Is it possible for the VSS to bypass the PCM?

[gpchris]

Being obd1.5 I am not able to turn my auto trans codes off but i have it running perfect with no check engine light only when vss is unplugged. When it's plugged in I get minor surging at speeds of 0-15mph. It would be nice if I could just bypass the PCM and send the vss signal straight to the cruise, speedo etc. From the diagram below I'm a little confused as to what the hell those 2 speedo wires do inside the PCM.

 

 

Any ideas?

[GnatGoSplat]

No, you can't bypass the PCM. The VSS generates a small AC sinewave voltage. The PCM takes that input, and converts it to a grounding output of 4000ppm. They are very different, electrically. You would need some kind of signal converter.

[RobertISaar]

depends on how good you are with electronics...

 

a 1995 4T60E as used in any 3100 or 3.4 W-body (not sure about the 3800s) produces a ~24,000 pulse per mile signal due to tire size and number of teeth on the VSS reluctor. your speedo, cruise and all that run on a 4,000 pulse per mile signal from the ECM, so if you accomplished hooking it up to where a signal can actually get through, everything would read 6 times faster than it should... that's where the ECM comes in, since it reads the 24,000 PPM signal, uses it for it's purposes, then divides the current frequency by 6(it's changeable in the tune, but all 94-95 3100/3.4 use a /6 operation) and sends it out as a 5V square wave signal.

 

but i believe the VSS can generate up to ~100 volts(i know it hits 30 at a minimum) or so in a sine wave depending on how fast and close the reluctor is spinning, so that's an issue, since the dash would probably go up in smoke...

 

the simplest things i can think of would be either something like a dakota digital converter or possibly wiring in another PCM and only connecting just what it needs to read the VSS and send that signal to the cluster/DIC/whatever... if something like a 555 timer set in frequency division mode would work, that would be uber cheap, but i don't know if those voltages would be within it's working range.

[BXX]

Convert to OBD2:thumbsup:

 

Or per what Mr Saar would recommend, convert to OBD1.

 

Both are simple enough operations.

[RobertISaar]

actually... i would recommend me getting my ass in gear and collecting enough BINs from the various speed-density 3100 and 3.4(iron head) OBD1.5 PCMs to start hacking through all of them to allow for OBD1.5 tuning the way the 94-95 LT1 guys can do.

 

so far, i've gotten ahold of a 94-95 f-body 3.4 automatic PCM, which used the 4L60E, but consider that the 94-95 f-body 3.4 manual trans applications used the same PCM as the 94-95 speed-density 3100s... so the capability for the factory PCM to work well with a manual trans is there, just not easily dealt with yet.

[BXX]

Yeah, not easily dealt with, yet:lol:

 

OBD2 swapping would easily take me less than 6 hours on a 3100 car including a read and reflash.