OBD1 PCM interchange/compatibility, 5.2 vs 5.9

fshd4it said
Hey all. I'm wondering about the negative effects (I've read fuel curve differences) of running a 5.2 PCM with a 5.9 engine. '92 Dakota 5.2 donor w/a 518. Motor was junk, hence the 5.9, and I do have a 94-95 5.9 PCM. I'm trying to figure this out because of the wiring change between the early 2-wire VSS, which also runs a speedo cable, and the later '93(?) up 3-wire no cable version. The combo unit works off sensor ground and speed signal to the PCM. The later unit, however, is wired to the 8v supply (in series w/the cam and crank sensors), the sensor ground, and the speed signal. Nutshell- if the newer 5.9 PCM won't recognize the 2-wire VSS, what issues am I going to see with the 5.2 unit?

I am doing something similar to my 91 Dakota, but rewiring the engine compartment to use the Magnum EFI.

Recently ran across a post on a different forum where the guy ran into the speed sensor issue and had a good fix.

He found that if you use the standard 2 pin sensor with the speedometer cable hookup, and run one pin to ground and the other pin to the PCM, it all works just fine.

Here is a direct quote: "What nobody ever mentions is that is it a Reed Switch sensor. Not a hall sensor nor a VR sensor as you might imply from the two wires. It's basically a little magnetic switch that switches between connecting the two inputs and open circuit 8 times per revolution. What this means is that with the VSS pin on the PCM pulled high internally, all you need to do is run ground to one pin and the other to the VSS wire in the C2 connector and you will get an 8K ppm digital signal. Speedometers are designed to turn 1000 times per mile. If you buy the longer speedometer cable to fit a 4x4 Dakota, it's the right size with the right ends for your older 4x4. You can also order the correct sealed plug from standard motor as well --- S727 or grab one from a junk Dakota."

https://ramchargercentral.com/proje...D=fea67c3529ae967c0d0f1f71aa9ed241#msg3502670