RhoXS

I want to test a theory. To do this I need the vehicle stationary with respect to the wheels not turning but have it actually change location enough to see the change on the map display. A ferry seems like the perfect way to do this but there is no ferry I have access to.

This is why I want to know:

It is known that simply disconnecting the vehicle speed sensor signal from the nav system will cause the nav system to stop updating the map position even though it is still a fully functional stand alone GPS system, not requiring any external input except for power. It appears obvious, at least to me, this behavior is intended to act as a deterrent from simply disconnecting the vehicle speed sensor signal to regain full control of the nav system.

I am trying to determine if the nav system behaves this way because it is programmed to not update unless it sees some small but finite speed from the vehicle speed sensor (no practical work around) or, if I am lucky, it looks for a small positive DC voltage that may likely be present at zero speed on the input from the speed sensor (an easy work around).

A little research located an Electrical Wiring Diagram with an ultra simplified schematic of the "C17 Combination Meter" (where the speed signal comes from). It clearly shows how the speed signal that the Nav system uses is derived. The signal going to Nav system is pulsed to ground (via a switching transistor and current limiting resistor) at a rate proportional to speed. This implies to me there is very likely a small positive voltage on this wire when the transistor is "off". At zero speed the transistor should be off.

In other words, if the Nav system updates the map when the wheels are stationary but the vehicle is actually changing location, there is very likely a simple work around to regain full use of the Nav system. If the map position does not change, then it is probably internally programmed to see a finite speed from the vehicle speed sensor before it will work as a GPS. In the latter case, my theory is in the toilet and I am SOL wrt having a usable system.

To test this theory I tried driving very slow so the vehicle is at least moving but the manual input icons are not yet greyed out. I wanted to see if the map updated as no wires were cut and everything is intact. I could not practically do this as the speed at which it locks out input is too low. Ergo, my idea to move the car via a ferry to test this.

Orzel

iTrader: (1)

Can't speak to factory nav system, but our Garimin 660 updates when traveling on a ferry.

ggebhardt

You NAV works off GPS and not the speed of your wheels.

RhoXS

Orzel, the ferry idea is only suggested as a practical method to facilitate moving the car to a different location to see if the map updates without the vehicle speed sensor sensing any speed. I suspect Lexus intentionally prevents their nav system from updating when it senses that the vehicle is stationary via the vehicle speed sensor. I believe this is done to prevent people from simply cutting the speed sensor wire so manual inputs are not locked out once the car is under way. A GPS uses satellites to determine location/speed/etc. and has no need for an external speed sensor unless the satellite signals are lost such as in a tunnel. Therefore, your Garmin, along with all GPS units, will update no matter if on a ferry or a flat bed truck.

RhoXS

ggebhardt, as I stated above, the Nav system should certainly be capable of working off only the GPS satellites. The vehicle speed sensor should only be needed for dead reckoning when the satellite signal is lost for some reason. However, when using the Lockpick3 device or a manual switch, to interrupt the speed sensor signal so one can make a manual input while underway, the system stops updating. Therefore, one cannot just permanently interrupt the speed sensor signal to regain full manual control of the Nav system.

lex

i think "borrowing" a tow truck would be easier than finding a ferry

dave2K

that is exactly what I was thinking. You'd only need about 5 minutes of towing to test the theory. That shouldn't result in too big of a bill. Of course I wouldn't be surprised if they prohibited having anyone in the vehicle while being towed. Maybe you could convince them otherwise at a low speed on a deserted road. Good luck, your theory is interesting and I, for one, would like to know the answer.