VSS issue in 6.0 swap in 94 4x4

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Originally Posted by 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.

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I did splice into the blue wire. Was that the wrong one in my picture? I spliced into the solid blue. Do I need to splice into the lt blue striped one?

yes, the light blue wire goes to the speedometer, the dark blue feeds something else

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Originally Posted by slow94

yes, the light blue wire goes to the speedometer, the dark blue feeds something else

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When I splice into the light blue wire, do i keep it still connected to the buffer box ( like i did with the dark blue wire in the previous pic), or do i cut it so its not connected to the box. Thanks!

I clipped the wire from the box, and it works fine

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Originally Posted by slow94

I clipped the wire from the box, and it works fine

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Is there a need to hook the solid dark blue wire back up to the box. ( the one that i tapped into the first time)

I believe that dark blue wire went back to the tbi pcm, if you removed the tbi pcm, id just cap that one off

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Originally Posted by slow94

The light blue goes to the guage.
http://pbr992.photobucket.com/albums...g?t=1375412060

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The light blue/black stripe wire goes to your speedometer
The red/white stripe wire goes to your cruise box (if you're using cable throttle, you'll need this hooked up, if drive-by-wire you dontl
Brown c9 is fused power feed for the drac buffer box (just leave alone)
Brown c13 is trans output speed to the tbi pcm (not needed)
Black/white stripe is ground for the drac buffer (just leave alone)
Dark blue goes to the Tbi pcm (not needed)

Purple and green are the vss input wires from the old tbi harness (not needed)

Thats all I can get from my Chiltons.
On my first 94 swap I used a dbw setup and just hooked into the light blue wire. The 94 I'm working on now is a cable throttle, so I spliced the light blue and the red/white wire into the green wire from the lq4 pcm, its not finished yet to tell if cruise will work though.

Quote:

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Originally Posted by slow94

The light blue/black stripe wire goes to your speedometer
The red/white stripe wire goes to your cruise box (if you're using cable throttle, you'll need this hooked up, if drive-by-wire you dontl
Brown c9 is fused power feed for the drac buffer box (just leave alone)
Brown c13 is trans output speed to the tbi pcm (not needed)
Black/white stripe is ground for the drac buffer (just leave alone)
Dark blue goes to the Tbi pcm (not needed)

Purple and green are the vss input wires from the old tbi harness (not needed)

Thats all I can get from my Chiltons.
On my first 94 swap I used a dbw setup and just hooked into the light blue wire. The 94 I'm working on now is a cable throttle, so I spliced the light blue and the red/white wire into the green wire from the lq4 pcm, its not finished yet to tell if cruise will work though.

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Awesome info, thanks alot. Speedo is working right now ( just way off with my big tires and 4.56 gears) and it is shifting all by itself. Now I just need to figure out if it will shift in 4wd. I got it to go in 4wd in my yard, but didnt want to take it down the road. Thanks for the help!

Just an update for my swap. It only took about 14 months but better late than never.

I had everything hooked up and working except the 4 Low circuit to the PCM. Well, after some testing and lots of research, I simply hooked the dark blue wire from the transfer case switch to the gray/black wire from C-2(Red), position 16 of the PCM. Works like a charm. Turns out the GM wiring diagram for the OBS trucks is wrong where is describes circuit # 1493(dark blue) gets power when the 4 wheel drive is activated. When I disconnected the wire and checked it out, it gets grounded when in the 4 low position. It's open in all the other positions. That's just what the new PCM needs at C-2 position #16 in the 4 low position. After checking a few sites, it turns out that the 4 wheel drive diagram is mistaken.

Figured I'd let others know in case they hit the same stumbling block.

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Originally Posted by Mickeymotormount

Just an update for my swap. It only took about 14 months but better late than never.

I had everything hooked up and working except the 4 Low circuit to the PCM. Well, after some testing and lots of research, I simply hooked the dark blue wire from the transfer case switch to the gray/black wire from C-2(Red), position 16 of the PCM. Works like a charm. Turns out the GM wiring diagram for the OBS trucks is wrong where is describes circuit # 1493(dark blue) gets power when the 4 wheel drive is activated. When I disconnected the wire and checked it out, it gets grounded when in the 4 low position. It's open in all the other positions. That's just what the new PCM needs at C-2 position #16 in the 4 low position. After checking a few sites, it turns out that the 4 wheel drive diagram is mistaken.

Figured I'd let others know in case they hit the same stumbling block.

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Thanks for sharing this...I completed my swap 6 month ago and not finding time to wire my 4 low! I'm very happy to see that the diagram is wrong because I don't want to activate a manual switch to ground and if I can avoid to wire it trough a relay it will be cleaner.