Fix one thing , break another - P1875

[fixkick]

your car has 2 VSS sensors, unlike modern cars and the cluster is 100% computer logic, and only has a VSS on the transmission tail (somewheres)
no, yours has cave man speedo VSS2, that is 100% mechanical.

The ECU sees them both, the VSS1 and VSS2,
yours VSS1 is wrong, wrong at VSS1 rear of you 4AT tranny the rear of said tranny is hard gear locked to the rear axle. and that can not be change.
only the VSS1 signal above must be changed some way, (analog way or digital way)

what you did not do is change and cure them both.
also you 4SP trans box is now shifting 13% late but I sure you did not notice that nor would I.

[thebanjoman]

Arduino, + ZCD , zero crossing detector is a must for VR coil sensors (most) as the signal  are tiny for sure low speed. of car. (a HALL sensor would be cake walk )

here is one example of the first part of the job, measuring frequency.

the next stop is scale up this new Freq. by 113% just using math.
freq-out = freq x 1.13  (now 13% more)
the set timer to freq-out


google how to use  Arudio to output a timer clock you control. (using freq-out)
and send it to a pin and connect that pin to VSS pin on TCM.

now the bad news, no spec on TCM
if lucky at least one pin might be ground and 2nd is input to TCM, vss1.

at the worse Ive done this and used transformer to emulate the VR sensor into TCM.
a 1:1 turns ratio transformer very very tiny one, 600ohms  rated, for audio work;.
I did the once to fool a FORD ECM to like my fake CRANK CKP sensor.

the VSS1 pins on the TCM
are not documented. pin # and color are, but not how TCM processes those pins, but in most cases is a ZCD in the TCM,   .

 Making changes at the trans speed sensor seems way above my skill level.

 Couldn't I make my adjustments on the speedometer side as that's what I am changing with my gear box device ?  

 Read the reed valve pulses and reduce them by 13%    ??

The speedometer is cable driven, and the suzuki parts used to have great sets to fit many head.
for different axle ratios used world wide. is the purpose.
that is G16 car
the J18, only has 1 gear set (plastic) for  only the one axle there are no other gear sets for the VSS2 cable drive to the cluster.
the SPORT is only sold in USA /Canada and Puerto Rico . MPH and KMH and use the same ratio drive, all. on the cable.  so there is no suzuki cure here, like G16s

you did it the only way on earth possible at the cable, a gear up box. it is. on the cable
VSS1 is not mechanical at all, it is electromagnetic, and  a pulse, only that is all it is. for speed.


There are only 2 VSS1 cures,  one is Analog electronics and other is digital processing and using some kind of re-scaling methods.

the reason nobody I know makes this is because, there are so many sensor types and worse so many TCMs.


here is what a 2 wire VR mag sensor looks like very small signal

I learn more with each conversation on this topic but I still may not be getting the full picture.   Apologies if I appear dense.

I understand the cable drives the speedometer and moves the needle depending on how fast/slow the cable is turned.  My Model A ford operates this way 90 year old technology so I think it's right.  

I'm spinning the cable faster by adding the speedo box.  spinning faster, needle moves higher on dial.  

And if I'm spinning the cable faster then isn't the reed valve opening and closing faster (pulse) now too ??

The pulses from the reed switch are sent to the computer aren't they?   I think so because I can see MPH reported on my ODBII scanner.   
While I am driving I can look at the ODBII MPH data and the needle of the speedometer and they are comparable.

I still can't get past the idea that both speed sensors are compared and if they differ more than 10% the P1875 code is generated. 
(the 1800 supplement book states this) 

I think i prove this, if I put in my 'speedo correction box' on the cable I get the errors.  if I remove it the errors go away.

I was hoping that if I could make the reed pulses appear as if they were never changed by adding the 'speedo box'  then the comparison would be within the 10% and the P1875 code would not be generated.      

I could also hypothesize that if my "speedo box" corrected say 8% and not 13% the errors would never be generated but the only way to prove this is to get another 'speedo box' of a different ratio change and put it in and try, but that's too expensive of a test.

This is not to scale but merely an example.

Reed switch pulses per second is 100 to be sent to computer.
Instead of being sent to computer it is sent to arduino. 
Arduino then sends 87 pulses to the computer instead of 100.

What is sent from the arduino is compared against the speed sensor in the transmission and it 'thinks' it's in spec and remains happy - 
Do not generate code.  

Im probably not thinking of this problem properly but I don't see why this wouldn't work.

[fixkick]

Arduino, + ZCD , zero crossing detector is a must for VR coil sensors (most) as the signal  are tiny for sure low speed. of car. (a HALL sensor would be cake walk )

here is one example of the first part of the job, measuring frequency.

the next stop is scale up this new Freq. by 113% just using math.
freq-out = freq x 1.13  (now 13% more)
the set timer to freq-out


google how to use  Arudio to output a timer clock you control. (using freq-out)
and send it to a pin and connect that pin to VSS pin on TCM.

now the bad news, no spec on TCM
if lucky at least one pin might be ground and 2nd is input to TCM, vss1.

at the worse Ive done this and used transformer to emulate the VR sensor into TCM.
a 1:1 turns ratio transformer very very tiny one, 600ohms  rated, for audio work;.
I did the once to fool a FORD ECM to like my fake CRANK CKP sensor.

the VSS1 pins on the TCM
are not documented. pin # and color are, but not how TCM processes those pins, but in most cases is a ZCD in the TCM,   .

 Making changes at the trans speed sensor seems way above my skill level.

 Couldn't I make my adjustments on the speedometer side as that's what I am changing with my gear box device ?  

 Read the reed valve pulses and reduce them by 13%    ??

The speedometer is cable driven, and the suzuki parts used to have great sets to fit many head.
for different axle ratios used world wide. is the purpose.
that is G16 car
the J18, only has 1 gear set (plastic) for  only the one axle there are no other gear sets for the VSS2 cable drive to the cluster.
the SPORT is only sold in USA /Canada and Puerto Rico . MPH and KMH and use the same ratio drive, all. on the cable.  so there is no suzuki cure here, like G16s

you did it the only way on earth possible at the cable, a gear up box. it is. on the cable
VSS1 is not mechanical at all, it is electromagnetic, and  a pulse, only that is all it is. for speed.


There are only 2 VSS1 cures,  one is Analog electronics and other is digital processing and using some kind of re-scaling methods.

the reason nobody I know makes this is because, there are so many sensor types and worse so many TCMs.


here is what a 2 wire VR mag sensor looks like very small signal

I learn more with each conversation on this topic but I still may not be getting the full picture.   Apologies if I appear dense.

I understand the cable drives the speedometer and moves the needle depending on how fast/slow the cable is turned.  My Model A ford operates this way 90 year old technology so I think it's right.   (I love Ford Model A cars, always have)

I'm spinning the cable faster by adding the speedo box.  spinning faster, needle moves higher on dial.  sure does. you can use  drill motor (elect drill) and do that at cable end. and see all that happen in real time, (2 man job that, one looking one doing)

And if I'm spinning the cable faster then isn't the reed valve(switch) opening and closing faster (pulse) now too ?? sure does . all 3 do , reed, dial and odom all go 13% faster.
all 3 are pure mechanical, The reed is too, it is dry reed switch magnetically (mech. connected)

The pulses from the reed switch are sent to the computer aren't they?  YES<  and both are so the FALL back mode on the TCM can work,  there is not one COMPUTER Box on this car it is 2 BOXes, ECM and TCM and they also talk to each other (communicate they do and never stop)
 I think so because I can see MPH reported on my ODBII scanner.   
While I am driving I can look at the ODBII MPH data and the needle of the speedometer and they are comparable. (make sense this and is true)

I still can't get past the idea that both speed sensors are compared and if they differ more than 10% the P1875 code is generated. 
(the 1800 supplement book states this) GOOD question, and is all software that, all Firmware hard coded in both the TCM and ECU ((unchangeable too)
The designer of said car ECU, did that with a purpose,   that purpose is to make sure both sensors are not bad or weak. (or no congruent)
The cable can fail too, dead broken or rust out and jumps like mad, the ECU will not like that either,
That error means  that VSS1 and VSS2 do not agree, and this is in fact a hard error. you can not change the firmware and till it to lie.
The OBD2 has what are call hard class (super hard failures) (dead things) and soft errors, things that are marginal.
YOUR  PC HDD and W10 checks for Smart errors all the time just like your cars OBD3 constant checking for errors. (all the time)
The only odd things is many errors take 2 driving cycle to see the CEL glow.  (the scan tool will tell you this if you look deeper)

Most cars have no redundant sensors,  very rare this and is here. thati why many see this and get confused, it is REDUNDANT.
The 4speed will not ever upshift, with any DEAD VSS, it can not, so Suzuki put in 2 sensors,  (IMO a great idea)
The VSS2 is the FALL BACK sensor, and that sensor is the weaker one. it is not all that accurate., unlike VSS1 super accurate.

call it the limp home sensor but at full 4th gear speeds,.



wow very good question you ask, all GOOD.
I think i prove this, if I put in my 'speedo correction box' on the cable I get the errors.  if I remove it the errors go away.
OK you forget 2 things, 1875 is differential failure.  only, A not equal to B. failed.
The ECU and TCM can both report dead VSS, for sure.
fact #2 the ECU has no GPS, to know the 2 sensors are lying,  sorry no such thing, as 3 sources of true speed , just like that vary bad air craft killed so many Airbus, Brazil to France.  (icing caused, but Gee GPU does not Ice up and those folks died for stupid engineers, IMO or bad cost cutters at airbus.
and lots more,  dead with bug guts plugging up PITO air speed tubes. or ice.
if you get ,my point here, 3 sensors,  VSS1, VSS2, and GPS.   just like all air craft need to use and some do not,. and far far worse ends..

I was hoping that if I could make the reed pulses appear as if they were never changed by adding the 'speedo box'  then the comparison would be within the 10% and the P1875 code would not be generated.      NO, the speed up gear box, speedo works, only for VSS2 sorry only VSS2.  you saw then needle match GPS so in fact know  it works.


I could also hypothesize that if my "speedo box" corrected say 8% and not 13% the errors would never be generated(true) but the only way to prove this is to get another 'speedo box' of a different ratio change and put it in and try, but that's too expensive of a test. Buy a GPS, or use TOMTOM drive car see speed off 13%^ then put your expensive, box there, and see error is now near 0% using GPS that does not lie..

This is not to scale but merely an example.

Reed switch pulses per second is 100 to be sent to computer.
Instead of being sent to computer it is sent to arduino.  NO this is wrong, your gear box works why are you playing with VSS2 when VSS2 works great, prove that with GPS)
Arduino then sends 87 pulses to the computer instead of 100.
this is yes for VSS1, only.  VSS1 reads wrong, and you fail to know that. hard hard failure..
This car need 2 corrections to  do speed right , if not done right 1875 , is for ever..

VSS1 sensor to Arduino and it raises RPM  by 13% (or as GPS told you was wrong and off) and now the TCM sees true speed.)
one other experiment can be  done , unplug VSS1, now the TCM fails over to VSS2, (cluster) and now you get VSS1 OBD2 error just for it dead. now. see>?




What is sent from the arduino is compared against the speed sensor in the transmission and it 'thinks' it's in spec and remains happy - 
Do not generate code.   NO. sorry
the Arduino does not know land speed ever (unless you have  GPS shield or 5G network card (very hard later) to know land speed.
The Arduino is programmed to be a gear box, it takes pulses input, RPM and then upscales them by 13% and send them to the TCM
if the ECU and TCM seen the new VSS 1 speed wrong for VSS2, then you then put the gear box in at 13 = 13, and all is happy in 1875 land.

Im probably not thinking of this problem properly but I don't see why this wouldn't work.

i will answer last line , in 2nd post.

answered in line and in red color....

[fixkick]

Here is the problem, more clear,
1: this car has no integrated GPS, so never ever knows the truth ever. (GPS does not lie and is your own truth , not using stop watch and mile markers for speed)
2: it knows dead sensors for sure. dead takes no CPU brains at all, just 0v = dead. or stuck at 5v. etc/
3: the TCM+ECU pair do know if the 2 sensor do not agree by 10km/mile or? and 1875 happens. (both wrong is good, one wrong is fail) note both wrong is FAIL too, (ticket getter car)
4 It never ever knows which one is off in RPM see line 1. (it can not know this)
5: The fact is both sensors must read the same (even if both lie the same) that is a fact (within the 10km/m rule spec)

there are more DTC errors here not seen by you, unplugging VSS1 then VSS2 and drive 2 driving cycles each from cold engine, see new DTC you will..
The FSM manual show just it takes to fail any DTC, driving cycles and all that. (no Car like this knows truth or things off (inaccurate) it only know dead,things. there are some crude exceptions, like MAF reading wrong or 02 sensor is off, one or the other are off it thinks and is. (this is complex here)
all sensors can be good , dead, stuck or off and intermittent. it takes a good tech to find truth (if not good or easy dead)

P0500 , VSS dead.
P0722 ,.VSS dead
or other, in your FSM.
building any custom cars (heavy mods) you need the FSM on both cars, if using donor electrics, if not just the J18 car FSM. I can only assume you do ..

[fixkick]

While fuel is kept cut at lower than 4000 r/min. for
longer than 4 sec.
VSS montor rule, and 2 driving cycles, seems they want fast , coasting in gear, for 4 seconds to trip it. (off off gas pedal; = cut fuel. mode. zero fuel flow injectors)

all books even the Esteem J18 shows 1875 as Hi/low 4wd error. but SQ418 cars (true name of sport) are odd in vast ways and I have no books on it, I have most the others, up to 2005.
P0720 is 4speed AT VSS1 error. in most books, by suz. and this trans.

[thebanjoman]

Arduino, + ZCD , zero crossing detector is a must for VR coil sensors (most) as the signal  are tiny for sure low speed. of car. (a HALL sensor would be cake walk )

here is one example of the first part of the job, measuring frequency.

the next stop is scale up this new Freq. by 113% just using math.
freq-out = freq x 1.13  (now 13% more)
the set timer to freq-out


google how to use  Arudio to output a timer clock you control. (using freq-out)
and send it to a pin and connect that pin to VSS pin on TCM.

now the bad news, no spec on TCM
if lucky at least one pin might be ground and 2nd is input to TCM, vss1.

at the worse Ive done this and used transformer to emulate the VR sensor into TCM.
a 1:1 turns ratio transformer very very tiny one, 600ohms  rated, for audio work;.
I did the once to fool a FORD ECM to like my fake CRANK CKP sensor.

the VSS1 pins on the TCM
are not documented. pin # and color are, but not how TCM processes those pins, but in most cases is a ZCD in the TCM,   .

 Making changes at the trans speed sensor seems way above my skill level.

 Couldn't I make my adjustments on the speedometer side as that's what I am changing with my gear box device ?  

 Read the reed valve pulses and reduce them by 13%    ??

The speedometer is cable driven, and the suzuki parts used to have great sets to fit many head.
for different axle ratios used world wide. is the purpose.
that is G16 car
the J18, only has 1 gear set (plastic) for  only the one axle there are no other gear sets for the VSS2 cable drive to the cluster.
the SPORT is only sold in USA /Canada and Puerto Rico . MPH and KMH and use the same ratio drive, all. on the cable.  so there is no suzuki cure here, like G16s

you did it the only way on earth possible at the cable, a gear up box. it is. on the cable
VSS1 is not mechanical at all, it is electromagnetic, and  a pulse, only that is all it is. for speed.


There are only 2 VSS1 cures,  one is Analog electronics and other is digital processing and using some kind of re-scaling methods.

the reason nobody I know makes this is because, there are so many sensor types and worse so many TCMs.


here is what a 2 wire VR mag sensor looks like very small signal

I learn more with each conversation on this topic but I still may not be getting the full picture.   Apologies if I appear dense.

I understand the cable drives the speedometer and moves the needle depending on how fast/slow the cable is turned.  My Model A ford operates this way 90 year old technology so I think it's right.   (I love Ford Model A cars, always have)

I'm spinning the cable faster by adding the speedo box.  spinning faster, needle moves higher on dial.  sure does. you can use  drill motor (elect drill) and do that at cable end. and see all that happen in real time, (2 man job that, one looking one doing)

And if I'm spinning the cable faster then isn't the reed valve(switch) opening and closing faster (pulse) now too ?? sure does . all 3 do , reed, dial and odom all go 13% faster.
all 3 are pure mechanical, The reed is too, it is dry reed switch magnetically (mech. connected)

The pulses from the reed switch are sent to the computer aren't they?  YES<  and both are so the FALL back mode on the TCM can work,  there is not one COMPUTER Box on this car it is 2 BOXes, ECM and TCM and they also talk to each other (communicate they do and never stop)

 I think so because I can see MPH reported on my ODBII scanner.   
While I am driving I can look at the ODBII MPH data and the needle of the speedometer and they are comparable. (make sense this and is true)

I still can't get past the idea that both speed sensors are compared and if they differ more than 10% the P1875 code is generated. 
(the 1800 supplement book states this) GOOD question, and is all software that, all Firmware hard coded in both the TCM and ECU ((unchangeable too)
The designer of said car ECU, did that with a purpose,   that purpose is to make sure both sensors are not bad or weak. (or no congruent)
The cable can fail too, dead broken or rust out and jumps like mad, the ECU will not like that either,
That error means  that VSS1 and VSS2 do not agree, and this is in fact a hard error. you can not change the firmware and till it to lie.
The OBD2 has what are call hard class (super hard failures) (dead things) and soft errors, things that are marginal.
YOUR  PC HDD and W10 checks for Smart errors all the time just like your cars OBD3 constant checking for errors. (all the time)
The only odd things is many errors take 2 driving cycle to see the CEL glow.  (the scan tool will tell you this if you look deeper)

Most cars have no redundant sensors,  very rare this and is here. thati why many see this and get confused, it is REDUNDANT.
The 4speed will not ever upshift, with any DEAD VSS, it can not, so Suzuki put in 2 sensors,  (IMO a great idea)
The VSS2 is the FALL BACK sensor, and that sensor is the weaker one. it is not all that accurate., unlike VSS1 super accurate.

call it the limp home sensor but at full 4th gear speeds,.



wow very good question you ask, all GOOD.
I think i prove this, if I put in my 'speedo correction box' on the cable I get the errors.  if I remove it the errors go away.
OK you forget 2 things, 1875 is differential failure.  only, A not equal to B. failed.
The ECU and TCM can both report dead VSS, for sure.
fact #2 the ECU has no GPS, to know the 2 sensors are lying,  sorry no such thing, as 3 sources of true speed , just like that vary bad air craft killed so many Airbus, Brazil to France.  (icing caused, but Gee GPU does not Ice up and those folks died for stupid engineers, IMO or bad cost cutters at airbus.
and lots more,  dead with bug guts plugging up PITO air speed tubes. or ice.
if you get ,my point here, 3 sensors,  VSS1, VSS2, and GPS.   just like all air craft need to use and some do not,. and far far worse ends..

I was hoping that if I could make the reed pulses appear as if they were never changed by adding the 'speedo box'  then the comparison would be within the 10% and the P1875 code would not be generated.      NO, the speed up gear box, speedo works, only for VSS2 sorry only VSS2.  you saw then needle match GPS so in fact know  it works.


I could also hypothesize that if my "speedo box" corrected say 8% and not 13% the errors would never be generated(true) but the only way to prove this is to get another 'speedo box' of a different ratio change and put it in and try, but that's too expensive of a test. Buy a GPS, or use TOMTOM drive car see speed off 13%^ then put your expensive, box there, and see error is now near 0% using GPS that does not lie..

This is not to scale but merely an example.

Reed switch pulses per second is 100 to be sent to computer.
Instead of being sent to computer it is sent to arduino.  NO this is wrong, your gear box works why are you playing with VSS2 when VSS2 works great, prove that with GPS)
Arduino then sends 87 pulses to the computer instead of 100.
this is yes for VSS1, only.  VSS1 reads wrong, and you fail to know that. hard hard failure..
This car need 2 corrections to  do speed right , if not done right 1875 , is for ever..

VSS1 sensor to Arduino and it raises RPM  by 13% (or as GPS told you was wrong and off) and now the TCM sees true speed.)
one other experiment can be  done , unplug VSS1, now the TCM fails over to VSS2, (cluster) and now you get VSS1 OBD2 error just for it dead. now. see>?




What is sent from the arduino is compared against the speed sensor in the transmission and it 'thinks' it's in spec and remains happy - 
Do not generate code.   NO. sorry
the Arduino does not know land speed ever (unless you have  GPS shield or 5G network card (very hard later) to know land speed.
The Arduino is programmed to be a gear box, it takes pulses input, RPM and then upscales them by 13% and send them to the TCM
if the ECU and TCM seen the new VSS 1 speed wrong for VSS2, then you then put the gear box in at 13 = 13, and all is happy in 1875 land.

Im probably not thinking of this problem properly but I don't see why this wouldn't work.

i will answer last line , in 2nd post.

answered in line and in red color....

 I thank you for all this worthwhile information.  The more I read and try to understand It's clearly beyond my scope of talents to solve.

So I'll be looking for a decent GPS based speedometer with an odometer so I can track maintenance and install it and hopefully it will do what I need to do with reasonably fast updates.    Maybe I can 'ebay' my 'speedo box', take a bath on what I paid, and see if it can be used by someone else who can make it work for them. 

If this was a 5sp then my problems would be solved.

[fixkick]

 Making changes at the trans speed sensor seems way above my skill level.

 Couldn't I make my adjustments on the speedometer side as that's what I am changing with my gear box device ?  

 Read the reed valve pulses and reduce them by 13%    ??

The speedometer is cable driven, and the suzuki parts used to have great sets to fit many head.
for different axle ratios used world wide. is the purpose.
that is G16 car
the J18, only has 1 gear set (plastic) for  only the one axle there are no other gear sets for the VSS2 cable drive to the cluster.
the SPORT is only sold in USA /Canada and Puerto Rico . MPH and KMH and use the same ratio drive, all. on the cable.  so there is no suzuki cure here, like G16s

you did it the only way on earth possible at the cable, a gear up box. it is. on the cable
VSS1 is not mechanical at all, it is electromagnetic, and  a pulse, only that is all it is. for speed.


There are only 2 VSS1 cures,  one is Analog electronics and other is digital processing and using some kind of re-scaling methods.

the reason nobody I know makes this is because, there are so many sensor types and worse so many TCMs.


here is what a 2 wire VR mag sensor looks like very small signal

I learn more with each conversation on this topic but I still may not be getting the full picture.   Apologies if I appear dense.

I understand the cable drives the speedometer and moves the needle depending on how fast/slow the cable is turned.  My Model A ford operates this way 90 year old technology so I think it's right.   (I love Ford Model A cars, always have)

I'm spinning the cable faster by adding the speedo box.  spinning faster, needle moves higher on dial.  sure does. you can use  drill motor (elect drill) and do that at cable end. and see all that happen in real time, (2 man job that, one looking one doing)

And if I'm spinning the cable faster then isn't the reed valve(switch) opening and closing faster (pulse) now too ?? sure does . all 3 do , reed, dial and odom all go 13% faster.
all 3 are pure mechanical, The reed is too, it is dry reed switch magnetically (mech. connected)

The pulses from the reed switch are sent to the computer aren't they?  YES<  and both are so the FALL back mode on the TCM can work,  there is not one COMPUTER Box on this car it is 2 BOXes, ECM and TCM and they also talk to each other (communicate they do and never stop)

 I think so because I can see MPH reported on my ODBII scanner.   
While I am driving I can look at the ODBII MPH data and the needle of the speedometer and they are comparable. (make sense this and is true)

I still can't get past the idea that both speed sensors are compared and if they differ more than 10% the P1875 code is generated. 
(the 1800 supplement book states this) GOOD question, and is all software that, all Firmware hard coded in both the TCM and ECU ((unchangeable too)
The designer of said car ECU, did that with a purpose,   that purpose is to make sure both sensors are not bad or weak. (or no congruent)
The cable can fail too, dead broken or rust out and jumps like mad, the ECU will not like that either,
That error means  that VSS1 and VSS2 do not agree, and this is in fact a hard error. you can not change the firmware and till it to lie.
The OBD2 has what are call hard class (super hard failures) (dead things) and soft errors, things that are marginal.
YOUR  PC HDD and W10 checks for Smart errors all the time just like your cars OBD3 constant checking for errors. (all the time)
The only odd things is many errors take 2 driving cycle to see the CEL glow.  (the scan tool will tell you this if you look deeper)

Most cars have no redundant sensors,  very rare this and is here. thati why many see this and get confused, it is REDUNDANT.
The 4speed will not ever upshift, with any DEAD VSS, it can not, so Suzuki put in 2 sensors,  (IMO a great idea)
The VSS2 is the FALL BACK sensor, and that sensor is the weaker one. it is not all that accurate., unlike VSS1 super accurate.

call it the limp home sensor but at full 4th gear speeds,.



wow very good question you ask, all GOOD.
I think i prove this, if I put in my 'speedo correction box' on the cable I get the errors.  if I remove it the errors go away.
OK you forget 2 things, 1875 is differential failure.  only, A not equal to B. failed.
The ECU and TCM can both report dead VSS, for sure.
fact #2 the ECU has no GPS, to know the 2 sensors are lying,  sorry no such thing, as 3 sources of true speed , just like that vary bad air craft killed so many Airbus, Brazil to France.  (icing caused, but Gee GPU does not Ice up and those folks died for stupid engineers, IMO or bad cost cutters at airbus.
and lots more,  dead with bug guts plugging up PITO air speed tubes. or ice.
if you get ,my point here, 3 sensors,  VSS1, VSS2, and GPS.   just like all air craft need to use and some do not,. and far far worse ends..

I was hoping that if I could make the reed pulses appear as if they were never changed by adding the 'speedo box'  then the comparison would be within the 10% and the P1875 code would not be generated.      NO, the speed up gear box, speedo works, only for VSS2 sorry only VSS2.  you saw then needle match GPS so in fact know  it works.


I could also hypothesize that if my "speedo box" corrected say 8% and not 13% the errors would never be generated(true) but the only way to prove this is to get another 'speedo box' of a different ratio change and put it in and try, but that's too expensive of a test. Buy a GPS, or use TOMTOM drive car see speed off 13%^ then put your expensive, box there, and see error is now near 0% using GPS that does not lie..

This is not to scale but merely an example.

Reed switch pulses per second is 100 to be sent to computer.
Instead of being sent to computer it is sent to arduino.  NO this is wrong, your gear box works why are you playing with VSS2 when VSS2 works great, prove that with GPS)
Arduino then sends 87 pulses to the computer instead of 100.
this is yes for VSS1, only.  VSS1 reads wrong, and you fail to know that. hard hard failure..
This car need 2 corrections to  do speed right , if not done right 1875 , is for ever..

VSS1 sensor to Arduino and it raises RPM  by 13% (or as GPS told you was wrong and off) and now the TCM sees true speed.)
one other experiment can be  done , unplug VSS1, now the TCM fails over to VSS2, (cluster) and now you get VSS1 OBD2 error just for it dead. now. see>?




What is sent from the arduino is compared against the speed sensor in the transmission and it 'thinks' it's in spec and remains happy - 
Do not generate code.   NO. sorry
the Arduino does not know land speed ever (unless you have  GPS shield or 5G network card (very hard later) to know land speed.
The Arduino is programmed to be a gear box, it takes pulses input, RPM and then upscales them by 13% and send them to the TCM
if the ECU and TCM seen the new VSS 1 speed wrong for VSS2, then you then put the gear box in at 13 = 13, and all is happy in 1875 land.

Im probably not thinking of this problem properly but I don't see why this wouldn't work.

i will answer last line , in 2nd post.

answered in line and in red color....

 I thank you for all this worthwhile information.  The more I read and try to understand It's clearly beyond my scope of talents to solve.

So I'll be looking for a decent GPS based speedometer with an odometer so I can track maintenance and install it and hopefully it will do what I need to do with reasonably fast updates.    Maybe I can 'ebay' my 'speedo box', take a bath on what I paid, and see if it can be used by someone else who can make it work for them. 

If this was a 5sp then my problems would be solved.

the cars mech, odometer is plenty accurate enough,  60k real miles are 7800 short  -13% so drive 67800 miles for that service,
there are many GPS MPH  meters sold, even free with cell phone app, put  dash cell phone holder there in car and run the MPH APP.  even dedicated MPH APP , virtual gauge.

Im am sorry this is confusing and it is.  for sure.
simply both VSS sensors must agree and is 2 cures not one, 2 things must be cured,  2 speed amplifiers, at 13% 

keep in mind cluster 60mph on this car is 69mph real.  a 9  mph error is huge , and  ticket getter  if not  using GPS Gauge.(of any kind, even a TOMTOM does speed)


good luck to you and cheers.