Heres a good thread from CF. Altho this is for an OBDII PCM, the idea is the same. You can see were Pin A31 and A32 are for VSS Ground and VSS Signal. The DD unit takes that signal from the trans and translates into something that our ECM will understand. The micro switch settings on the unit are for that purpose.
Here's an interesting post from this thread:
"
If 85-89 used a 2k pulse per mile VSS, there must be a typo
in the 89 service manual.
It shows a block diagram lower right corner that the VSS is 4k pulses
per mile.
If he needs to correct the VSS frequency so it can be
used on the 87 Dash Cluster he can use an electronic ratio
adapter.
If the wires are connected, I would think he would get some kind of reading on the display even if the frequency was off.
Something similar to the Dakota Digital's SGI-5 may work.
The SGI-5 can convert a sine wave from 4K to 2K but the converted
output signal does not appear from their description to be a sine wave.
You would need to contact them to see if it would work."
Here's the thread.
http://forums.c**f**m.com/c4-tech-pe...gnal-help.html
