I just completed a VSS repair on my 1995 Coupe 6-speed and wanted to post my story and troubleshooting in the case that it may help others.

Last year I noticed that my Cruise Control was acting up, but since I don't use it I tabled the repair to the winter (which never happened). A few weeks ago I was on a quick run and on my way back noticed that the speedometer was stuck at zero (was fine on the way out). A few moments later the "Service Engine Soon" light came on, so into the garage and up on the lift. After reading through the FSM and looking online I had an idea where to look. I removed the VSS from the transmission - note to others: don't forget to drain the transmission before doing this, you won't like the gear oil bath!

After researching the part online I was lucky to have found a 3rd party replacement on Amazon! So I ordered it (could always return it if the old unit is good).
Here's the part:
I tested the existing VSS with my Oscilloscope & DMM, driving it with an electric drill turning at 1600 rpm this was the result:



Note that there is practically no waveform and the measured voltage is 1.147 VAC.

The I tested the replacement under the same conditions and this was the result:



The new one had a a waveform and the output voltage was 14.69 VAC. BIG DIFFERENCE.

So the new VSS went into the car with a fresh fluid change for the transmission and I now have my speedometer back!

In summary: I really hate throwing parts at a problem and prefer to troubleshoot and have evidence before I order anything. Unfortunately the FSM had not output specifications for the VSS and only spoke about using a test kit to simulate the VSS output and then watch the dash. I'm hoping that this information will help others with troubleshooting in case they run into a similar issue.

Thanks!

 

Think I am headed down the same road . . . . symptoms match what you had. Have not pulled the VSS out of my car yet. Like the use of an oscope!

My car: '92 ZO7, ~58K miles, with ZF6 and GM3 RPO [3.45 Rear Axle Ratio (8.5 in. Ring Gear)]. I see you bought the SC12. What axle gear ratio does your car have? Standard Motor Parts (SMP) also makes a SC13 and the specs on their page <https://ecatalog.smpcorp.com/STD/#/vehicles/parts/num/S-553?type=r>. Both the SC12 and SC13 apply to your and my cars. Only visual difference in one is made of black plastic and the other is white. Electo/Mecaniical differences are not evident to me. Were you able to determine if the SC12 was a match to your car' axle's ratio? If so how? What color was the VSS gear and did it transfer okay?

 

If you jack up the back high enough you won't need to drain the transmission.

 

Another test of a VSS is to check for continuity across its two pins. You should have roughly 400ohms resistance across them. When mine went bad, it still made some voltage when spinning, but there was infinite resistance across the two pins.