I have been trying to swap a 1996 LT1 engine and transmission out of a Cadillac Fleetwood into my 1986 el camino. I bought a digital dash because the old engine used a cable speedometer. The speed sensor in the transmission has 2 wires, and it seems to go to the ecm. The ecm has a speed wire that I hooked up directly to my dash. The speedometer has never worked. It sits reading 0 mph all the time. I used a multimeter on the wire out of the ecm, and it did have voltage that changed with the speed of the car, and changed when I was stopped. I sent the dash in, and they verified it works. Does anyone know why my speedometer does not work? I wonder if I need a speedometer filter, but there is no info on the internet that I could find on these things. It could be a ground issue, i guess, but I have no idea where it would need one that it does not have. I did replace the speed sensor while assembling the project, and had to change the plug on the 94-95 wiring harness I got with the engine to make it plug in to the speed sensor, but I was very careful to get the plug wires from getting hooked up backwards. The reading from the multi meter seems to prove everything is working somewhat, although I don't remember what range I was getting, and if its the right range for my dash. I do remember it was not exactly what I was expecting from reading the dash manufacturer's instructions to test for signal, but not that different. I think it was higher than expected.
One solution I thought of was to tap one of the wires from the sensor and take that to the dash, but I am afraid that would rob signal going to the ecm and cause problems. If I am wrong and I will have to do it this way, would I tap wire "A" or "B"? I did plan to hook up a cruise control unit, and that would possibly need to use the wire coming out of the ecm.
The car drives fine, except it shifts into overdrive at 50mph instead of 40mph, which I thought was odd, but could be right for that car it came out of. It goes up into the mid 2000's before shifting.
Thank you.

 

From what I remember there’s no voltage coming from the vss wire coming from the ecm to the Speedo it’s i might be wrong but in my application since I don’t run the electric speedo I have to run 12v switched power with a resistor to that same wire from the ecm just haven’t done it yet I’m hoping after that my transmission will shift normally

 

93 LT1 is the only one I'm aware of that used an external buffer, the rest will condition the signal coming out of the PCM. What sort of signal is the dash anticipating and what is the PCM putting out? You can't just assume this is all plug and play unless someone with an identical setup has it working.

 

I guess not. I don't know what kind of signal is expected or sent. All the instructions said to do was hook up the wire in the dash to the one out of the ecm. It has occurred to me that the problem could be here, but I don't know what to do if that is, or how to know.

 

Are you using a factory digital dash or aftermarket?

 

Aftermarket from Intellitronix. It says it should hook up to the speed wire from the ecm.

 

Any aftermarket cluster should specify how many pulses per mile it seeks. What does the manual say?

 

It says it is looking for 8,000 pulses per minute.

 

IIRC GM is 4k ppm directly from the VSS. I'm not sure what the PCM itself puts out.