Speed sensor wire from dash

[vex]

First and foremost for those who don't know what I'm talking about; look on page 50-40 and 50-41 of the FSM for the S4 model year. You'll notice that there is, what appears to be, a speed sensor input to the dash. The wire is Green/Red and is used in the process of the power steering, cruise control, and if you have it the Automatic Transmission. This ensures that the cruise control only functions below a specific speed. However when I attempt to figure out how it does this by looking at the dash I do not get any answers.

From my investigation I can see that the input comes from the power steering control unit that is taking readings from the A.A.S, as well as the tach (more precisely the trailing coil that feeds that tach)--50-78 for those following along.

My question is this: Where is the speed sensor (I assume it's within the dash), and how do I retain the function of the speed sensor even though I will not be running the dash?

[vex]

So I've isolated the sensor... and it's going to be a lot more complicated than I originally thought. The speed sensor switch acts as a DPO (I think closer to a sin wave). The issue is the output for this thing is going to act similar to a slowly raising/oscillating DC voltage. This means that

1. At 0v (that's infinite resistance) the power steering pump is fully activated
2. At 12v (that's assumed, but I'm fairly confident) the power steering pump is fully deactivated

This means I may need to find a way to to output a digital signal to directly turn the power steering pump on and off based on road speed.

[NoDOHC]

You have found correctly, the VSS comes from the speedometer.

I doubt that the digital dash has no means of generating a VSS. In fact, I would expect it to accept a VSS from an external speed sensor. Either way you have a source of vehicle speed.

You will want some form of VSS coming into your ecu so that you can determine which gear you are in on your data logs (it is nice for acceleration data too). That is why I think that the dash would supply such a signal (or accept it).Feed the same signal to the power steering and AAS. The scaling may be off, but I doubt that you will notice a difference (it is no big deal if the Power Steering module thinks that you are going 25 when you are really going 35).

Another option is to completely remove the power steering. I have no trouble steering my car, even in an autoX with the PS pump removed but the PS rack retained.

TTT's procedure for de-powering the rack is optimal.

[vex]

I have a speed sensor already set to install on the transmission to be wired into the ECU. I however cannot find a 'speed output' as it were. I can find speed switches, generic frequency outputs, and so on and so on (PS1k). But I can't find an output to mimic the signal.

I really want to avoid splicing the signal wire going to the ecu, but it's looking like it's becoming more and more likely that I will have to to accomplish this.

[NoDOHC]

How does the digital dash know your speed? Is it reading it from the ECU?

If so, you may have to splice the wire. It is not a very big issue, as this is not a high frequency signal.

Most speedometers are 4,000 pulses/mile. At 100 mph (not that you will ever go this fast ), you will get 400,000 pulses per hour or 111 pulses per second (111 Hz). This is well below the speed where you will have to worry about any transmission dynamics of the wiring (typically around 20 kHz). Correct wire impedances and termination are not necessary at that speed (although I would recommend twisted-pair wire and probably a shield).

[vex]

Originally Posted by NoDOHC

How does the digital dash know your speed? Is it reading it from the ECU?

Yes, it's pulling it from the ECU.

If so, you may have to splice the wire. It is not a very big issue, as this is not a high frequency signal.

Most speedometers are 4,000 pulses/mile. At 100 mph (not that you will ever go this fast ), you will get 400,000 pulses per hour or 111 pulses per second (111 Hz). This is well below the speed where you will have to worry about any transmission dynamics of the wiring (typically around 20 kHz). Correct wire impedances and termination are not necessary at that speed (although I would recommend twisted-pair wire and probably a shield).

I'm looking to avoid the wire splicing (i'm anal about that). There looks to be a software upgrade in the pipeline which might be able to do what I want.

[vex]

Something else I just considered about going this way with the speed sensor: The pulses will not match the original output. I'd have to dig up that thread I made about calculating gear ratios and what not, but from what I can remember the pulses from the VSS will not match the pulses from the electronic speed sensor, which means my power steering will shut off and turn on at different speeds than normal. Not that it's a major problem... but it's a problem I'll have to deal with at some point later on. Who knows... maybe I will just depower the power steering rack and call it a day?

[NoDOHC]

Originally Posted by NoDOHC

The scaling may be off, but I doubt that you will notice a difference (it is no big deal if the Power Steering module thinks that you are going 25 when you are really going 35).

Originally Posted by vex

The wire is Green/Red and is used in the process of the power steering, cruise control, and if you have it the Automatic Transmission. This ensures that the cruise control only functions below a specific speed.

The VSS is a feedback for the cruise control (vehicle speed), more than a limit switch, it is required for the cruise control to operate. The stock cruise control can still control vehicle speed with a different pulse/mile count, as you will be setting the desired speed to a different setpoint as well - the setpoint and feedback are shifted equally. (If your desired pulse frequency was 100 Hz before and has gone to 145 Hz now, the cruise control can still regulate to the new speed.) The cruise control does have an effective maximum, but it is well in excess of the speed limit. The cruise control also has a minimum engagement speed, but I think you will still be able to use it on the highway.

[vex]

I've dumped the cruise control awhile ago. I'll eventually setup an aftermarket one, but until then I don't really need it.

[NoDOHC]

I ditched mine too, but I miss it a lot. I take 10 hour trips every so often and I really miss the cruise then. The issue is that the Haltech makes the engine come alive a little faster than the stock ECU, so the cruise was actually unstable (you could feel the car accelerating and decelerating and watch the TPS move significantly on the gauge page). This hurt my fuel economy significantly and finally forced my hand into ditching the 'extra weight'.

Basically, you are wise to remove the stock cruise if you remove the stock ECU.