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.
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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.
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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.
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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.
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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.
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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.
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