I'm in the middle of an engine swap that includes a Nissan t-case, TH700R4 and LB4 4.3 Chevy TBI. The engine/tranny combo is out of a 1990 S-10 Blazer (Z VIN). The Nissan t-case has an electronic speed sensor giving out square signal 0-0.5V. I know the TBI and TH700 also need a square signal (2000/mile), but at which voltage ?
I don't want to run closed loop as I want to keep a stable idle and lock the converter at Hwy speeds.

The '90 GM VSS provides a square wave by completing a connection to Ground. The VSS wire from the ECM hooks to the VSS sensor and the "other side" of the VSS is connected to Ground. The square wave is generated by an internal wiper in the VSS sensor that has a 50% duty cycle between 0 ohms and infinite ohms. Not sure about the 2000 pulses/mile but it should be something like 4 pulses/rev (might equal 2000 pulses/mile). Bottom line, it's a connection to Ground, you should NOT be feeding the VSS wire with a potential greater than 0V.

Thanks.
That makes sense, even when it means I need to run a seperate VSS for the ECU then:(

I don't want to run closed loop as I want to keep a stable idle and lock the converter at Hwy speeds.

What does closed loop have to do with a VSS?

You can run closed loop on the O2 sensor, which has nothing to do with VSS and run a closed loop with grounded VSS sensor wire which will eliminate lockup in the converter and may cause problems with the engine stalling.

Open loop or closed doesn't matter as far as I know but not running a VSS can cause engine stalling issues for some. If I understand you right I think you just want to run a VSS so you can use the lockup in your torque converter?....I don't know the answer to your question about the voltage..

Yes, it's all about torque converter lock up and the stall problem.