Hello all. First time poster. Would love some input.

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?

TONY

 

Any help would be great.


Tony

 

What is the speed difference between the speedometer and ECM reading?

 

GPS speed ... ecm speed
13 is 21
17 is 26
28 is 43
33 is 55

An average difference of .6

I have a 3.73 rear end. 15 tooth drive gear. 45 tooth driven gear. 24.5 inch tires.

I know this is the best I can get unless I lower my rear-end ratio or increase my tire size. Otherwise I need to get speed calibration module by Dakota digital or hypertech.

That's where this testing of the buffer box comes in. The buffer box splits the the vss signal between my cruise control, ecm and speedo. I will put this module between the speed sensor and buffer box. So I need a working buffer box.

 

Anyone?

Tony

 

thirdgen.org says the factory rear end gear was a 2.73

http://700r4.com/speedoCalc/speedocalc.shtml

says (with your specs in post #4) drive gear 19 and driven 34

maybe worth a read

https://www.thirdgen.org/forums/tran...eedometer.html