I bought an electronic speedo conversion from D&M Restoration. Seems good. No complaints on that. I think he buys the electronic part from somebody else and just puts it on the speedo face. I asked him this question and he hasn't been able to answer me. Here's my question: I also bought a 12V pulse generator off ebay that I am considering trying to use to "run-up" the odometer to match what my mechanical odometer displays now. The speedo is "looking for" a sine wave from a "2-wire magnetic sensor". The pulse generator puts out a 12V square wave. I'm not worried about the square vs sine wave. I'm thinking that'll be "close enough". I'm worried that the output wave will have too much power and will fry the speedo's chip. I am attaching a link to the ebay pulse generator, the instructions for the speedo, and the description of the Tremec speed sensor that I'd be mimicking with the pulse generator.

Anybody done this? Will this work? Or, will I fry my speedo?

pulse generator: https://www.ebay.com/itm/192863557531

 

As long as the voltage is correct, your pulse generator won't draw anymore power than it needs. Think of it as a simple Ohm's Law: Current = Voltage / Resistance. Current is what will fry a circuit, but as long as you keep the voltage what it should be, the pulse generator won't pull more than it should because the resistance is fixed. Voltage is the variable and a long as you keep that where it's supposed to be, you'll be fine.

Also, I agree that square vs sine should be fine as well.

 

With 12V and ground connected to the speedo, the two yellow wires have 4.8V on them. So, that's what the speedo would send to the speed sensor in the tranny. The pulse generator will produce a 10V peak-peak square wave if I connect it to a 12V car battery. I'm not sure if the 4.8V on the unconnected yellow wires from the speedo means it'd want a 4.8 V-PP or a 9.6V-PP square wave. If I give it a square wave with double the peak-peak voltage is that enough to fry it? I can try it first using 4 D batteries which would produce a 5V peak-peak square wave...

 

I took a look at the ebay ad and the pulse generator does vary its output voltage based on the input voltage, so yeah, you might need to be a little careful. I'd suggest trying your 4D battery idea first and see if that works. You could also look and see if you have an AC to DC converter that puts out 5v, something like this:

 

Are you certain that the 4.8v goes TO the speed sensor? I thought that a GM two wire sensor has a ground and a signal wire.

 

FYI for those who tried to help, I gave up on the whole pulse generator to run up the odometer idea. If the new electronic speedo does have a way to use a pulse generator to do that, the guy who sold it to me is not helping and won't give me contact info to talk to the people who actually make it. What's funny is that while screwing around with it, I noticed that when you hold the button to get to the calibration mode, there is a mode that causes it to display "pulse". But, in the instructions, they just say "don't do that". Ok whatever. So, I will just do what they said to do and drive a mile to calibrate it. That means the odometer will display zero miles and will count up from there. I guess that's not such a huge deal. I just thought it'd be nice if I could have the odometer go in with the correct number matching the old one. Of course, my car is not of the ilk where the odometer reading is really going to make any difference. If/when I ever put mine up for sale, the odometer reading will be irrelevant. So, I don't know why I was so worried about that anyway.

After I fiddled with it, I basically checked to confirm it was behaving the same way it was before I did anything which it was. So, I don't think I did any damage. Just wasted my time and you guys' time. When will I learn?

 

Yeah, I'm not sure that "to" is the correct verb. There was 4.8V across the two wires. I Googled a little more and I believe the way this VSS works is that it basically shorts the two wires 17 times each revolution. So, the speedo will "see" that. Whether it's measuring amps or volts I don't know.