VSS - Understanding?

RootBreaker

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So I am still working on my 8.1L swap..getting farther though.
Today, I started wiring it up and got stuck with Crank sensor wire, maf and vss...
ok maf, i need inlet tube and sensor, crank...wrong plug..grrrr

anyway..the vss part. Been trying to understand it but its not sinking in.

The guy that did my harness knows I have a manual (sm465) and there is a vss wire. I thought it went to the trans but the more I read, on 4x4s it goes to the xfer case.

Now questions....
1. Does the vss simply supply speed via wire vs speedo cable?

2. Is vss required by ECU? will computer function without this?

If it is required... I could get the adapter with no cable pass through and get the signal back to ecu.
or
I could get the passthrough to connect my speedometer cable up. This is possibly just a waste...Unless...I can find who/how to calibrate it

So I have an np205, then a T adapter with gears in it to calibrate speedo....
If I went with passthrough I would have...

NP205 - T adapter - Vss with passthrough - speedo cable

and like said speedo wont be right...
so need to understand where to go from here...
I am hoping @Larry chimes in as I know he had a sm465 behind his 8.1L

thx
 

HankScorpio

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I think I remember reading in the 8.1L swap thread that VSS is irrelevant. Read it all, I have an 8.1L in the shop waiting for my V3500. I think TBI needs the VSS, my Blazer needed the ORD conversion when I swapped a 205 in to get the check engine light to shut off. The 8.1 should not care how fast the truck is moving, especially with a manual.

This gets electronic VSS into your 205:
 
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RootBreaker

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I think I remember reading in the 8.1L swap thread that VSS is irrelevant. Read it all, I have an 8.1L in the shop waiting for my V3500. I think TBI needs the VSS, my Blazer needed the ORD conversion when I swapped a 205 in to get the check engine light to shut off. The 8.1 should not care how fast the truck is moving, especially with a manual.

This gets electronic VSS into your 205:
ive read the thread but yeah time to go back through it... lol
 

Chevy305

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VSS should be a twisted pair of wires one being a type of green, the other being a type of purple. With a manual, all the ECM needs to know is if the vehicle is moving, it doesn't care if the speed is correct or even close for that matter. If the ECM sees you moving it tends to help the return to idle to prevent the rpms from dropping too low and stumbling. Is it completely necessary? No. Are there plenty of people driving around just fine without it? Sure. If you have return to idle while moving issues, can it help? You bet.
 

RootBreaker

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VSS should be a twisted pair of wires one being a type of green, the other being a type of purple. With a manual, all the ECM needs to know is if the vehicle is moving, it doesn't care if the speed is correct or even close for that matter. If the ECM sees you moving it tends to help the return to idle to prevent the rpms from dropping too low and stumbling. Is it completely necessary? No. Are there plenty of people driving around just fine without it? Sure. If you have return to idle while moving issues, can it help? You bet.
thx.. I will keep that in mind..
 

dyeager535

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It depends how things are setup. Different years required different VSS input. Up until 1990 it was all run off the back of the speedometer in the c/k/r/v, with a buffer. 1990-91 was twisted pair, vss off the tcase, no speedo cable, used a DRAC module.

Later on I don't know, probably twisted pair straight to the pcm.

Its pretty common that stalling on deceleration is a symptom of no vss. But the later you get (or more advanced) the more likely it is to exhibit other symptoms I'd bet. Trans and driving habits probably matter too. But while TBI for instance only seems to exhibit lack of VSS with the staling problem, TPI, same vintage, will "lose" idle without VSS being hooked up.
 

ZooMad75

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@RootBreaker - you don't need to have VSS for it to run with the manual. Tuck it back in the harness and move on to the next issue.

Both of Larry's trucks and mine lack the VSS input to the PCM for the 8.1. They drive fine without it. No stalling or odd idle issues when coming to a stop. We are using cable throttle, but the dbw shouldn't be looking at speed either.
 

Magikal

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@RootBreaker - you don't need to have VSS for it to run with the manual. Tuck it back in the harness and move on to the next issue.

Both of Larry's trucks and mine lack the VSS input to the PCM for the 8.1. They drive fine without it. No stalling or odd idle issues when coming to a stop. We are using cable throttle, but the dbw shouldn't be looking at speed either.
Good to hear. Also getting ready to do the sm465 swap and ditch the 4l80e behind my 7.4l vortec and I've been pondering this very issue about the vss.
 

RootBreaker

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Good to hear. Also getting ready to do the sm465 swap and ditch the 4l80e behind my 7.4l vortec and I've been pondering this very issue about the vss.

dude...are you crazy on that swap? u in big boy pants??? cuz sm465 is big boy territory.... love mine!!!!
 
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