what the hell does this thing even do and why do I need it?

To preface, the part is already ordered and will be here this week, so I'm replacing it anyway.

So, Saturday I pulled hte Rio (08 Rio LX auto) in and pulled the knuckle/bearing/hub and ball joint assembly out to replace. the car doesn't have ABS, so the right knuckle is hte only one to have a speed sensor. Being 10 years old, it was of course rusted/siezed/joined forever. Shattered as I tried to pull it apart. Great. No one in hte midwest has one in stock. Its Saturday before christmas.... crap. Found one on Amazon, ordered it and unlike everything else on these Korean cars this bastage was expensive. Since my truck is down with no brakes and a leaking radiator that left us down to one car. Well, I'll try it out and see if I can nurse it back and for to work for 3 days. 

Took it for a drive.... everything works. Speedo, odo, trans shifts better than ever. Nothing out of whack. So.... what the hell does this sensor even do? CEL finally came on this morning on theway to work. Trans shifts better than ever. Normally, its VERY slow shifting when it's cold. hangs/slips until it warms up.  Now, it shifts crisp and when it should. Maybe unrelated? just seems odd that it would choose to change when it looses a speed sensor. 

So I ask the hive, what the hell does that thing even do?

I'm going to go out on a limb here and say... Vehicle Speed Sensor.

Maybe find a parts diagram online and see what the part name is, then google that name with your  vehicle make and model to find out?

All the parts places refer to it as a Wheel speed sensor. But it's not affecting the speedo, odo so who gets the signal and why?

In reply to Dr. Hess :

duh! the actual VSS is on the transmission above the diff. there.s also another output and input speed sensors on the trans. and then this thing on the wheel. 

The VSS on the trans, is that for the speedo?  It sounds like your mystery sensor is the actual VSS that the ECU uses.  What is the code the ECU threw with the CEL?

Maybe if the old & busted wheel speed sensor was providing old & busted information to the transmission, that's why it took a while to shift properly.  Now that it's gone, the trans is free to act as it sees fit, until the supervisory brain sees that the input from the sensor is missing and throws a light.

The bad news in this scenario is that a brand new sensor will give you the same performance as no sensor (not counting the CEL, of course.)

As for what the sensor actually does... maybe some kind of mechanical safety warning in case wheel speed is grossly disproportional to what the transmission thinks vehicle speed ought to be?  Would that help anything besides deterring burnouts (a, cough, known problem with an 08 Rio automatic)?

Each wheel may still have a VSS involved with the safety systems.  I cant remember the years of implementation for each manufacturer (for sure), but wheelspeed is something that is monitored in the precrash data for the event data recorder (only records <30seconds before crash.)   An 08 is right in the range where it might have the full suite that would include that (a brand new car would for sure). The EDR was mandated around that year range and I struggle to remember what was required each year as they were phasing stuff in.  (If the car having a black box scares you, you own the data and can decline having it read, IIRC)  they are used a lot in accident reconstruction and I have been involved with some. Cant tell you about your car specifically

I dont *think* it would be too closely tied into the algorithm that determines any airbag or seatbelt pretensioner deployment as they use other sensors to determine delta-v in much faster intervals than wheelspeed could.

it may also be at the good doctor and others are saying and be an ECU tie-in that determines some characteristics on how certain things work (trans shift tables, engine management, emissions if they are like VW)

*holiday inn express, information worth only what you paid me for it

As ABS was an option, I wonder if this is a sensor for a system that the car doesn't use.  It is often more expensive to make multiple different versions of a component, so they make one to accommodate different options.

This thing is, the way the vehicle's electrical system is wired up may be looking for a voltage drain from that component, even if its isn't needed otherwise, which could lead to faulty readings elsewhere in the circuit.  The system may also throw a code because it is reading an error for a sensor used in a safety system you don't have.  Do you know what the code associated with the warning light is for?

It just tripped the CEL on the way into work this morning so I haven't had time to read it. The scanner is in the truck IIRC. This is the only knuckle with a sensor and I remember the 02 Elantra I had was similar as well. No ABS but it had one wheel speed sensor on the right front.

This is more curiosity at this point. If everything works, why are you there? and why the hell do you cost 3 times as much as most normal replacement parts.

I think it's there to check the other ones, kinda like the rear O2 sensor is there to confirm cat efficiency and front O2 .

So a friend got curious and did some digging. This appears to do nothing of importance. The ECU looks for the signal, and as long as it's within a specified range does nothing with it. The signal exceeds that range or stops getting a signal and it trips a code. So it's a sensor that gets replaced for the sake of replacing it. 

The other theory is this is what sets the top speed limiter at 115. Once the ECU sees that max value shuts down. Now.... this car isn't a fast car and it's not going to see those speeds. Ever. so this is getting replacedjust to turn off the CEL. shiny happy people. 

Often redundancy seems wasteful, right up until you lose one engine in-flight on your airplane.

agreed. But its not an airplane. 

Bobcougarzillameister said:

agreed. But its not an airplane. 

cars do crash or fail a hell of a lot more often with more deadly consequences.

In reply to Stefan :

And that has about as much to do with this topic as whether the sun is yellow or orange. 

You mean to tell me there were USDM cars in 2008 without ABS?

Some cars calculate vehicle speed using ABS sensors. The one on the back of the trans may be an OUTPUT speed sensor used to see if the trans is slipping and the "ABS" sensor is used for vehicle speed. There are still three other inputs so your speedo would work normally. If this is the case. YMMV.

Guessing it has something to do with a trans performance/slip sensor - the reason your trans is shifting better now is without the signal it goes into limp mode, runs the line pressure to the moon and loses its PWM abilities on the shift solenoids just to make the car get home when a sensor fails.  So it shifts hard and fast like a proper trans should.  Driving like this long term will eventually burn up the pump or something else equally important, plus your torque converter clutch probably quit working so HEAT!

That being said it wouldnt hurt to do a relearn on the trans or talk to a trans guy and see if you can shim the line pressure higher.  Transgo may even make a kit for it.

In reply to Matthew Kennedy :

YEP. and it weighed about 2400lbs IIRC. 

Bobcougarzillameister said:

In reply to Stefan :

And that has about as much to do with this topic as whether the sun is yellow or orange. 

It's yellow.

WildScotsRacingCampbellCougarSeed said:

Bobcougarzillameister said:

In reply to Stefan :

And that has about as much to do with this topic as whether the sun is yellow or orange. 

It's yellow.

technically it's white, our atmosphere gives the tint our eyes interpret at yellow/orange. 

Bobcougarzillameister said:

So a friend got curious and did some digging. This appears to do nothing of importance. The ECU looks for the signal, and as long as it's within a specified range does nothing with it. The signal exceeds that range or stops getting a signal and it trips a code. So it's a sensor that gets replaced for the sake of replacing it.

 

As a government employee I understand this completely.

Nissan does this too sometimes.  ABS does wheel speed for the trans, trans VSS does the speedometer...

In reply to Matthew Kennedy :

I have an 06 Mazda3 without abs, pretty sure it was not standard on the first gen.