Vehicle Speed Sensor Location ('01, Job 1, 4WD)

[Yardsale]

Where is the VSS located?



If it's in the transmission and the differentials are re-geared, there should be no change in the tachometer reading at an indicated speed (because there is no change in the relationship between the engine and the transmission), right?

 

[Dan Long]

My understanding is that the rear axle ABS sensor is also used for vehicle speed. If the sensor was in the tranny, you WOULD need to reprogram with a gear change. Because of the location on the rear axle, you only need to reprogram with a tire diameter change due to the computer basing speed on a set tire diameter and number of axle rotations.

 

[Yardsale]

Oh, if this is true, I'm in good shape (and it explains why the tach. shows higher RPM after the change from 3.73 to 4.10).



Thanks, Dan.

 

[Dan Long]

Well, I think I need to retract my earlier statement somewhat based on a thread I just stumbled across while looking for something completely different:wacko:... the regular Explorers used the rear axle ABS sensor as a speed sensor, however the ST is apparently using the Output Shaft Speed sensor on the tranny as its source.



This means that you need to reprogram for your speedo to be correct. Your tachometer however, will be accurate regardless of any changes you make to the driveline because it is measuring your engine speed. By swapping to a higher numerical ratio though, your engine will be spinning faster at any given speed than it was before. If you went to a lower numerical ratio, your engine would spin slower at the same speeds.



Sorry for the confusion, I've been dealing with Explorers for years now and they're very similar, but occasionally a small difference like this pops up and bites me in the a$$.

 

[Yardsale]

Oh man ...



Before the re-gear, when I was traveling at about 75 MPH the tach. would show about 2600 RPM, now it shows about 2800 at that speed. This is about the difference I would expect from the gear change (about 10% (3.73x1.10=4.103; 2600x1.10=2860)), give or take.



Now I'm confused.

 

[Todd Z]

The engine RPM will be higher due to the higher numbered gears. If you did not correct your speedo for the new gears it will read a Faster speed then your actually going. The rpms will also be higher....



But going from 3.73 to 4.11 is not a huge jump at all.



Todd Z

 

[Yardsale]

The engine RPM will be higher due to the higher numbered gears.

This is true only if the VSS is at the differential and the speed is taken off the ring gear. If it's at the transmission, there should be no change in the engine RPM-speed relationship since nothing between the the two have changed.



Put another way, the relationship with regard to the revolutions of the engine and drive shaft (or transmission) has not changed (it is the relationship between the drive shaft (pinion gear) and wheels (ring gear) that has changed). If the speed is determined by a sensor at the transmission, then there should be no difference in the relationship between what the tach. and speedometer show because there is no change between them. If the speed is determined by a sensor at a differential (ring gear), then there should be a difference in the relationship between what the tach. and speedometer show because there is change between the drive shaft (pinion gear) and wheels (ring gear).



My original question: Where is the VSS located?

 

[Dan Long]

I don't think you understand the concept of this, no offense.



If you change your tire diameter or rear end ratio you will change the RPM's required to attain any given speed. Using a 4.10 gear, your driveshaft will spin 4.10 times for every 1 time your tire spins. Using a 3.73 gear, your driveshaft will spin 3.73 times for every 1 time your tire spins. Therefore, the 4.10 gears means your motor must turn faster to go the same speed.



If you change your tire size to a larger size, one revolution of the tire will cause your truck to roll farther, effectively lowering the rear end ratio, and lowering the engine RPM at any given speed. The opposite goes for changing to a smaller tire size.



The sensors are simply measuring either how fast the output shaft (drive shaft) is turning, or how fast the axle shafts are turning. If measured at the output shaft, the computer must know what gear ratio is in the rear end and what the tire diameter is to calculate how far one revolution of the driveshaft will move the truck. If measured at the axle shaft, the computer only needs to know how big the tire is to calculate distance moved for each revolution because the gear set has already multiplied the driveshaft rotation.



The VSS sensor is only telling your truck how fast the speedo should read. The RPM's are measured at the engine. Unless your speedo is accurate though, you don't know exactly how much difference you have in RPM at any given speed. The VSS also contributes to when your tranny shifts based on speed and throttle, so you want to have it reading correctly.

 

[Todd Z]

Actually the ring gear is going the same speed, it is attached to the axles through the carrier housing...



DID you speed up your axles, answer is NO.....



You speeded up your pinion gear which is bolted to the drive shaft....The drive shaft will turn faster thus turning the transmission and engine faster....



So the speedometer will read the same as long as you don't change the tire size,, and the engine RPM will be higher...



BUT you should change it in the computer via a Xcal just to make sure all the computers air fuel and transmission stuff works properly.....



From my research on the ST the VSS is read off the rear end housing.... IT is part of the ABS sensor.....



Todd Z

 

[Yardsale]

Yes.



So, what Dan is noting is what I have been trying to convey - there is a difference between whether the VSS is taking a measurement at the transmission or at the differential.



First, the actual vehicle speed and the accuracy of the indicated speed are not of concern. The concern is the difference in the displayed speed at a given RPM (and vice versa) due to gear ratio change.



Assume no tire size change.



Assume that for a given engine RPM, the output shaft of the transmission turns a certain RPM. The transmission output shaft, in turn, drives the drive shaft at the same RPM of the transmission output shaft.



Assume that the VSS takes a measurement from the transmission output shaft. We now have a relationship between the engine RPM and the (displayed) vehicle speed (as calculated by the computer). This relationship, while not linear, can be compared at different engine RPMs and (displayed) vehicle speeds. Examples: 2500RPM@65MPH, 3000RPM@75MPH, etc.



Assume all comparisons are at freeway speeds (65 to 75 MPH) with the transmission in fifth gear, overdrive engaged.



The gear ratio is changed from 3.73 to 4.11 (9.25%).



If, as assumed, the VSS takes a measurement from the transmission output shaft there should be no difference in the displayed vehicle speed at a give engine RPM because there is no change between the components at the two sensors (RPM and VSS).



Now, assume that the VSS takes a measurement from the ring gear (or ABS sensor) in the differential.



In this case, there should be a difference in the displayed vehicle speed at a give engine RPM because there is a change between the components at the two sensors (RPM and VSS) – the VSS should now be reading fewer axle revolutions for a given engine RPM.



This relationship change can be expressed two ways:



- for a given engine RPM, the displayed speed will be lower

- at a give displayed speed, the engine RPM will be higher.



Yes?



Well, Todd, you beat me to it. This response was to Dan Long or (anybody else who might be able to assist). (And thank you, Dan, really.)



If we are in agreement on the mechanics, and since I have noticed that at a given speed the RPMs are higher, I have to assume that the VSS is at the differential.



As Todd points-out, I have noticed some shifting differences - it's takes a bit more foot to down shift (probably due to the higher RPMs). It's not drastic, but noticeable.

 

[Dan Long]

Here is the post I was referring to that stated the VSS is in the tranny. Look to reply #24.



Yardsdale, I think we're on slightly different trains of thought, although we're getting close to a collision. Your logic seems correct about the indicated speed. If the VSS is in the tranny, you should be showing the same speed and RPM correlation unless you reprogrammed anything, although the speed shown won't actually be accurate. The difference you noted in RPM does seem to indicate the VSS as being in the rear end though.



Also, having the correct speed will make for a much easier to drive vehicle as you've noted the shifting issues.

 

[Yardsale]

Woohoo!



I just called a local Ford dealer (who'd a thunk?) and asked the parts person (figuring it has to be attached somewhere and they'd probably have a picture of the location) - (drum roll) ... rear differential.



Perhaps there is a difference in 2WD and 4WD.



Getting there ...

[Broken External Image]:

 

[Todd Z]

Yardsale,

I think your still looking at it backwards....



The change you made DOES not spin the tires or the rear Ring gear at any different RPM...

IF the VSS was on the tranny then you will see a MPH change on the speedo on the truck...

IF the VSS is in the rear you will NOT see a MPH change on the speedo...



It changes the speed of the drive shaft and pinion gear, which changes the rpm of the transmission and motor...



Thats why you could put 5:56 in your truck and the speedo will still read correct.... Just the engine will be doing 5,000 rpm at 60 !!!!.....



I think we are thinking the same as the speedo goes, But I think the reason is back wards..



Todd Z

 

[Dan Long]

If the VSS is on the tranny, MPH to RPM correlation will stay the same (although not correct MPH)



If the VSS is on the ring gear, MPH and RPM correlation will differ (although both will be correct).

 

[Yardsale]

Remember, I noted that accuracy is not of concern.



As Dan notes, we are thinking the same, but expressing our thoughts differently.

Thats why you could put 5:56 in your truck and the speedo will still read correct.... Just the engine will be doing 5,000 rpm at 60 !!!!.....

Translated means changing the ratio from lower to higher, results in a difference between the engine RPM and displayed speed, and that difference, in this case, is that for at a given displayed speed, the RPMs will be higher than before the change. Conversely, for a given RPM, the displayed speed will be lower than before the change. It's the same relationship expressed differently.



I am not implying that the speedometer is accurate or inaccurate, only that the relationship between the engine speed (RPM) and the displayed speed is changed when the differential ratio is changed and the VSS is at the differential.



It depends on whether the focus is on keeping the engine RPM or displayed speed constant and then checking the other (if one is viewed as not changing, then other changes).

 

[Dan Long]

OK, if you haven't been following the thread I linked to above, we've been trying to determine what's going on here.... we've come to the conclusion using my 2001 Wiring Diagram manual that the 01 Job 1 uses the RABS sensor for speed, whereas the Job 2 and later use the OSS sensor to determine speed.



This means that Yardsdale's speedo should be reading correctly right now, and the tach is reflecting the slight increase in RPM's at given speeds. I'll try to unplug my RABS plug and see if my speedo still works or not, I've got a Job 2, so that might help finalize things.

 

[Yardsale]

Yes Dan!



Thanks to all for contributing!