What does the VSS reading tell the PCM?
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What does the VSS reading tell the PCM? I'm putting an LQ4 6.0L in a Jeep, so this will have a transfer case with a low range (basically a two speed 2.7:1 underdrive after the trans). I'm reading and getting conflicting advice about where to put the VSS sensor.
It seems logical that this sensor should be measuring driveshaft speed, which could be converted to road speed no matter what gear you are in. However some folks have told me that it should go between the transfer case and the transmission because the Low Range will confuse the PCM. I don't understand why the PCM would care if you are in low range or not. Road speed is what it is, regardless of the gear you are in.
The only reason I could see that making sense is that maybe the PCM would never shift an auto out of 1st gear until a certain speed, regardless of RPM? So I suppose with a manual transmission then, you would want the VSS after the transfer case, right?
Advance Adapters says the following in quite a few places:
"Vehicles with an engine conversion using a 1993 & later GM computer controlled engine and an automatic transmission requiring a VSS “should not” use the True-Pulse VSS kit. When using or installing this specific type of drive train combination, we highly recommend a VSS be located in the transfer case adapter* or the automatic transmission. This will give the ECU a correct 1 to 1 reading when in 4wd low range."
* Transfer case adapter being between the transfer case and the transmission.
http://advanceadapters.com/product/2...-cv-yoke).html
Can anyone help me through my confusion?
It seems logical that this sensor should be measuring driveshaft speed, which could be converted to road speed no matter what gear you are in. However some folks have told me that it should go between the transfer case and the transmission because the Low Range will confuse the PCM. I don't understand why the PCM would care if you are in low range or not. Road speed is what it is, regardless of the gear you are in.
The only reason I could see that making sense is that maybe the PCM would never shift an auto out of 1st gear until a certain speed, regardless of RPM? So I suppose with a manual transmission then, you would want the VSS after the transfer case, right?
Advance Adapters says the following in quite a few places:
"Vehicles with an engine conversion using a 1993 & later GM computer controlled engine and an automatic transmission requiring a VSS “should not” use the True-Pulse VSS kit. When using or installing this specific type of drive train combination, we highly recommend a VSS be located in the transfer case adapter* or the automatic transmission. This will give the ECU a correct 1 to 1 reading when in 4wd low range."
* Transfer case adapter being between the transfer case and the transmission.
http://advanceadapters.com/product/2...-cv-yoke).html
Can anyone help me through my confusion?
it should be in the T-case or on the drive shaft. i just replaced the VSS in my 01 silverado and the sensor is on the T-case, and i would assume by its placing that it is reading right off the output shaft. you would want the computer to be aware of the vehicle speed when in a 4L range.
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Vss The VSS needs to be on the output of the 4L60E. I've done two jeep swaps and the adapter from novak comes with a VSS hole drilled and tapped.
The VSS supplies a pulse the that computer needs to shift the trans at the right time. It doesn't care if you are in two wheel high or four wheel low.
I'm not sure but I bet the gear that the Vss reads needs to be the correct number teeth per 360 degree revolution of the trans output shaft.
If you put the vss it on the transfer case out put shaft the shifts will be off in four wheel low.
Just saying what worked for me.
The VSS supplies a pulse the that computer needs to shift the trans at the right time. It doesn't care if you are in two wheel high or four wheel low.
I'm not sure but I bet the gear that the Vss reads needs to be the correct number teeth per 360 degree revolution of the trans output shaft.
If you put the vss it on the transfer case out put shaft the shifts will be off in four wheel low.
Just saying what worked for me.
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From: Downers Grove, IL
I think you hit it right there. I had always heard that the VSS should be after the trans but before the transfer case..but I didn`t ever really think about why. Now it makes sense, because at WOT the trans shift is based off of 2 variables, with an or equation IE: on my car the WOT first to second shift is when the engine gets to over 5600rpm OR if the car goes above 40mph. Well the PCM would tend to get confused if you switched to low range and it saw the engine high in the RPM range but the vehicle speed was only 20 mph (because the PCM would look at the VSS and calculate when the vehicle speed should be). So if the VSS is placed after the transfer case the PCM may think that the transmission is slipping when the transfer case is in low range....or at least that is my analysis at 2:33AM lol. But I do usually come up with my best ideas way after I should have been asleep.
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Thanks for the replies guys. I think most of the info I've seen says it should be in the transmission before the transfer case, but yet GM does put it in the tailshaft of the transfer case in their 4x4 trucks. I wonder how they get that to work?
Perhaps there is a switch in the T/C that tells the PCM that it's in 4low and it uses a different variables to calculate shift points?
I'm sorry if this sounds ****, I understand the workarounds (I like the dual VSS idea and may actually go with that) but my goal is to to make my installation look and work as factory as possible so I'm really trying to learn and understand the OEM way of doing it. One of the reasons I'm doing this project is for the hands-on education.
Perhaps there is a switch in the T/C that tells the PCM that it's in 4low and it uses a different variables to calculate shift points?
I'm sorry if this sounds ****, I understand the workarounds (I like the dual VSS idea and may actually go with that) but my goal is to to make my installation look and work as factory as possible so I'm really trying to learn and understand the OEM way of doing it. One of the reasons I'm doing this project is for the hands-on education.
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From: Downers Grove, IL
What year/bodystyle truck uses the VSS behind the transfercase? I can check if there is a PCM tune for one of those posted in the HPTuners site and I can see if there are 2 different shift tables in the tune for high and low range.
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That would be great if you could look up the shift tables for those trucks!
I'd imagine all factory 4x4s do that or have two VSS inputs. Otherwise how do they get the correct speedo reading in 4low?
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Verdad Gallardo Joined: Sep 2004
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From: Downers Grove, IL
Well I pulled up a PCM file from an 01 4x4 and there is only 1 MPH vs gear WOT shift table. However, there is a switch from the transfer case that sends a signal to the PCM to let it know when its in low range, so the PCM may look at that and recalculate the shift point MPH. The only thing that I don`t understand with that is that there is no where that I see in the programming to specify what the low range gear ratio is. I`m not a 4x4 guy so I don`t know much there, but I`m assuming that there is more than 1 gear ratio available for transfer cases.
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All GM trucks with a VSS from 1988 up have one VSS and it's in the transfer case for the 4x4's. Its no different than if you keep it first gear in the trans. it reads the revolution of the reluctor wheel. So factory location is in the output of the transfer case. The NP241 I used is in the output shaft of the case.
As long as the VSS reads the revolutions of the driveshaft/output shaft then it will read correctly no matter if its in high or low. If its in the adapter its reading the trans ratio not the low range so the VSS and the speedo will be off in low range. And if you have doubler will be worse.
Just like if you change tire size or gears the PCM doesn't know it unless you program it in because they are after the VSS, but it reads the low range gearing based off the VSS just as if it was another transmission gear.
THe signal lets the PCM know its in low you limit speed wise phsically by the low gears and I believe lows the shift points. But it knows the gear ratio based of the RPM of the VSS in the transfercase.
I think they AdvaceAdapters and others just try to up sell their adapters to make more money, I mean $250 buck just for a VSS WTF! If not then why does GM use a transfer case output mounted VSS???? If your using a stock GM case with a VSS, just use the VSS aready installed.
As long as the VSS reads the revolutions of the driveshaft/output shaft then it will read correctly no matter if its in high or low. If its in the adapter its reading the trans ratio not the low range so the VSS and the speedo will be off in low range. And if you have doubler will be worse.
Just like if you change tire size or gears the PCM doesn't know it unless you program it in because they are after the VSS, but it reads the low range gearing based off the VSS just as if it was another transmission gear.
THe signal lets the PCM know its in low you limit speed wise phsically by the low gears and I believe lows the shift points. But it knows the gear ratio based of the RPM of the VSS in the transfercase.
I think they AdvaceAdapters and others just try to up sell their adapters to make more money, I mean $250 buck just for a VSS WTF! If not then why does GM use a transfer case output mounted VSS???? If your using a stock GM case with a VSS, just use the VSS aready installed.
Last edited by Bo185; May 15, 2009 at 08:37 PM.
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All GM trucks with a VSS from 1988 up have one VSS and it's in the transfer case for the 4x4's. Its no different than if you keep it first gear in the trans. it reads the revolution of the reluctor wheel. So factory location is in the output of the transfer case. The NP241 I used is in the output shaft of the case.
Well I pulled up a PCM file from an 01 4x4 and there is only 1 MPH vs gear WOT shift table. However, there is a switch from the transfer case that sends a signal to the PCM to let it know when its in low range, so the PCM may look at that and recalculate the shift point MPH. The only thing that I don`t understand with that is that there is no where that I see in the programming to specify what the low range gear ratio is.
I wonder if the 4Low switch tells the shift routines in the PCM to just ignore the road speed from the VSS (or plugs some constant in there) and shift according to RPM only? Like sort of an "off road only" mode. That would account for the switch and the single table. I'm not sure how this theory could be tested though.
If that's true, I wonder if it would have some performance benefit, maybe to just leave it in that mode all the time or rig it to a switch on the dash. Just thinking out loud...
Last edited by YeepieJeepie; May 16, 2009 at 07:23 AM.
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I used the VSS from the AA conversion trans adapter for ECM because I'm using the original Jeep 231 TC, so the 231 VSS probably wouldn't have been compatible with the GM ECM anyway. The VSS sensor was included in the AA adapter kit. Plus my engine/trans was from a 2wd donor, so the harness wiring/connector was an easy plug in at that location.
Also wasn't sure how the auto shifting would be in low range. It shifts great the way I'm using it now. Used the TC VSS to run the elec speedo since I wasn't using it for anything else. I could care less if my speedo is off while in low range. I'm pretty sure I can gauge what 5mph really is
I do recall seeing a '4wd low' discrete wire in the ECM wiring, but just assumed that it was only for dash status. Perhaps it's also used in the trans shift calcs. Not sure.
Also wasn't sure how the auto shifting would be in low range. It shifts great the way I'm using it now. Used the TC VSS to run the elec speedo since I wasn't using it for anything else. I could care less if my speedo is off while in low range. I'm pretty sure I can gauge what 5mph really is
I do recall seeing a '4wd low' discrete wire in the ECM wiring, but just assumed that it was only for dash status. Perhaps it's also used in the trans shift calcs. Not sure.
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That's the part that puzzles me. That obviously works for the OEM, with correct speedo readings and shift points in 4Low, but yet it seems that few people hook it up that way when they swap these drivetrains into other 4x4s (of course, maybe 50% even hook up the VSS at all).
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You're right though, the Atlas is very expensive and an OEM GM T/C would be a much more cost effective way to go.
Last edited by YeepieJeepie; May 17, 2009 at 09:28 AM.
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Originally Posted by 98blackSS
I do recall seeing a '4wd low' discrete wire in the ECM wiring, but just assumed that it was only for dash status. Perhaps it's also used in the trans shift calcs. Not sure.
Interestingly, here is what AA says about it in their Atlas transfer case guide. This would seem to support that it does more than just light the 4x4 indicator on the dash. The Atlas is available with various low range gear ratios, so I'm sure that's what the last sentence is about.
When ordering this tailhousing with an Atlas, we also include a standard shift control unit that is drilled and tapped to accept a low range switch. This allows the Chevy computer to re-calibrate the output shaft signal when in low range. Note: The Chevy computer may require some wiring and computer program changes for the different low ratios.
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Well I pulled up a PCM file from an 01 4x4 and there is only 1 MPH vs gear WOT shift table. However, there is a switch from the transfer case that sends a signal to the PCM to let it know when its in low range, so the PCM may look at that and recalculate the shift point MPH. The only thing that I don`t understand with that is that there is no where that I see in the programming to specify what the low range gear ratio is. I`m not a 4x4 guy so I don`t know much there, but I`m assuming that there is more than 1 gear ratio available for transfer cases.
Regarding the T/C gear ratio, it's just a two speed with four shift positions (including neutral). There's a 1:1 gear in the 2high and 4high ranges (making it invisible gear ratio wise), then there's an under drive in 4low range. That's usually somewhere between 2:1 and 3:1. The Jeep Wrangler stock 4low ratio is 2.71:1 for example.
Multiply this by your axle gear ratio and whatever gear you shift the transmission to, it obviously lowers your final drive ratio considerably.
Last edited by YeepieJeepie; May 17, 2009 at 09:44 AM.
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At this point I'm not sure what T/C I'm going to use. I was considering beefing up the stock NP231 with a wide chain and HD planetaries, but the VSS issue has me stumped. I would rather not tack a VSS externally on the tail shaft, drive shaft or rear end.
I may actually bite the bullet and go with an Atlas just to make things bolt in and hook up easier, plus it's short (length is always an issue in a Jeep) and I know it's bulletproof. I also like the ability to shift to 2Low on just the front or rear.
I'm keeping a drivers side front pumpkin and driveshaft. The Dana 30 is staying in the front and I'm going with a Ford 8.8 in the rear, this way my wheel bolt patterns match. The D30 will be beefed up with Superior 30 spline axles and ARB lockers front and rear. Somewhere in the future, the 8.8 may get a Superior Super 88 upgrade too.
This definitely isn't a low buck project!
I may actually bite the bullet and go with an Atlas just to make things bolt in and hook up easier, plus it's short (length is always an issue in a Jeep) and I know it's bulletproof. I also like the ability to shift to 2Low on just the front or rear.
I'm keeping a drivers side front pumpkin and driveshaft. The Dana 30 is staying in the front and I'm going with a Ford 8.8 in the rear, this way my wheel bolt patterns match. The D30 will be beefed up with Superior 30 spline axles and ARB lockers front and rear. Somewhere in the future, the 8.8 may get a Superior Super 88 upgrade too.
This definitely isn't a low buck project!
Last edited by YeepieJeepie; May 17, 2009 at 12:34 PM.
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Just find a drives drop GM NP241 (mech shift), they will have 40 pluse VSS and you can get a slip yoke eleminator kit for the short wheel base Jeep. It has a low ratio of 2.72:1 and is common an pretty cheap! Just use the stock GM adapter and 4x4 4l60. And wire in the transfer case low and be done with it!!! It will work just like the stock GM trucks do!!!