[ KB ] How to Install a Navigation System Defeat Switch

[Danny]

Category: 3 Electronic Modifications
Type: Modifications

Article Name: How to Install a Navigation System Defeat Switch
Author: efusco
Description: Temporarily disable the speed sensor and thus allow a passenger to enter navigation instructions while the vehicle remains in motion

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[efusco]

This is not really new--unless you never read it the first time then it's new to you. I just finally got around to posting it in the KB myself, under my own name. I made a couple minor additions/modifications, but nothing major, just some polishing to really make the install slick.

[Danny]

Posts by cyclone that were lost when I deleted the old article:

Hi, All,  

I did the modification, it went very well (with a couple of caveats below). Thanks for the awesome information! My wife was anxious when I said I'd be cutting a hole, but she's thrilled with the outcome. Having gotten used to our MDX where the nav system isn't disabled during movement, it was frustrating to have to sit in the driveway while my wife enters our destination. This is a gas-saving modification!  

Here are my comments:  

1) I had two violet-ish wires in the bundle under the console. One was clearly thicker than the other and was near the surface - that's the wrong one. You want the thinner violet one.  

2) I originally started with the Radio Shack Momentary Pushbutton Normally Closed (NC) Switch Part # 275-1548, but after installing discovered that you have to keep the button pressed in order to override the speed sensor, which makes entering anything far less convenient, and almost impossible for the driver to do (not that you would!). The Radio Shack SPST Rocker Switch Part # 275-693 works much better, plus (at least with the brown interior) looks like it's original equipment, as it looks much more like a car rocker switch ought to.  

3) When first installing the rocker switch I held the soldering iron to one of the contacts for a fair amount of time (maybe five - ten seconds) in order to heat it up well and ensure good solder melting. this apparently melted something inside the switch, as it didn't work properly and I had to replace it. So try to minimize the amount of heating of the switch contacts.  

Hope this info helps...  

Bill

 
 


Anyone know how this modification might affect the warranty? If something went wrong with your nav system, would you remove it and make up a story about the hole in your console?  

[efusco]

Anyone know how this modification might affect the warranty? If something went wrong with your nav system, would you remove it and make up a story about the hole in your console?

I guess they can use about any excuse to try to void the warranty, but I'd MAKE them prove that my switch somehow caused the problem. This is really nothing, it interupts the speed sensor wire and nothing else. You're not affecting the programing, you're not hacking the programing, you're not adding any additional power or inputs, nothing that can directly affect the NAV system or should cause any malfunction.

I think they'd be using it purely as an excuse and I'd fight them on it.
--evan

BTW, the EV button is slightly different as it could be reasonablely argued that use of that button may have/could have put undue stress on the hybrid system and esp. the battery and if there is any damage that could be remotely related to either of those two problems it would be hard to successfully argue the point.

[sibtrag]

Originally posted by efusco+--><div class='quotetop'>QUOTE(efusco)</div>

I don't know much about the specifics of auto electronics, but ...

Could your touching the switch cause a static discharge on that line?

What's the nature of the signalling on that line? +12/0 moving/stationary, pulsed (or analog) ? Or is it current-based signalling?

Can flipping that switch while moving cause too-wide/too-narrow a pulse? Or glitch to a level out of range, or too sharp a transition.... Will the absence of a load mess up the speed sensor (especially for current sensing).

Electronics can be pretty sensitive, especially if the designer didn't expect anyone to muck with that wire.

Were I doing this myself, I'd probably hook up the switch so it connected the NAV input to the inactive level (probably ground, but I'd investigate first) that is being sent while the car isn't moving and which presented the sensor input a load similar to the NAV input.... but I'm paranoid.

<!--QuoteBegin-efusco

I think they'd be using it purely as an excuse.

You're certainly right about that. They wouldn't know what caused the problem.

[efusco]

Were I doing this myself, I'd probably hook up the switch so it connected the NAV input to the inactive level (probably ground, but I'd investigate first) that is being sent while the car isn't moving and which presented the sensor input a load similar to the NAV input.... but I'm paranoid.

Sounds like a good thought, it also sounds like you know a lot more about electronics than I do (I know very little). I guess the only assurance I have that the current mod is safe is that it has been done on many cars (previous and current model Prii) without reported malfunctions. All the switch does is interupt the signal, AFAIK. I guess there 'could' be static discharge or some other odd thing that could cause a problem, but I have to assume the system's been designed to tolerate surges and interuptions in power/energy flow.

Your idea seems to be much better, if it's possible to implement it in an efficient manner. Meanwhile I think the mod as described is probably safe. Note, however, the disclaimer--obviously I have not assurance that I've covered all the bases.

[rflagg]

Just wanted to give you guys a thank you for putting this up. Went through the procedure yesterday, and it went off without a hitch - definately have to make sure you go for the right wire (the thinner purple wire in the middle), and it also helped to have two people when it came to the soldering, but otherwise very cool stuff.

Albeit a small one, I have taken the step to mod my prius, and it's a joy!

-m.

[brosnan]

Originally posted by sibtrag

I don't know much about the specifics of auto electronics, but ...

Could your touching the switch cause a static discharge on that line?  

What's the nature of the signalling on that line?  +12/0 moving/stationary, pulsed (or analog) ?   Or is it current-based signalling?

Can flipping that switch while moving cause too-wide/too-narrow a pulse?  Or glitch to a level out of range, or too sharp a transition....  Will the absence of a load mess up the speed sensor (especially for current sensing).

Electronics can be pretty sensitive, especially if the designer didn't expect anyone to muck with that wire.

Were I doing this myself, I'd probably hook up the switch so it connected the NAV input to the inactive level (probably ground, but I'd investigate first) that is being sent while the car isn't moving and which presented the sensor input a load similar to the NAV input....  but I'm paranoid.

For those that are curious, I find the signalling as follows:
Voltage toggles between 0 and 13.6V.
Changes state about once per foot or so if rolling above a min speed,
both forward and reverse.
Freezes at either 0 or 13.6 if you stop.
Electrical levels are not quite consistent with a pullup resistor in the nav
unit and a switch to ground in the speed sensor (wherever that is).
Too bad, because then a switch to ground from this wire would be fine
and you wouldn't need to cut the wire - just tap into it.
Short circuit current to ground is 8.2mA when in the high state.
Current through a 150 Ohm resistor to ground is 6.8mA (high state)
Current through a 1.5K Ohm resistor to ground is 2.2mA (high state)
Maybe someone who's cut the wire can say what the current
through the switch is in each state?

I might be tempted to ground the metal body of the switch to
improve the odds in an ESD hit. Maybe even a surge suppression
diode between the purple wire and ground for extra protection.
Another method might be to use magnetic window alarm sensors
so you could open the speed sense line without much chance
for zapping it with ESD.
2004 #9

[PriusLover]

I was very happy to find a thread on this subject - does anyone know if a mechanic would do this since I'd hate to screw something up during the modification. Thanks!

[efusco]

I was very happy to find a thread on this subject - does anyone know if a mechanic would do this since I'd hate to screw something up during the modification. Thanks!

I've advised that you can always ask around at stereo install places to see if they'd be willing to do it. I believe someone here found someone that agreed to do it for about $50. Just remember that they'll almost certainly never have done anything like this before either and are just about as likely to screw up as you are. They're biggest advantage will be their comfort level working with a soldering iron and pulling apart the dash and running wires.