VSS issue in 6.0 swap in 94 4x4

[HPICamaroSS]

All of this is assuming your still running the 94 ifs and np241 that came in the 94 originally

Correct. 94. IFS. But I switched to a 94 2500 tcase so the splines would match up with my 4l80

[HPICamaroSS]

Looks like mine on my '93. The tailshaft sensor is the VSS that goes to the PCM. As depicted above, that sensor shown goes to the floor shift lever.

I left everything original except for hooking up the 4wd Low wire from the PCM. That is something that needs to be done. Since the drivetrain will be spinning at a much slower RPM compared to engine speed in the 4 Low position, the 4 low wire from the PCM must be grounded so the PCM "knows" the drivetrain is in 4 low and shifts the trans accordingly. I mistakenly removed that from my modified original harness and now must repin the PCM and find how I can get the wire grounded in the 4 Low position. It will take some doing but I figured I'd mention it so someone else doesn't make the same mistake I did.

Thats really good info. I was having the same problem. Is there any wire specifics you can give me. Routing/ Splicing into what wire? Color of wires? Thanks!

[HPICamaroSS]

AND since my 94 tcase has a different style vss connector compared to nelsons new style vss pigtail, is that all you do is cut off their pigtail and put the old OBS connector on their harness so it plugs into the tcase? Does it matter what way the wires go?

[HPICamaroSS]

+1 to the above.

What do you guys think? I have my green speedo wire spliced into the blue wire at the vss buffer box. Do i need to cut the blue wire so its not connected to the buffer box? Here is a pic of how it is.

[Mickeymotormount]

I think you're hooked to the wrong wire. My '93 truck schematic has the light blue/black wire going to the speedo.The dark blue wire is the Rear Wheel Antilock Brake wire. If you pull the instrument cluster and look at the plug, there are letter designations for all the terminals on the plug. Letter designation "S" should be the light blue/black wire. This wire is the speedo wire. You can either snip it here and run your green wire directly to the plug or snip it at the buffer box being sure to eliminate the buffer box from the circuit.

[Mickeymotormount]

Thats really good info. I was having the same problem. Is there any wire specifics you can give me. Routing/ Splicing into what wire? Color of wires? Thanks!

Here's the schematics and a description of the operation of the transfer case for a '99 Silverado with the 6.0.. Following years may be the same or different. The 4 wheel drive wire is gray/black and is pinned at the red C-2 connector at location 16. From '03 on, it's the green connector(C-2) and the same gray/black wire at pin #16.









Still reviewing my '93 schematics to find out how I can ground this wire when I switch into 4 low. Stay tuned, I may be close to an answer.

[HPICamaroSS]

I think you're hooked to the wrong wire. My '93 truck schematic has the light blue/black wire going to the speedo.The dark blue wire is the Rear Wheel Antilock Brake wire. If you pull the instrument cluster and look at the plug, there are letter designations for all the terminals on the plug. Letter designation "S" should be the light blue/black wire. This wire is the speedo wire. You can either snip it here and run your green wire directly to the plug or snip it at the buffer box being sure to eliminate the buffer box from the circuit.

Whats weird is the way u see it hooked up in the pic, my speedo worked with my old vss harness connected at the rear transfercase. So I had the green wire spliced to the blue wire, then I had nelsons vss connector for the input, and my obs harness for the vss output....and the symptoms were it didnt want to shift ( obviouisly)....BUT the speedo worked back then. When I hook up nelsons rear vss output on the tcase, the speedo doesnt work.

[slow94]

The way vss is setup in 94, is the twisted pair wires from the vss going straight into that drac box, then the decoded speed signal is fed to the guages, tbi pcm, abs, and cruise. So when you hooked up the old tbi vss wires, you were feeding the guage, but your lq4 pcm did not have a clue how fast you were going causing the weird/no shift.

On the lq4, the vss sensors go straight into the pcm, the pcm decodes the signal based on tire size, gears, etc and then feeds a speed signal to the guages, cruise, abs, etc.

So now that youve hooked up the nelson harness to the transfer case, youre giving the pcm the correct signal it needs to shift, but you've taken the 94 drac box out of the loop by unhooking it. That is where splicing into the blue wire comes in, your bypassing the 94 vss/drac setup and using the speed output from the pcm. Hope that makes sense.

To get the proper ground to the pcm for 4wd low, you can build a simple relay circuit to apply ground when your 1994 transfer case applies 12v.

[slow94]

http://www.the12volt.com/12voltimages/pos2neg.gif

You would hook the 12v output(that went to the tbi pcm) from the transfer case into pin 86, when activated this will apply a ground to pin 87 on the relay, you hook that straight into the lq4 pcm pin 16. Sorry, im on my tablet and its hard to type out long messages.

[slow94]

The light blue goes to the guage.