I am using a Holley Terminator X on my 1991 Formula that was a 350TPI car originally. From the factory, the VSS signal was sent to the ECM and then the ECM sent a signal to the instrument cluster speedometer. I have read I can just wire the VSS direct to the gauge cluster and it will work and I have also read that I need either a Dakota Digital SGI-100BT or a Classic Industries SN74Z converter box to make this work.

when looking at the wiring diagrams, it seems like I can just wire the high signal to the gauge cluster and the low signal to ground. The ECM just intercepted the signal to use it for EGR operation etc and then sent it to the speedo.

 

If I remember right, stock gauge requires a 4000 PPM (pulse per mile) square wave that never goes below 0V. There's a better name for that kind of waveform but I can't find the word right now. But point is the speedometer cannot understand the trans speed sensor.

If you have a Holley ECM with auto transmission feature then there is an output that will deliver 4000 PPM from ECM to speedometer. If you have a basic Terminator X then you will need a signal converter (like from Dakota Digital) to convert the speed sensor sine wave to a 4000 PPM.

Some aftermarket gauges can be wired directly to the transmission speed sensor.

 

If your transmission has the same speedo gear tooth count (pulse per revolution) then theoretically it might be possible to re-use a stock yellow signal converter box (whatever it's called).

 

I think you're referring to the VSS buffer box? I used one for my original FiTech swap to drive my speedometer since the factory ECM was no longer present. Two wires for power, two to read from the VSS, one wire is a 4000ppm output, the other two are 2000ppm outputs. As I recall, you want the pin labeled "ECM Output" in this picture. If you hook up the wrong one it shouldn't hurt anything, it will just read wrong.

I believed I picked up one for $20 used, not sure what they're going for now.

 

I'm guessing though that it's got to be the same tooth count as 700R4 because all that is hardwired into design and not programmable. So for example it would not work with a T56 having 17 tooth, right?

 

I think it's more complicated than that, because wheel/tire size, rear gear ratio, and tooth count all factor in.

Again, I'm going off some hazy memories here, but as I recall the difference between the factory ECM using a 700R4 (my car was originally an auto) and this method with a WC T5 was either nonexistent or small enough that I didn't care. I do remember testing against some of those stationary speed warning signs, for what that's worth. I might have just gotten lucky with my choice in part. But if they're still relatively inexpensive it might be a better solution than going with a Dakota Digital box.

 

My car does not have the buffer box as shown in the posted wiring diagrams. The ECM receives a 4000 pulse per mile input and outputs a 4000 pulse per mile output to the speedo. if you have cruise control maybe there is a buffer box to convert a 2000 pulse per mile signal for that system but mine was not equipped with cruise control.

I have a Strange S60 rear end with 3.73 gears so I have a newer magnetic reluctor VSS with a tone ring installed on the output shaft of the 700R4 but it outputs the same 4000 pulses per mile for that gear ratio. (I purchased the VSS from Monster Transmissions). Originally the car has a plastic gear (red in color) on the output shaft and another gear on the VSS that generated the AC waveform signal.

I'll just try to hook up one wire to the speedo input and the other to ground and see if that works. If it doesn't, I'll buy the Dakota Digital Box. I'll keep you guys posted and let you know if it works.

 

Sorry, I didn't look at that wiring diagram and didn't realize the late cars were different.

I suspect the wording on that diagram is maybe misleading though. It says 4,000 PPM at speed sensor and ECM but I don't think those waveforms are similar enough to be interchangeable in function. But I guess I've never tried either so....