This is not a question, I'm putting this up in the hopes that it may help someone at some point.

Most people swap in a GM transmission with their LS. In that case it's usually pretty easy to configure the GM PCM to drive the vehicle speedometer.

My swap was into a Toyota keeping the Toyota 5 speed, but it should apply to any vehicle that uses a 3 wire VSS.

Why care about the GM PCM knowing speed? DFCO enable. Electric fan control. Cruise control.

As we know our GM PCMs connect to a 2 wire VSS from the factory. 2 wire being a reluctance type sensor so the PCM expects an AC waveform.



A lot of Toyotas use a 3 wire sensor. These are active devices and generate a 5v square wave output. This is then typically buffered by the speedometer before being sent to the ECM.

They even show you the open collector buffer transistor in the vehicle schematics:



One of the great things about the GM PCM is that it lets you tune the number of pulses per mile. So it doesn't matter that we won't be getting the exact pulse rate as a GM sensor (or that we have different gear ratios and larger tires).

Since the square wave is 0-5v and the GM PCM likely has some circuit or logic that looks for a 0 crossing we cannot simply connect the output of the 3 wire VSS or speedo directly to the PCM. The signal has to have some negative portion.

If you google around you see that many people buy an expensive little black box to convert between the 2 waveforms. But you can do the same with a 2 components worth cents.





This is how the signals look at 15mph in my Toyota:




The yellow waveform is the output from the Toyota speedo. Approx 0-5v square wave as expected.

The red waveform is the output of the above circuit when connected to pin C1 22 on a 411 PCM. The yellow waveform is the input. You'll notice that it's not a very big signal (~400 mv peak-to-peak) but the important part is that it does go negative for the low part of the signal, meaning there is a zero crossing for the PCM to detect.

In HPTuners I set Speedo -> Calibration -> VSS Pulese per mile to 4200 and the GM PCM had an accurate speed.

 

Nifty!

 

So you installed a capacitor and a resistor, is that right?

Andrew

 

Cool,so the other VSS wires go to 12V and the other ground,right?

 

Zizzle-this worked on my Porsche swap .Thanks for posting ! By the way an excellent write up .
Dave

 

 

just how in the heck did u do this what sort of relay and resistor did u use ive tried all I could for day to get the t5 3 wire vss to work with a vt speedo with no luck any help would be great sick of using my phone around town and then going flat battery on me

 

can anyone chime in here to help a little more. i bought the resistor and capacitor. what does C1 22 have to do with this? can't i just tie into the high and low signal input wires from the pcm to this OP's resistor/capacitor diagram and then the other two wires on the 3-wire VSS get 5v signal and ground from the pcm empty slots? am i missing something?
also, i have heard that the ford gt500 2-wire VSS works in the tr6060/t56....can't you just pop one of those in and voila?

 

What are you trying to do?

 

i'm trying to use the 3 wire hall effect sensor, but my PSI harness and 411 pcm demand a VR signal from a 2-wire sensor.

 

Zizzle,
thanks for sharing.

 

I have a two wire Ford sensor in My T56 magnum transmission. Does anyone know if the ford vss uses the same square wave form as the GM sensor?

I have changed the number of teeth in the tune to compensate for the difference, but something still seems like it might be off.

 

if it uses a 2-wire its a VR signal not a square wave. square wave requires 3 wires (5v, "ground"/low voltage, signal)

if your 2-wire doesn't work let me know. i'll buy it for testing lol

 

did you figure it out? I’m just getting truck running and needing to figure this out.

 

Thank you!!! I ran into this problem with a cam sensor on an engine swap. I found a million solutions for VR to Hall but nothing the other way til this. You totally saved the day!!!!!

 

This is an old thread, but I could use some help. Mirrored wiring above but still ECM does not see speed. Not sure if I got the correct parts and settings. Can anyone confirm these are right, 1uf 50V capacitor and 1M Ohm 1/4 Watt Carbon Resistor?

What "VSS Pulse Per Rev" setting do I need with a Jeep 8k PPM VSS to E40/LS2 ECM? I only need to get the speed to ECM, Jeep has a separate GPS speedometer.

Thanks for any/all help!