VSS Buffer

The Vehicle speed sensor adapter I had finally broke (shaft sheared off) and I figured it was time to revisit my attempt at using the stock VSS in the speedo cluster. I didn’t want to try the home built buffer again, so I started searching the web. I found that there is an actual GM part that is reported to be on many of the F body cars and Cadillacs with 5.0, 5.7 and some 6cyl with TBI. The part number is 25071437 and is called the “4 out buffer”. I found mine on ’87-’88 Pontiac Trans Ams. It takes in the 4kppm signal the VSS generates and splits it into a 4k signals and two 2k. The two 4k is for the electronic speedometer and the 2k goes to the ECM and cruise control computer. The biggest problem was finding one of these at the wrecking yards.

VSS buffer pinout
VSS buffer diagram

More description and pictures can be found at GMThunder (bottom of page) Also, read the rest of the GMThunder web site for great info on EFI conversions and tuning.

To wire it up, I connected the pink/black wire on the VSSB to the pink/black wire going to the ECM (hot in start and run). I connected the black/white wire to the black/white going to the ECM (ground). The purple wire connects to the green/black wire at the Toyota “Emission Computer” connector. The brown wire connects to the brown wire going to the ECM VSS signal.

I still haven’t gotten it to work properly yet. I get the signal from the VSS, just no outputs from the buffer. Will have to check my wiring.

One reply on “VSS Buffer”

This system is so complex it is shocking!
Great article explaining how it all works, when it works !


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