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Vehicle Speed Signal and Navigation


snoman
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I'm hoping to clear up the confusion regarding the Legacy's vehicle speed signal. Many aftermarket navigation systems require a connection to the vehicle's speed signal. While the Legacy lacks a conventional speed sensor, it does have a conventional speed signal.

 

The antilock brake control module (ABSCM) calculates the vehicle speed from the 4 ABS wheel sensors. The calculated vehicle speed is sent to the gauge cluster, or "combination meter" as a 0-5 volt pulse waveform. The ABSCM and gauge cluster are among the few locations where the vehicle speed signal can be tapped. This signal is not available at the ECU.

 

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I have found a very convenient location for tapping the vehicle speed signal. It is located inside the cabin so you don't even have to run a wire through the firewall!

 

The ABSCM in the engine compartment connects to the bulkhead harness (connector B301). The speed signal is passed from the "bulkhead harness" to the "instrument panel harness" underneath the glovebox at connector B38. The vehicle speed signal is carried on Pin 18 of connector B38. It is a green wire with a black stripe. See the wiring diagrams below.

 

 

http://users.adelphia.net/~ajhuba/vss/pic1.jpg

 

http://users.adelphia.net/~ajhuba/vss/pic2.jpg

 

 

 

 

Okay, that last diagram is a mess! I have included the following pictures to show where you can find the B38 connector.

 

First, you'll have to remove the plastic cover underneat the glovebox. It is held in place by 3 push-in fasteners.

http://users.adelphia.net/~ajhuba/vss/pic3.jpg

 

 

 

 

 

 

 

After you remove the panel underneath the glovebox, you can access the connector. You cannot see it unless you almost put your head on the floor of the car and look up. The connector is recessed behind the black plastic on the far right side.

 

I was unable to take a picture at such an extreme angle, so I disconnected the connector and lowered it some to depict its location under the glovebox.

http://users.adelphia.net/~ajhuba/vss/pic4.jpg

 

 

 

 

 

 

 

This shows the unplugged connector. All Subaru connectors are keyed and unique, so you can compare the shape to the pin diagrams above to confirm you have the correct one.

http://users.adelphia.net/~ajhuba/vss/pic5.jpg

 

 

 

 

 

 

 

 

If you are installing a navigation unit, I would highly recommend placing the computer/sensors inside the glovebox. It is literally a foot from the speed signal wire, and it's a central location for other audio connections.

 

http://users.adelphia.net/~ajhuba/vss/pic6.jpg

 

 

 

 

 

 

For those of you using a Pioneer AVIC-N2 navigation server, you can easily confirm that it is receiving a speed signal from the Legacy's ABSCM. On the navigation menu, choose "Settings", then "Hardware Info", and then "3D Calibration". If connected correctly, the "speed pulse" counter should increase while the car is moving. The learning status will also say "3D Hybrid" when a valid speed signal is detected. If your AVIC-N2 was already calibrated before connecting the speed signal, you will have to reset the "Learning Status" before it switches to 3D Hybrid mode.

 

http://users.adelphia.net/~ajhuba/vss/pic7.jpg

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Does this have any effect on the ABS?

 

No. The ABSCM produces a rather strong waveform ranging from 0 to 5 volts. The navigation system's VSS input also has a high impedance, and shouldn't affect the operation of the ABS system as long as you tap the speed signal correctly. Your car will not know it is connected.

 

For simplicity, I would recommend using a "vampire tap". These plastic squeeze-type connectors are available at Radio Shack and allow you to obtain the signal on a wire without having to cut it!

 

http://rsk.imageg.net/graphics/product_images/pRS1C-2266845w345.jpg

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Incredible post! You really improved on my "green black wire" nugget to the nth degree. I bet your nav is a little more responsive now...

Well I did this last night and I've only driven 8 miles since. It's too soon to tell while it recalibrates. Hopefully there will be an improvement, because I was very disappointed with the accuracy of the AVIC-N2.

 

 

I did notice that the AVIC-N2 reports 50% confidence in distance after only 8 miles! Previously, without the speed signal it took over 1000 miles to reach this confidence level!

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  • 2 weeks later...
update?

Well it seems to be working somewhat better with the speed signal attached. The navigation system now tracks my position through tunnels and in the downtown area between the skyscrapers.

 

I've been doing mostly highway driving, so the system hasn't completely learned left/right turns yet. Positioning should only continue to improve.

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I'm going to be anal here - Vehicle Speed Sensor.

 

(if anyone sees my OCD meds laying around let me know, can't find 'em anywhere...)

 

:confused:

 

The whole point of this thread is that the Legacy does not have a conventional speed sensor, and the speed signal must be taken from the ABSCM instead.

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way to actually find the wire! Snoman gets a +2 from me.

 

 

Good finding, I was about to give up on finding the actual wire due to the CAN system and I thought the actual sending unit and such were built IN the gauge cluster.

 

With this new findings and what not, I could have kept my HKS CAMP unit :( oh well.. onto newer and better things with this new-found knowledge.

 

Thanks for the work and documentation.

Keefe
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With this new findings and what not, I could have kept my HKS CAMP unit .

-Hijack-

What did you think of that bad boy? I was looking around for something with a video output to use on my indash screen for virtual gauges.

ignore him, he'll go away.
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-Hijack-

What did you think of that bad boy? I was looking around for something with a video output to use on my indash screen for virtual gauges.

 

PERFECT! All your gauges on one screen. Nuff said ;)

 

I was even desperate enough to install the LCD on my sunvisor (rice style) but I sold it because I got tired of looking for the Speed Sensor wire and I didnt think I wanted to deal with messing with the ABSCM.

Keefe
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What do they go for used?

 

They are now discontinued. The set I sold for was $750 with the Junction box, the main unit, the 4 senders (2 psi readers, 1 EGT probe, 1 Low-temp water/oil probe) and all the wiring that went with it.

 

I bought the MattJK 7" double din screen for my WRX at the time. So for a total of over 10 gauges for the price of $1150, It's worth it.

 

The speed signal wire will be helpful later when I will be mating such things as the TRAQMATE ( http://www.traqmate.com/ ) later on and give the GPS unit something better to read about. It's something to calibrate the GPS instead of depending on the accelerometers to do the work. I'll need some fancy data-acq stuff to make use of the driving techniques and car performance use when I am running the GT.

 

Dynos can only do so much for you when it comes to tuning. On-road applications just make it that much more worth the money to know that there are other ways to go fast than to depend on just pure power alone (especially for one that pussy-foots the throttle halfway on the exit of a turn or at a turn where the driver could have gone full-throttle).. Just because a car can make 500 whp at 5000~6000 rpms, it means nothing when the car is in gear and your foot isnt pushed down all the way. ;)

 

Again, thanks for the finding of the wire. I hope that when I get around to installing some video equipment that requires some splicing for some data to mate with it to get some real-time feedback, then it's going to be soo worth it to replay and watch it all to know where you can just that much faster and quicker. Here are a list of units I was looking into:

 

http://www.g2xtreme.com

http://www.racereye.com

http://www.advantagemotorsports.com

http://www.aimsports.com/auto

http://www.pixpress.com

Keefe
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Thanks for this documentation. You actually saved me some experiments that I was about to do. I tapped into the B38-18 wire, and my AVIC-N1 is now at 3D hybrid mode too. I also tapped the same wire to Defi HUD, and now I have speed in my HUD. Thanks!
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  • 1 month later...

I just installed an XM Navtraffic receiver. During this installation I discovered the "optional connector". This connector, B228 on the front bulkhead harness, is called the "OP Connector" in the Subaru tech manuals.

 

This connector is a one-stop shop for wiring a navigation system. It has a +12 V unswitched power lead (up to 7.5 A), vehicle speed signal, backup light signal, and about 10 other signals.

 

The connector is dangling underneath the steering column, not connected to anything. I'm guessing that Subaru's OEM navigation system uses this "Optional Connector". Either way, it's another option for locating and tapping these signals when installing aftermarket equipment.

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I just installed an XM Navtraffic receiver. During this installation I discovered the "optional connector". This connector, B228 on the front bulkhead harness, is called the "OP Connector" in the Subaru tech manuals.

 

This connector is a one-stop shop for wiring a navigation system. It has a +12 V unswitched power lead (up to 7.5 A), vehicle speed signal, backup light signal, and about 10 other signals.

 

The connector is dangling underneath the steering column, not connected to anything. I'm guessing that Subaru's OEM navigation system uses this "Optional Connector". Either way, it's another option for locating and tapping these signals when installing aftermarket equipment.

 

 

do you happen to know what B41 connects to? looks like from the diagram that it may be connected to something near or similar? that's cool that B228 can be tapped for more uber cool stuff, (im still thumbing through my manual to find out the pinouts for B228s 10-pin connector to see what signals are running out of there..)

Keefe
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do you happen to know what B41 connects to? looks like from the diagram that it may be connected to something near or similar? that's cool that B228 can be tapped for more uber cool stuff, (im still thumbing through my manual to find out the pinouts for B228s 10-pin connector to see what signals are running out of there..)

I'm out of town right now, but I'll take a look later this week for B41.

 

There is a page in the Subaru tech manual dedicated to the "OP Connector" that shows a large diagram of this connector's pinouts.

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  • 2 weeks later...

QUESTION.... Regarding the stock navigation unit... is there a way to allow the navigation unit to run while the vehicle is in motion.

 

It is a huge pain to have to pull over and completely stop every time I want to use the navigation. My buddies New honda accord does not do that. We often times ride together for business and it woudl be nice if the passenger could input destinations while you drive. This is horrible.

Thanks

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