Monday morning I had 4 lights come on, the red brake light on the left, the (!) symbol, antilock brake and VSC lights. I took it in to my local Toyota dealer (still under warranty) and they found a bad speed sensor. They will order it and replace it in a few days. They told me to drive it, but the anti lock brakes and VSC will not function. I wonder if someone else had this problem before? They said the sensor costs over $100.
It's probably a Hall Effect or reluctor assembly to sense shaft rotation and the logic determines speed. On my former 2000 GMC Sierra pickup with "AutoTrak" 4WD, I had a lot of trouble with the front and rear reluctor sensors. Whenever they would fail, the Driver Information Center would scroll "SERVICE 4WD" in scary amber letters and it would be stuck in 2WD. And if I was trying to drive up the hill at my former lake cottage, I'd also be stuck thanks to 2WD. It's probably a rare problem on the Prius. The reluctor has an air gap, so its not like a brush contact. Similar sensors are used on crankshafts and camshafts to calculate ignition timing for distributorless ignition systems and Crank Triggered systems. Also to calculate camshaft phasing for VVT systems. In theory it should never wear out.
The speed sensor works by having a serrated rotor attached to the axle on the front and inside the hub in the rear. The sensor itself is a permanent magnet attached to an iron core and wound with wire. As the rotor passes the sensor it sets up a varying voltage output. They can be watched with a volt meter ( watched at slow speed such as spinning the tire by hand ) or is better with an osiliscope when it is turning faster. As the Brake course book tells it, the system works like the magnetic pickup in your typical distributor.
Considering how the brake system works, I don't think I would want to drive it without the speed sensor. The skid (brake) ECU has full control of the wheels, you only tell the car how you want to stop. Now in the case of catastrophic failure, such as complete power loss, then you would have manual foot control of the front wheels.
I had the red triangle come on when I was going home. I called the dealer and he said it is part of the error. Not sure if that is correct, but the Prius seems to run fine. They said that the part should be in today. I'll ask to see the old part, curious to see how it works...
I'm surprised something so simple costs over $100. At least it is under warranty. Thanks for posting the image.
Hall effects don't use coils and magnets for the sensing. They use a solid state device to detect the presence of a magnetic field, usually by a magnet that moves toward or away from it. That's how the shifter works if memory serves me.
Dan: Right you are. I was going by an old shop manual for a Ford truck which had a caption for an ignition system. It clearly shows a magnetic pickup with the coil, then calls it a "Hall Effect" sensor. On the next page, it shows a semiconductor sensor and the label is "Reluctor Sensor." It's now obvious the shop manual has a typo and I should have researched that a bit more carefully. The circuit schematic that you posted is a Variable Reluctor sensor, not a Hall Effect sensor. To clear up any confusion: Variable Reluctor Sensor: This is a passive sensor (Needs no external power to operate) and works by way of Induction. The inductor is made by wrapping fine wire around a core with a small magnet, and the inductor is kept stationary. The rotating part (A shaft say a distributor, crankshaft, camshaft, etc) has an assembly called a Shutter with precisely-separated notches or teeth as a timing wheel. The rotating part is called the Reluctor. When the Reluctor/Shutter turns, it creates changes in the magnetic flux of the Inductor. If you were to put the output of the Inductor on an oscilloscope, you will notice a sinusoidal AC output from the Inductor. Everytime the shutter portion of the Reluctor breaks the field, the polarity will change and you will see regular sinusoidal + and - transitions. The higher the RPM of the Reluctor, the higher the output voltage of the Inductor. These devices are *very* reliable and are typically "lifetime" units. Hall Effect Sensor: The Hall Effect also depends on changes in magnetic field. The rotating shutter interrupts the magnetic field in it, and the IC sensor is stationary. The semiconductor sensor is also "active" in that it needs a power source to operate and detect the changes in magnetic field. The output of a Hall Effect sensor is constant voltage and square-wave constant polarity. It's not sinusoidal at all. The square wave output is much more precise than a Reluctor sensor and can more precisely determine things like camshaft rotational angle. The Hall Effect isn't quite as durable as the Variable Reluctor but is usually also a "lifetime" device.
Turned out to be the wire going to the sensor also. They have to order the wire. They replaced the sensor and the lights still came on. They found that the wire had a cut in it. I guess it opened up the circuit. I guess this was from the factory? They were not to sure how it got cut.
<div class='quotetop'>QUOTE(thorn\";p=\"85269)</div> So more than likely it had nothing to do with the Reluctor, and it's probably still good. A Reluctor is highly resistant to a short on its output leads. A Reluctor is also very easy to test once off the car. You simply attach a multimeter to the output leads and stimulate the Inductor, either by using the shutter wheel or even just the tip of a screwdriver. You hold the flat tip of the screwdriver up to but not touching the Inductor, then move it away. The multimeter should register voltage, or if set on a low range current scale, a very slight current. A nick in the wire could have been made during manufacture, but since the wire wasn't carrying more than a minute signal, it still worked. All it takes is a bit of corrosion or a wire break and the signal stops.
The sensor probably did still work. They showed me it, they had damaged it alittle getting/prying it off. It was bowl shaped. They ordered the wire. It was damged up in the other wires, I guess a harness. So it must have been at the factory or maybe it had been rubbing against something. I hope that is the only one!
<div class='quotetop'>QUOTE(thorn\";p=\"85408)</div> That is very rare to have a broken wire. The Prius is generally very well built from the factory and normally has very few problems. Hope that is the the only problem you encounter, :wink:
I have a 92 Toyota truck and 02 Highlander, and had a Corolla and never had any lights come on before. Really not much problems with them at all. Have you seen this in your shop before?
That sort of problem is extremely rare on a new car. Usually you will find wiring harness problems on older cars and trucks, especially if road salt is dumped on the roads in winter.
<div class='quotetop'>QUOTE(thorn\";p=\"85538)</div> Never had one with wheel speed sensor problems. Like I said before,the Prius is really well built and problems, if they do come up, are few and far between. Really rare.Most wiring issues on any Toyota's are generally a bad pin fit in one of the connectors.
The wiring problems must be rare. They had a hard time finding the part. They said next week sometime. Maybe they had to get it from Japan?
Japan would be my guess also. With very few problems happening on the Prius' , some parts are not stocked in this countries regional warehouses. More common parts would but a lot are not. Guess this is the price for being at the front of technology, :lol:
