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Old 08-17-2011, 03:51 PM   #1
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Default Na VSS signal and wiring

OK.

So after reading a few posts and doing some searches. I decided to put a wire on my 97s RSW screw on the back of the cluster and send it into the VSS(vehicle speed sensor) input on my ECU.

THIS DIDN'T WORK....regardless of what I read other places.

Is there another place where I can find it?? Does my stock gauge somehow take the mechanical signal and have a electronic output somewhere on it? Either way I would LOVE to have this kind of input.

It would make Boost feedback, Boost by gear, launch control ect... a walk in the park.
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Old 08-17-2011, 03:56 PM   #2
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The RSW terminal is just a switched closure to ground (there is literally a magnetic reed switch.)

Thus, it requires a pullup.

Did you provide a pullup?
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Old 08-17-2011, 03:57 PM   #3
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the VSS wire already goes back to the stock ecu.... dunno why you're fishing for one.

the VVS wire needs a pull-up.


EDIT: dammit joe.
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Old 08-17-2011, 04:08 PM   #4
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Quote:
Originally Posted by Braineack View Post
EDIT: dammit joe.
Cat picture time:

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Old 08-17-2011, 04:26 PM   #5
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Quote:
Originally Posted by Braineack View Post
the VSS wire already goes back to the stock ecu.... dunno why you're fishing for one.

the VVS wire needs a pull-up.


EDIT: dammit joe.
Stupid newb question, what is a pull-up?? I was looking at the 97 electrical manual and saw the VSS wire as well, after the fact... Still don't know what a pull-up is.

...and once that turbo spools, that kitty is toast.
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Old 08-17-2011, 04:41 PM   #6
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Originally Posted by Preluding View Post
Stupid newb question, what is a pull-up??
In its simplest form, a resistor connected between +5 and a signal wire.

You've got an ECU that's expecting a pulsed voltage input. And you've hooked it up to a device that does not provide a voltage output, it provides a switched closure to ground. So to make this circuit work, we place a resistor (1k usually) between a supply of power and the signal line in question.

Now, when the switch is open, the line will be energized to whatever voltage we're pulling it up to, and when the switch closes, that voltage drains to ground and the line goes to 0v. Thus, we achieve a pulsed voltage.

This method is quite common. Here's an example of a pullup applied in a launch-control situation- look at the "activate low" diagram, and ignore the diodes: http://msextra.com/doc/ms1extra/MS_E...l.htm#launchin


Quote:
...and once that turbo spools, that kitty is toast.
Actually, that's an attempt to control boost the same way they do with centrifugal superchargers, by placing a restriction inline with the compressor inlet. Kitty is precisely calibrated to achieve 12 psi at redline.
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Old 08-18-2011, 11:35 AM   #7
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Quote:
Originally Posted by Joe Perez View Post
In its simplest form, a resistor connected between +5 and a signal wire.

You've got an ECU that's expecting a pulsed voltage input. And you've hooked it up to a device that does not provide a voltage output, it provides a switched closure to ground. So to make this circuit work, we place a resistor (1k usually) between a supply of power and the signal line in question.
From this response. I have no idea how the VSS signal works....

I thought it would be a voltage output based on how fast I'm going, like most of all the other sensors in the car.

Your speaking of a switch/resistor combo and I don't understand the diagram.
What are the arrows with a line in front of them?? Why do I need my TPS signal at all? 0-volts = Ground yes?? Is the switch my existing wire or is that extra?


My IQ is falling by the second. I may try to find a friend who haz better edumacationz in this then me.
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Old 08-18-2011, 11:44 AM   #8
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take a 2.2K resistor. Connect one end to VSS wire and the other to 5v. have a nice day.


The pull-up keeps the input high (5v) when the signal is not grounded. Otherwise, as your vss pulses to ground, it's not going back to voltage so the CPU can distigish anything from it. When the VSS signal grounds, the voltage drops from 5V to 0V. So now the CPU sees pulses.

Does that make sense?
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Old 08-18-2011, 12:04 PM   #9
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Here's the simple version:

The VSS (on '90-'97 cars) is just a switch. When it's closed, it conducts to ground. When it's open, it does not. It opens and closes many times per second as the speedometer cable rotates.

Here are the two states of the VSS, which I have symbolized by drawing a SPST switch at the left side. In both of these drawings, I have added a pullup resistor at the top. At the right is the pin which feeds into the ECU (or the cruise control, or the A/T controller)

First, here is what it looks like when the VSS is open:



Voltage is coming in through the resistor, but it has nowhere to go, so the ECU sees +5v at its input.


Now, here is the VSS closed:



Voltage flows through the resistor, but it has a path to ground through the switch contacts. Thus, the voltage all drains out to ground, and the ECU sees 0v at its input.


If you imagine this system cycling back and forth between the two states at a high speed, the result (as seen by the ECU) will be a squarewave signal alternating between 0 and 5 volts.


The exact value of the resistor is not critical. You could use 1k, 2.2k, whatever you have lying around. 1k would be brown-black-red-(gold/silver), and 2.2k would be red-red-red-(gold/silver).


The key point is that without the resistor providing a pullup, the ECU will see 0v all the time, regardless of whether the VSS is open or closed. In the original design, the pullup resistor was inside the ECU (or the cruise control box or the A/T controller), however since you have none of those things, you need to supply the pullup yourself.



Quote:
Originally Posted by Preluding View Post
Why do I need my TPS signal at all?
You don't.

I took that image from the MegaSquirt documentation. On that ECU, there is a pin labeled "TPS Vref" which is not specifically related to TPS, it's just a convenient source of +5v that it commonly used to provide a pullup for the TPS, as well as for whatever other external sensors need a +5 pullup. Your AEM probably provides a similar supply of external +5v. I don't know what pin on the AEM it is, but it should be going to pin 3I of the ECU harness (a light green / white wire) as this is how +5 is supplied to the TPS (as well as the EGR sensors and the TCM) in the factory harness.
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Old 08-18-2011, 02:13 PM   #10
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Dear Joe,


THANK YOU


I am definitely understanding this quite a bit better.

To clarify a little more for dummies:
ECU = My AEM EMS +5V input
+5V = ? Not quite sure where this 5V will be coming from but its going through a resistor before being tied into the wire that goes between the ECU and
Switch = This is technically the VSS Output from the gauge cluster, the wire is already there.

So really, I need to find the input wire (Should be 3I as you've graciously told me) and splice another wire to it with a resistor to a +5V output (Should be one on the ecu somewhere)
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