Hey everyone,

A while back, there was a posting on how to access a diagnostic menu via the "Display" menu, by touching the corners of the screen alternately. This takes you to a menu where you can access some of the Navigation system info.

One of these screens is called vehicle sensors if I recall correctly, and it shows you your current speed, and a "pulse count" that continually counts upward. I think people had speculated that this is the speed being reported by the GPS unit, but I am not convinced that this is true. I think it is the actual wheel speed sensor, which doesn't show the error that the speedo shows. Mine is off by about 5 mph while I am at about 70mph.

Does anyone have documentation that this sensor reading is GPS based or wheel speed based?

Here is a link to another thread showing how to access this screen. The one that I am referring to is under the Navigation Check button, then there is a Vehicle Sensors button you press.

You can get out of the diagnostics screens by pressing the display button and holding it for about 5 seconds. This "reboots" the nav system and brings you back to the toyota logo screen.

Thanks!

 

To answer your question directly - no, I don't have the documentation.

But don't wheel speed sensors detect the rotational speed of the wheel, not the forward speed of the car? Such that the diagnostics screen would have to calculate the speed displayed (and perhaps introduce the same error that the speedo has) rather than just pass on the raw value from the sensor?

I understand the value in the diagnostics screen to be the lateral velocity as calculated from GPS triangulation and not via the wheel speed sensor.

One way to prove it would be to disable the GPS receiver (by either driving underground or just unhooking it?)

 

Actually my theory that this is the wheel sensor is that it is labeled "SPD". I thought that this was a sensor in the wheel that could be used for "dead reckoning" when the GPS signal is lost. This way it estimates your location on the map by using your speed/distance traveled, coupled with the gyro sensor that gives a relative bearing.

That brings up another question. That same diagnostic screen shows the relative bearing from the gyro sensor, but this never seems to match your compass heading. I am curious what this value is actually relative to. Is it relative to the direction you were going when you entered diagnostic mode, or is it relative to the direction you were going when you started out in (when you started the car)? I think it is the former, since the latter would be tricky if you started out in reverse...

I think that the speedo probably uses the input from this SPD sensor, but for some reason it is calibrated strangely to cause the low readings.

I recall reading somewhere about speedo error, and that European cars had their speedo calibrated in such a way that they NEVER read low, even if someone puts a huge set of wheels/tires on the vehicle. This is due to some European regulations that stipulate this "NEVER low" requirement. Perhaps Japan has a similar mandate? (I think this was from a Car and Driver editorial column a few years ago).

 

I believe that the SPD (speed) signal sent to the NAV unit by the Combination Meter is probably the speed signal computed from MG2's rotation sensor in the hybrid transmission (and not from the wheel rotation sensors). Since MG2 is coupled to the front wheels through fixed gearing, its rotation rate is directly proportional to the average wheel (and hence the car's) speed. And since it's sent to the NAV unit from the Combination Meter, it will almost certainly be the same data that the speedometer displays.

Stan

 

The pulses are from the wheels. The NAV does count them but actually only uses them during times it "dead reckons". Note the "calibration" option in the NAV setup which you are suppose to run when you replace tires. Thay calibrates the distance per pulse by using the NAV to measure the distance and the pulses to determine wheel rotation.

 

schmidtj — The TCH's Overall Electrical Wiring Diagram (attached below) shows quite clearly that the four wheel speed sensors are connected only to the Skid Control ECU. The NAV unit receives the SPD signal from the Combination Meter. The Combination Meter is connected to various ECUs and to the CAN bus. The NAV unit is also connected to the CAN bus but not to the wheel speed sensors. I reiterate what I said in my post above.

Stan

 

Why does the NAV care when I change tires and need to be recalibrated?

 

schmidtj — Presumably the NAV does the same as ScanGauge does. It allows recalibration of the speed signal (a single signal, not four) that it receives on the CAN bus from the car's ECUs. That is, the SPD signal is adjusted by the user's dialed-in correction percentage.

Stan

 

I haven't actually done a recalibration but from the book you don't really dial in any corrections. You tell the NAV to calibrate and then drive until it says it's done. It measures wheel pulses and GPS satellite derived distance travelled during the calibration and automatically developes a miles/pulse algorithm from that. At least that's how I read it.

 

The dealer installed NAV units in Canada are not connected to the CAN bus, but of course are connected to the SPD signal. They can be calibrated.
Hope this info helps in your discussion.