the following steps: Raise back end of car, so rear wheels are off the ground. Red probe of the voltmeter goes to brown wire from the buffer box and black goes to ground. Turn car on (engine not running), place transmission in neutral and rotate the tires by hand. Meter should jump up and down from below 1 volt to over 3 volts DC. Mine read from .93 volts to almost 10 volts. Manual says buffer box is good. The brown wire is 2000ppm output.

Next, I tried the Light green w/black wire, 4000ppm output. I did NOT see any voltage change. Since the manual only talks about the brown 2000ppm wires. Does anyone know if this procedure also works on the Light Green w/black wire? If not, how do I test the light green w/Black wire? I also looked at my Buffer Box circuit board. There were no signs of defects or burnt circuits.

Right now, I'm guessing something is wrong at my Buffer box. I have my aftermarket dash speedo hooked up to the Purple wire input from the speed sensor and it works. That's a 4000ppm count. But when I hook it to the Light Green w/black I get no reading at all while in motion. Which is also 4000ppm count. So again, does anyone know how to test the light green w/Black wire output from the buffer box?


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