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A New Idea For A Permanent Nav Overide

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It has been established interrupting the vehicle speed signal normally input to the Nav system will restore the ability to manually enter data when under way. This fact is taken advantage of by products such as Lockpick3 and some hardware “hacks”. Interrupting this signal also has the negative effect of causing the Nav system to stop navigating, even though it is a full function GPS with no justifiable need for an external speed signal. Therefore, none of these techniques are true elegant solutions that can be implemented once and then forgotten. They all cause the Nav system to stop navigating temporarily and some action must be taken to restore it.

I was thinking about this and some thoughts came to mind.

I made two assumptions for how the speed signal is probably internally used by the Nav system based on what I would have done if I was the Engineer that designed the system:

1. The nav system probably does not normally use the external speed sensor signal. It probably only uses GPS based data for calculating position and updating the map unless the GPS data is not available. Only then does it revert to the vehicle speed sensor signal for temporary dead reckoning.

2. The speed sensor signal has a secondary function (maybe primary in the warped sick mind of a lawyer). This secondary function is to lock out manual input to the Nav system once the vehicle exceeds some very low speed. There must be some finite threshold value, albeit very low, so the system does not inadvertently lockout input when not in motion (i.e. avoids “chatter” by using a small “dead band”). As a deterrent to simply removing this signal to override the manual input lockout, the Nav system is programmed to stop navigating, even using GPS derived position information, if it sees no input from the vehicle speed sensor input.

If I was brave enough to experiment (I am not) with my brand new three week old 2009 RX350, I would try one of the following two experiments. Using a good high impedance digital DVOM to measure the voltage on the speed signal line at rest might help focus on which might be the best method below.

1. I would take a relatively large value electrolytic capacitor and connect it between the wire containing the speed signal and ground. Doing this would avoid having to cut the wire but it would maintain a mostly constant positive DC voltage on the input to the Nav system at no higher than the voltage it normally sees. This should be relatively safe to do (with respect to potentially damaging something) and might just simulate a constant zero speed but intact input to the system. Depending on how the speed signal going to the Nav system is isolated from other circuits, it might be necessary to insert a diode behind it (this means cutting the wire) to avoid effecting other Nav system functions. This approach assumes the vehicle speed sensor signal is a positive DC voltage that is interrupted at a rate proportional to the vehicles speed. There might even be a positive constant DC voltage present at zero speed. OR

2. I would cut the wire containing the speed signal and then ground the end going to the Nav system (insulate and secure the end coming from the wiring harness). It is possible the vehicle speed signal is a negative/grounded signal that is interrupted with a small positive DC voltage at a rate proportional to the vehicles speed. At zero speed it is possible the speed signal input is grounded.

If my car was not so new and out of warranty I would be experimenting as I think one of these approaches may put a quick elegant end to this foolishness. Anyway, I hope these thoughts provide some useful ideas for someone braver than me.

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