VSS/Speedometer Wiring

[Neeraj Ahluwalia]

Hi all,

When I was originally dieting my wiring harness, I ended up removing the wiring for the vehicle speed sensor. The ECU is not happy with this - the rev limit is cut to 5k & the radiator fans stay on.
I have the 3-pin sensor and I was wondering if anybody knows what voltage its supposed to receive. I understand it gives the ECU 5V pulses, so I would assume it needs a 5V power supply, however I have read conflicting information online.

After I give it power I also assume I can just run a wire to pin A1 on the ECU.

Thanks for the help.

[RPGs818SNA]

Hi Neeraj,

A quick check of the schematic for your ECU donor will show what power the sensor uses (probably 12v) and where to connect it (usually to the speedometer and ECU, and perhaps a cruise control module). If you can post ECU and wiring harness source details (year, model), I can provide those details.

RPG

[Neeraj Ahluwalia]

Hey RPG,

My donor was a 2003 WRX. I've found this ECU pinout: https://www.northursalia.com/modific...ng/wrxpin.html
Unfortunately it does not show what voltage the speed sensor needs - only that it gives 5V pulses.

I have this wiring manual: http://ken-gilbert.com/wrx/mans/7%20-%20WIRING.PDF
You can see on page 20, the vehicle speed sensor is on power supply FB-16. I'm not sure if that means it shares 12V with the rest of the loads.

[RPGs818SNA]

Hey Neeraj,

Here's the schematic for the speed sensor for a 2003 WRX with turbo.

speed sensor 2003 Turbo.jpg

Pin 1 Green Yellow wire goes to Pin A1 on the ECM as you said, and through a connector pair to Pin C2 of the instrument panel.
Pin 2 Black Red goes to engine ground
Pin 3 Green Black goes to 12v power from Fuse Block-17 Fuse #18.

The speed sensor regulates the 12v power down to 5v for the signal pulses.

That should make everything work better.

Good luck with it,
RPG

[Neeraj Ahluwalia]

Hey RPG,

Thank you for the very useful information. It's unfortunate that the signal goes to both the ECU and the instrument cluster - requires pulling off the side sails to hide the wire

However I looked at my connector and it doesn't have those colours that you stated. It has white, brown, and green. Am I looking at the right thing?

20230725_204531.jpg
20230725_204540.jpg

[RPGs818SNA]

Hmm. I scrolled through the entire diagram looking for a black 3 pin connector with white, green, and brown wires. No luck. Was the wiring harness from the 2003 WRX your engine came from? Does your speedometer work? Can you add a photo of the pin side of that connector? It appears that those 3 wires plus a blue one go into a light grey connector plugged into a black connector. Can you see the color of the wires coming out of the black connector. Can you tell where they go? Can you tell where the blue wire comes from? Sorry for all the questions, but I'd really like to help determine for sure what that connector is.

RPG

[Neeraj Ahluwalia]

My wiring harness was for sure taken from the 2003 WRX that my engine came from. My speedometer does not work.
The pin side used to be a black square-ish connector that I did not hook up when I was building the car. I was under the false impression it was the gear position sensor.
I switched it to a deutesh connector so I could add another wire of my own- the blue wire is my own wiring to my trunk latch switch. It is a custom harness throughout and the only stock wiring is in the center console (presumably where the other side of the VSS connector is buried).

[Neeraj Ahluwalia]

In addition, pin C2 on the combination meter seems to be a brown and white wire, not a green black wire as shown on your schematic.
There is a green black wire but it is in pin C7
20230725_232848.jpg

[Neeraj Ahluwalia]

I looked more closely at the schematic I had found and it shows the the VSS is connected to pin A2 on the combination meter - which matches a green yellow wire on my harness. In addition, if you look up images of the Subaru VSS, they all have that brown, green, white wire colour.
I found an image that shows which ones are ground, +12V, and signal. I'll start the car tomorrow and see if that solves my issue.
Screenshot_20181029-125129_Dropbox_zpsooz7ltgd (1).jpg

[RPGs818SNA]

Good sleuthing, Neeraj. I found the same thing last night but while I was writing it up my laptop ran out of battery. Sigh. What was confusing is that the schematic shows connector B17 with GB, GY, and BR wires. It turns out that B17 plugs into a pigtail with G, W, and Br wires that go to the connector that attaches to the VSS sensor.

VSS pigtail.jpg

You replaced the 4 pin pigtail connector to add the blue wire. That should plug into B17 with the GB, GY, and BR wires. GB goes to fused 12v power, BR to ground, and GY to A2 on the combo meter and A1 on the ECM as you mentioned, plus to pin 19 on the cruise control module if you still have it. That should do it.

RPG

[Neeraj Ahluwalia]

So i ran the wires to the dash pin A2 and ecu pin A1 according to the image I found and success!
The speedometer is now working and the ECU is no longer angry. I can proceed with tuning.
Thanks for your help RPG.

Side note-this thing is menacingly fast. It is also in desperate need of an alignment.

[RPGs818SNA]

Great news Neeraj. Hope you can fish the wires up to the front without having to take the side sail off.

Be safe driving that thing.

RPG