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 Post subject: Three wire speed sensors vs. two wire speed sensors
PostPosted: Tue Aug 07, 2012 9:21 pm 
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I was wondering if anyone has run across the different type of speed sensors on the newer cars. The mid-to-late 90's Subarus appeared to have two-wire speed sensors that just screw into the transmission. From what I can tell from the wiring diagrams, both wires go back up to the speedometer circuit. So I'm assuming the speedometer circuit is supplying a voltage and then reading the output signal from the speed sensor.

With the newer three wire sensors, it appears that the supply voltage is fed from a switched ignition source, another pin is ground, and the third goes to the ECU, TCU (if equipped), and the combination meter micro computer.

Interestingly enough the Liberty manual I have that shows the speed sensor separate from the combination meter shows it to have a three wire speed sensor with shielded wire.

I guess if I keep the 6spd speed sensor, I'll need to find the proper connector for it.

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 Post subject: Re: Three wire speed sensors vs. two wire speed sensors
PostPosted: Tue Aug 07, 2012 11:05 pm 
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I'm not sure about the years, but I noticed when researching the speed sensor on my '98 Outback that manual transmission cars use a different sensor then automatic cars. The manuals has a sensor with a pigtail to the harness connector while the autos had the harness plug directly into the sensor.

I'm not sure how many wires, but maybe that helps. First gen overseas Legacy/Liberties had electric speedos on some models (possibly all?)

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 Post subject: Re: Three wire speed sensors vs. two wire speed sensors
PostPosted: Wed Aug 08, 2012 5:39 pm 
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Thanks for the feedback PhyrraM.

From the research I've done, only the turbo models overseas had the electronic speedos. I ended up snagging one from the UK and will be putting it in my Legacy.

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 Post subject: Re: Three wire speed sensors vs. two wire speed sensors
PostPosted: Sat Aug 11, 2012 8:12 am 
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I'm guessing the two wire one is similar to bicycle speedometer sensor. Reed switch. Like the VSS used within the instrument cluster. It basically opens and closes the contacts each time the magnet fly by and faster it is, the more pulses you get per minute.

The output is then outputted into meter by A/D converter.. or considering the age of our cars maybe an analog integrator.

The 3 wire one could be providing the same output via solid state switch or it could be a module that generates pulse.

Either way, its not hard to convert the signal. If it's putting out voltage, put it through a transistor and you'll have an open-collector output which you could then mate with traditional system.


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 Post subject: Re: Three wire speed sensors vs. two wire speed sensors
PostPosted: Wed May 03, 2017 7:44 pm 
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I'm tacking on to this thread because I'm up against this issue with my Vanagon conversion project. I have a 2006 Impreza 5MT transmission with an integral 3 wire VSS. I need to adapt the 3 wire VSS system to the 91-94 OBD1 legacy system which receives its vehicle speed via a speedometer reed switch which simply connects ECU pin B11 to ground. My plan is to connect ignition switched power and ground to the 3 wire VSS and then use the VSS pulse output to control an NPN transistor. If I understand the open collector concept, I'll connect the VSS pulse output to the NPN transistor base, the NPN collector to ECU pin B11 and the NPN emitter to ground. Do I have that right?

From what I've found online, the Subaru 3 wire VSS is generating the pulse output via a rotating magnetic resistance element which means that the square wave output is going to have a high voltage at something less than ignition voltage and a low voltage at something slightly above 0V. Does anyone know what those max/min voltage values are assuming a +12V to +14V ignition power source? If not, no big deal, I'll find out with an oscilloscope down the road. But I'll need to know those voltages so that I can select a transistor with a gate threshold voltage less than than the max VSS pulse but greater than the minimum VSS pulse. Make sense?

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 Post subject: Re: Three wire speed sensors vs. two wire speed sensors
PostPosted: Thu May 04, 2017 6:21 pm 
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Maybe I'm missing something, but why do you need the NPN transistor? Is that so the output works with the vanagon speedometer? Based on the data I see in the FSM the output from the speed sensor is changing between 0v and 5v.

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 Post subject: Re: Three wire speed sensors vs. two wire speed sensors
PostPosted: Thu May 04, 2017 7:23 pm 
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Not worried about the speedometer. The newer VSS outputs a square wave that has a max voltage greater than 5V. I want to use the new VSS signal but I'm weary of connecting that VSS output directly to the ECU because we know that our older ECUs are designed to have only 5V on pin B11.

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 Post subject: Re: Three wire speed sensors vs. two wire speed sensors
PostPosted: Fri May 05, 2017 6:10 pm 
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Have you tested the output signal from the VSS to see if it's actually higher than 5v? If so, how much higher is it.

Again, the info I saw in the factory manual for the newer VSS shows 0 & 5v being what is inputted to the ECU. There was no modification of that signal between the VSS and ECU.

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1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

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 Post subject: Re: Three wire speed sensors vs. two wire speed sensors
PostPosted: Fri May 05, 2017 6:26 pm 
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Interesting. I have not tested it yet, but I will before deciding on the appropriate next step. Do you know if the supply voltage to the newer three wire VSS is +12V ignition switched power?

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 Post subject: Re: Three wire speed sensors vs. two wire speed sensors
PostPosted: Fri May 05, 2017 7:21 pm 
First Gear
Joined: Thu Feb 10, 2005 8:48 pm
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Location: Portland, OR
Found the 2006 FSM wiring diagram and I'm seeing that new VSS does indeed output a 5V square wave using 12V ignition sourced power and ground. I still don't think that it would be best practice to connect the newer VSS output directly to the older ECU speed sensor pin since the ECU is already outputting 5V and expecting a switched low impedance ground path. But, it's tempting to just try it and see if it works.

Knowing that I have a 5V VSS control signal to interface with is still progress. I'll report back once I get this drivetrain spinning and can do some testing.

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 Post subject: Re: Three wire speed sensors vs. two wire speed sensors
PostPosted: Tue May 16, 2017 5:58 pm 
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I would give it a try and see how it works. I just checked my Link ECU to see how it's setup and I have the pull up resistor enabled. I'm not 100% sure whether that is still enabled from when I had the original reed switch speedometer or not. Either way the speedo input to the ECU works with the pull up resistor turned on, which is more or less how the stock ECU is setup.

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1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

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 Post subject: Re: Three wire speed sensors vs. two wire speed sensors
PostPosted: Mon Jun 18, 2018 8:03 pm 
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Location: Portland, OR
I have this one verified.

Short answer: The 3 wire VSS output pin can be connected directly to pin B11 on our 91 to 94 ECUs

Explanation and Testing:
The 3 wire speed sensor takes +12V power and ground and outputs an alternating low resistance/high resistance pulse. With the VSS output disconnected from the ECU, the output during the high resistance portion of the cycle is about 1.5V. During the low resistance portion, it's in the milivolt range. So the VSS itself does not output +5V. I verified this operation using my oscilloscope.

The ECU outputs +5V continuously and "waits" for the VSS to pull the ECU down to near-zero during the low resistance portions of one complete revolution.

When the VSS output is connected to the ECU speed sensing pin and the signal is probed again, now you can see the square wave oscillating between +5V and about 150mV.

For reference, on the connector that plugs into the VSS:
Pin 1=Output
Pin 2=Ground
Pin 3=+12V

The wire colors used for pins 1 and 3 seem to vary over the model years. Don't trust the colors! Verify the wire colors to pins 1 and 3 before connecting power.

The snip below explains the VSS sensor operation. It's important to note that the alternating magnetic reluctance devise output is buffered by an internal integrated timer circuit which "latches" low/high to generate the square wave needed by the ECU. A MRE alone would generate a more sinusoidal output.

VSS Operational Description:
Image

2006 Impreza 2.5i VSS Wiring Diagram:
https://drive.google.com/file/d/1y5jPJTM5W_XBnjs6LjxSilHxcgACbMK8/view?usp=sharing

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 Post subject: Re: Three wire speed sensors vs. two wire speed sensors
PostPosted: Tue Jun 19, 2018 5:42 pm 
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Thanks for the detailed reply and information. I guess just to confirm and make sure I understand correctly.....you're saying that you could hook up one wire to the output pin of the 3-wire VSS and that would work with the 90-94 ECU's and what they expect to see from the VSS input pin?

I am not clear whether the power and ground need to be hooked up to the 3-wire VSS.

Thanks

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1990 Legacy (AWD, 6MT, & EJ22T Swap)
1997 Impreza OBS

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