Now that you've fixed the switch, take the speedometer correction gearbox out and see if that fixes it.

I've tried to wrap my head around the problem and did some more reading and understanding.  There are 2 pages in the FSM on the P1875 code.   

First page explains what would trigger the code conditions with the Vehicle Speed Sensor (VSS) and the Transmission vehicle speed sensor (TVSS).

The 2nd focused on diagnosing the 4Lo circuit. 

And in my haste for a quick fix I failed to understand the problem fully and completely ignored the first page and focused ONLY on the 2nd. 

Fordem,

You pointed out fully the 1st page and I promptly ignored it and went onto the 2nd page.  Apologies

I have confirmed the 2nd page,  The 4Lo Circuit is complete and works as it is  intended.

1st page.

DTC DETECTING CONDITION
While driving at higher than specified vehicle
and with more than specified throttle valve opening,
following conditions are satisfied for specified time.
. Vehicle speed sensed by vehicle speed sensor
speedometer) is more than 10 km/h lower than
sensed by transmission vehicle speed sensor when
4WD low switch is OFF, or
• Vehicle speed sensed by vehicle speed sensor (in
speedometer) is more than 10 km/h higher than that
sensed by transmission vehicle speed sensor when
4WD low switch is ON.the

Fordem,

As you stated at the beginning of this thread.  Adding the Speedo cable correction box changes what is sent to the Speedometer and odometer and VSS
but does NOTHING to change anything at the TVSS (transmission vehicle speed sensor) so there is going  to be more than a 10 km/h difference between the VSS and the TVSS,   

And that is WHY according to page 1  the condition is met (10 km/h lower) and the code is thrown.  The condition is met and the code is thrown lots of times and after a threshold it will trigger the check engine light.

SO  If I want to continue to use my speedo correction box with the cable, I will also have to correct the TVSS too. 

I have corrected the Speedometer/odometer by 12% so I will have to see if I can adjust the TVSS by 12% too so there will be no difference and it won't throw the code.

'Yellow box' https://yellr.com/yb_inst.htm  may have a way to adjust the TVSS . 

Further research is needed. 

I was wondering about that.

https://www.jaycar.us/speedo-corrector-module/p/AA0376

I haven't used these personally, but, I've assisted (via forums) a couple of people with the previous version, which was a kit of parts that needed to be assembled.

Been following along on this & what I wonder is: Is it possible to just splice the corrected signal from the VSS & fool the ECU/TCU into thinking it is seeing both? It just wants the signals to be the same on both, so why couldn't you just send the same signal to both? Just a thought.

I was wondering about that.

https://www.jaycar.us/speedo-corrector-module/p/AA0376

I haven't used these personally, but, I've assisted (via forums) a couple of people with the previous version, which was a kit of parts that needed to be assembled.

I'll pull the correction box this weekend and take a trip.  If the code goes away then it'll be more proof.

 As the Speedo is manual and for now seems to be correcting the speed/odometer with the box I bought inline with the cable.   Should / could I try and correct the TVSS with this box you suggest?

I'm a bit confused. 

I'm not certain what or where the TVSS is - what was in my mind before I read your post about the yellow box - was to install the mechanical corrector between the transfer case and the speedometer to correct the calibration for the larger tires, and then use a digital corrector between the speedometer and the ECU to reverse the correction done by the mechanical corrector.

I'm not certain what or where the TVSS is - what was in my mind before I read your post about the yellow box - was to install the mechanical corrector between the transfer case and the speedometer to correct the calibration for the larger tires, and then use a digital corrector between the speedometer and the ECU to reverse the correction done by the mechanical corrector.

The Manuals are really poor about referring to the parts in multiple ways. 

When referencing the P1875 section it calls it a 'transmission vehicle speed sensor'  (TVSS) its a sensor that is on the left side of the trans about in the middle.
In another section of the manual it refers to it just as a Vehicle Speed sensor.

Taken from the A/T section of the FSM supplement.

Vehicle Speed Sensor
This sensor is a pulse generator type that detects revolution of the
output shaft (vehicle speed) in the transmission case.
The pulse generator is a non contact sensor consisting of a permanent magnet, coil and gears.

As the gear of the output shaft turns, the magneflux from the permanent magnet varies and a voltage of the frequency corresponding
to the rotor revolution occurs in the coil. This voltage is inputted to the PCM (ECM) where PCM (ECM) judges the output shaft
revolution or the vehicle speed. The vehicle speed is also detected from the speed meter.

Which is why I wondered if the box you referenced was thought to be added to this sensor to make the adjustment electrically where
the external box I added was to handle the speedometer mechanically.

If I read this right... i 'might' be able to use a correction box. 

Putting aside the manual for the moment - in my mind, the VSS, or vehicle speed sensor is the one that measures the speed of the vehicle - on my car (a second gen), it's located on the transfer case, pretty much where the speedometer cable is on your car.  On your car, the VSS is internal to the speedometer, so the point in the drive train at which the measurement is being made is more or less the same.  Any sensor measuring at a point prior to the transfer case output will not see the "vehicle" speed, because of the gearing in the transfer case.

On my car, the sensor is electronic and feeds the ECU and the speedometer unit in the dash cluster - what the people I have worked with have done is insert the digital corrector between the sensor and the speedometer, so it alters the signal going to the speedometer only - it's a situation where two wires run from the sensor to the ECU and the cluster and you cut the wires where they connect to the cluster to fit the corrector - that way the signal to the ECU remains undisturbed - in theory this requires cutting one wire behind the speedometer and would be easily reversible, should the need arise.

The Jaycar unit can be configured to alter the signal to compensate for both larger or smaller tires, so my thought is to compensate mechanically for the larger tire and then reverse that compensation digitally - I'll try to find the wiring for the cluster on your car and take a look at it - I should have the documentation for the SV620 which is a V6 engined version of your SV418 - the clusters should be interchangeable.

Putting aside the manual for the moment - in my mind, the VSS, or vehicle speed sensor is the one that measures the speed of the vehicle - on my car (a second gen), it's located on the transfer case, pretty much where the speedometer cable is on your car.  On your car, the VSS is internal to the speedometer, so the point in the drive train at which the measurement is being made is more or less the same.  Any sensor measuring at a point prior to the transfer case output will not see the "vehicle" speed, because of the gearing in the transfer case.

On my car, the sensor is electronic and feeds the ECU and the speedometer unit in the dash cluster - what the people I have worked with have done is insert the digital corrector between the sensor and the speedometer, so it alters the signal going to the speedometer only - it's a situation where two wires run from the sensor to the ECU and the cluster and you cut the wires where they connect to the cluster to fit the corrector - that way the signal to the ECU remains undisturbed - in theory this requires cutting one wire behind the speedometer and would be easily reversible, should the need arise.

The Jaycar unit can be configured to alter the signal to compensate for both larger or smaller tires, so my thought is to compensate mechanically for the larger tire and then reverse that compensation digitally - I'll try to find the wiring for the cluster on your car and take a look at it - I should have the documentation for the SV620 which is a V6 engined version of your SV418 - the clusters should be interchangeable.

After some further discussions with someone who has knowledge (Fixkick) on his forum,  He has provided me links to more details.   For the purposes of the discussion  There are 2 VSS in this configuration.

vss1  at the cluster
vss2 at the transmission.

Both can be scaled.    vss1  at the cluster,  I'm doing that now with the cable adapter  so I should focus on vss2 at the transmission.

VSS2  is coil sensor 2 wire and not a 3 or 4 wire hall sensor sadly, that means the cable is shielded and is very tiny signal, with weak 1volt pulses, sinusoidal shaped,
The only way to work this type of signal as did suzuki TCM is using a Zero crossing detector ZCD.  effectively making it HALL(like) sensor like but inside the TCM.
producing a clean square wave, for RPM. on the ZCD output pin.

https://www.elprocus.com/zero-crossing-detector-circuit-and-working/

Im trying to read and understand this process and even if it could apply to an off the shelf box for this. application.   

More reading and trying to wrap my head around all this.



More reading and

Just so it's clear my suggestion does not involve the vehicle speed sensor in the transmission - it reverses the correction made to signal from the speed sensor in the speedometer.

Just so it's clear my suggestion does not involve the vehicle speed sensor in the transmission - it reverses the correction made to signal from the speed sensor in the speedometer.

Raise the bridge.. Lower the river..   Both sides of this are worth looking into and to see which is going to be the easiest.  Nothing will happen overnight but with
every bit more of information gets me closer to understanding the problem and possibly solving it.

Raise the bridge.. Lower the river..   Both sides of this are worth looking into and to see which is going to be the easiest.  Nothing will happen overnight but with
every bit more of information gets me closer to understanding the problem and possibly solving it.

More reading, more searching and conferring with others     I can see that I am just going to have to get an oscilloscope hooked up to both sensors and see exactly what is coming out.
I'm working on locating a scope that can be used.  I think I can get the signals off the wires at the PCM end without having to tear a lot apart.

More reading, more searching and conferring with others     I can see that I am just going to have to get an oscilloscope hooked up to both sensors and see exactly what is coming out.
I'm working on locating a scope that can be used.  I think I can get the signals off the wires at the PCM end without having to tear a lot apart.

I haven't done any more work on this other than reading.     Wife has been in the hospital a lot these last few weeks and that has been where my focus is.

The P1875 code is misleading. The manuals point you to 4lo switch problems but its not. It's really detecting the difference between the speedo VSS and the automatic transmission speed sensor
which changes gears. the 4sp auto is shifted by the computer.

If they are not within 10% of each other then throw the code but its also not a constant check either.  I will throw the code intermittently .  I have an 'ultragauge' I am using to watch temps and other
readings and the P1875 code shows up 'pending' a lot across the screen it then goes away. It's hit or miss as to when that pending code will turn into the 'check engine light"   
I have never been able to purposely drive the vehicle and 'make it happen'.  I can drive 36 miles to work one way and see the pending code come up about 5 or 6 times and then the check engine light will come on.   I can drive to the store
some 10 miles away and never get the check engine light but will see p1875 pending codes show up.   It's really intermittent.

This exercise was also tried 10 years ago on this forum but no results were ever posted if they ever got it solved.

https://www.zukiworld.com/forum/technical-discussion-beginner-repair/p1875-on-sidekick-sport-1997-at-with-33'/

As Fordem has suggested, approaching this on the VSS inside the gauge cluster side is easier than at the transmission side for sure.  The detail I am focusing on right now is, according to what I have been able to discover is.

The reed switch (VSS)  Inside the dash unit  supposedly  clicks  4 times per Speedo cable revolution.     So that is a constant.   And now to find out how many revolutions per MPH on the needle will also be one.

 I'll have to figure out how I can measure this. 

I'm going to try and get ahold of a good gauge cluster, locate the VSS and see how I can use a cable/hand drill and try to measure the output of the vss and  then see if one of the correction boxes can be used
inline to  adjust the VSS output.

If the VSS output can be adjusted without driveability issues then I may have it and keep both the speedo VSS and the transmission VSS  within  the 10% and stop throwing the codes.

Most people would just live with the difference and use a GPS or some sort of Phone APP for adjusting the MPH while driving but I want to see if it is possible to do this..

The reed switch (VSS)  Inside the dash unit  supposedly  clicks  4 times per Speedo cable revolution.     So that is a constant.   And now to find out how many revolutions per MPH on the needle will also be one.

According to a note I made many years ago, 1026 rev/mile - distance not speed - unfortunately I no longer recall the source of the information.  Try pulling the instrument panel bezel out far enough to see the bottom edge of the speedometer - it may be marked there - it is on the cluster for the GVs.

According to a note I made many years ago, 1026 rev/mile - distance not speed - unfortunately I no longer recall the source of the information.  Try pulling the instrument panel bezel out far enough to see the bottom edge of the speedometer - it may be marked there - it is on the cluster for the GVs.

With everything else I have going on and never getting time to look at this problem I am probably just going to find a GPS Speedo with Odometer and use it for my daily driving and mileage,  I need accurate mileage for oil changes and maintenance and of course keeping the cops at bay.  I'm calling defeat on this one unless a simple solution comes along.

With everything else I have going on and never getting time to look at this problem I am probably just going to find a GPS Speedo with Odometer and use it for my daily driving and mileage,  I need accurate mileage for oil changes and maintenance and of course keeping the cops at bay.  I'm calling defeat on this one unless a simple solution comes along.

Well this 'adventure' is a big BUST.    After review, discussion, and research,  with the way that the Suzuki has designed the transmission VSS and the Speedo VSS there is going to be no way to correct the things without generating the P1875 code.

I do not possess the technical talent to change this, even with trying to change the VSS at the speedometer to match the VSS at the transmission. 

I've pulled the correction box inline with the speedo cable and will have to use a GPS based speedometer to do my best to maintain proper speeds.

If this was a 5sp my reduction box would work with no issues at all   

I've given up.