VSS signal and scaling
Thread Starter
Teching In
Joined: Oct 2002
Posts: 49
Likes: 0
From: NE Ohio
VSS signal and scaling A few questions on the VSS. I'm not going to use a GM tranny, so I'm not going to have the standard speed sensor.
Does anyone know what the speed signal is used for besides the cruise control and speedometer output? Is it used for other engine control functions? Is the VSS required to make the engine run correctly?
Does anyone know what the VSS (vehicle speed) signal is, and what scaling it has by default in the PCM?
Is the signal a DC square wave, or an AC type signal?
What is the standard scaling for the signal in the PCM? In know that the reluctor ring on the tranny output has 40 teeth. I figure this to give us about about 8000 pulses per mile with average rearend and tire sizes.
I know we can change the scaling using LS1Edit to adjust for rearend and tire changes. How much of a change can be made?
If I find a speed sensor that only gives me 4000 or even 2000 pulses per mile, can the PCM be adjusted that much?
Thanks!
Perry
Does anyone know what the speed signal is used for besides the cruise control and speedometer output? Is it used for other engine control functions? Is the VSS required to make the engine run correctly?
Does anyone know what the VSS (vehicle speed) signal is, and what scaling it has by default in the PCM?
Is the signal a DC square wave, or an AC type signal?
What is the standard scaling for the signal in the PCM? In know that the reluctor ring on the tranny output has 40 teeth. I figure this to give us about about 8000 pulses per mile with average rearend and tire sizes.
I know we can change the scaling using LS1Edit to adjust for rearend and tire changes. How much of a change can be made?
If I find a speed sensor that only gives me 4000 or even 2000 pulses per mile, can the PCM be adjusted that much?
Thanks!
Perry
Joined: Nov 2001
Posts: 1,707
Likes: 33
From: Anderson, Indiana
Re: VSS signal and scaling Yes, the VSS is used for other engine control functions. It is used by several things, but the most important that I have found is that it's used heavily in determining the IAC position during coastdowns. When you coast down a hill with your foot off of the gas, the PCM sees zero throttle, high vacuum, and high RPM. With no VSS signal the PCM thinks "why is the engine idleing so fast with no throttle; close the IAC until the idle speed is correct". This often results in a stall when you finally do stop, because the IAC won't recover quickly enough from its closed position during the coastdown.
WITH a VSS the PCM sees the same conditions as above, but now it makes a different decision; "I see zero throttle, high vacuum, high RPM, AND a VSS signal, aha we're coasting, don't close the IAC, the high RPM is correct!"
The VSS signal out of the transmission is basically an AC sine wave. Out of the PCM it is a conditioned square wave.
It seems like you are low by about a factor of 5 on your PPM number, but my brain is already off for the night. <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />
You can change the scaling quite a bit with LS1 Edit, but what I've found is that if you aren't using a GM electronic trans anyway, the accuracy isn't that important, just that there IS a VSS signal of some sort.
WITH a VSS the PCM sees the same conditions as above, but now it makes a different decision; "I see zero throttle, high vacuum, high RPM, AND a VSS signal, aha we're coasting, don't close the IAC, the high RPM is correct!"
The VSS signal out of the transmission is basically an AC sine wave. Out of the PCM it is a conditioned square wave.
It seems like you are low by about a factor of 5 on your PPM number, but my brain is already off for the night. <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />
You can change the scaling quite a bit with LS1 Edit, but what I've found is that if you aren't using a GM electronic trans anyway, the accuracy isn't that important, just that there IS a VSS signal of some sort.
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91 Z28 LS2 408CI, LS9 Supercharger, LPE GT7 cam, Yank3000, 3450 raceweight.
Latest numbers: 9.71 ET, 141.42 MPH, 1.40 60' , 610 RWHP Mustang Dyno
www.speartech.com
91 Z28 LS2 408CI, LS9 Supercharger, LPE GT7 cam, Yank3000, 3450 raceweight.
Latest numbers: 9.71 ET, 141.42 MPH, 1.40 60' , 610 RWHP Mustang Dyno
www.speartech.com
On The Tree
Joined: Apr 2002
Posts: 112
Likes: 0
From: Ottawa, ON
Re: VSS signal and scaling The VSS outputs 40 pulses per driveshaft rotation and the signal is A/C.
The VSS signal has one side grounded at the PCM (VSS LOW). The PCM takes the VSSHigh signal and modifies into a square wave and outputs 4000 pulses per mile for use by the speedometer/cruise/radio and miscellaneous others.
The VSS signal has one side grounded at the PCM (VSS LOW). The PCM takes the VSSHigh signal and modifies into a square wave and outputs 4000 pulses per mile for use by the speedometer/cruise/radio and miscellaneous others.
