# Conversions & Hybrids - vss voltage square wave voltage peaks

11-04-2004, 07:18 PM
could anyone tell me what the vss voltage peak is after it is converted to square wave. I have a ls1/t56 in an 89 s-10 and I want to make my speedometer function properly. I believe the t-56 vss gives off 4000 pulses per mile and my trucks cluster receives 2000 pulses per mile. If Im wrong, please correct me. I am making a divide by 2 circuit on a small board with a few terminals and parts from radio shack to make this work. If anyone else has an easier way to do this without changing the reluctor wheel on the tranny, please let me know. thanks.

Scooter70
11-04-2004, 10:26 PM
That sounds about right. The 4k/mile is correct for the T56, I'm not sure about the S10. I couldn't tell you the voltage peak of the wave though. It should be a pretty simple curcuit if it's just a frequency divider.

11-04-2004, 11:47 PM
i believe it will be. Now all I need to do it is the voltage peaks, hope someone can chime in with it, i think it will be in a helms manual or something. thanks for the reassurance on the ppm scooter!

Speartech
11-05-2004, 10:20 AM
The T56 doesn't really "put out" 4000 PPM. How would it know when it's gone a mile?
It simply puts out a pulse train to the PCM. How many pulses per mile that turns out to be is calculated in the PCM based on the pulses per rotation of the output shaft, the tire size on the vehicle, and the rear axle ratio.

The PCM calculates the info and it generates a square wave of 0-5 volts at a rate of 4000 pulses per mile.

I haven't seen an electric GM speedo that wasn't set up for 4000 PPM, so I don't think you need to make a divider.

The problem you run into on truck swaps is with the ABS. On early trucks the VSS signal from the trans is sent to the DRAC module. It then conditions the signal and sends 4000 PPM to the speedo, 2000 PPM to the cruise control module, and X number of pulses to the ABS system. If not set up right you might lose the ABS function, but still have normal brakes.

If you have ABS I think it would be difficult to keep it functioning correctly using a T56 due to the low pulse count out of the T56 VSS, compared to the 4L60E's 40 teeth per revolution.

As for the speedo, if you take the SPEED output from the PCM and hook it to your speedometer, and program the PCM with the correct gear ratio and tire size, your speedo should be right on.