EJ25 Speed Sensor

[totorro]

Hello, I have questions around speed sensors and EJ25 ECUs, does anyone have any experience?

A bit of background - my van has a EJ25 conversion which was done by a previous-previous owner. When I bought the van I was told it didn't have a speed sensor fitted, and it came with a still boxed GPS speed sensor. The engine runs well and the only issue is that after ~10 minutes at a constant speed (e.g. motorway / dual carriageway) the revs will drop, but coming off the throttle and back on makes everything normal again. It's a little annoying but all in all not a big deal. The seller of the van was upfront about this when I bought it.

Since getting the van and doing some investigating I've found that there is a speed sensor fitted to the speedo. It's wired up with 12v and earth, and the "VSS" signal is connected into the wiring loom (yellow wire).

At the ECU end there's at least two yellow wires which come out the wiring loom and are connected to the ECU, but I've no idea if these are the speed sensor signal or not. Also at the ECU end are two wires which are not connected to anything, I'm guessing one of these is the neutral position switch, which from what I've read sounds like the right setup. These two wires have a handy sticker which reads "Don't worry about these if van runs funny etc ring me" - no phone number

Using a OBDII reader there's no speed reported by the ECU. So the signal isn't getting picked up.

I don't know who did the conversion / wiring or even exactly which model of Subaru the engine came out of, so that's a bit of an unknown. I've tried looking for a ECU pin out but haven't been able to find one which matches exactly - seems like Subaru had a lot of variations. The ECU is marked "4R / 22611 AH25 / A18-000 D9R / 2508".

Seems like there's a few options:

• The speedo VSS isn't connected to the ECU
  
• The speedo VSS is connected but is broken
  
• The speedo VSS is connected and working, but there's some other problem
  

If I can verify that the speedo VSS is broken I can use the GPS one as an alternative. I've thought about connecting up a meter to the VSS signal then try to monitor it, but it's tricky while driving and I've not sure what the signal should be (e.g. a 12v pulse, or is it a pull up/down resistor)? I could potentially knock up a data logger from an Arduino / Pi Nano. If the VSS looks good I can try and identify where the wire comes back out the loom, or could run a new wire back to the ECU, assuming I know where it's supposed to be connected.

Does anyone have any knowledge on how the sensors are supposed to work, and does anyone have a good resource for finding / identifying Subaru ECU pin outs?

[RJES]

Aidan asked me to reply to this. Usually this question is not straight forward to answer, as a lot of times folks don't really understand how to test a speed sensor, so I end up having to talk them through every step. The ECU part number is incomplete (one digit missing), so I cant definitively tell which one you have. I suspect it is probably 22611 AH250? If so that would make it an EJ25 Legacy engine from MY02 or MY03 with EGR. Does that sound right? You wiring work was not done by me - I've never written a label like that. 

The fact that you know what a pull up resistor is will almost certainly make testing the sensor easy. But it partially depends what type of sensor you have . The correct spec of Subaru sensor is open collector, as are the ones I make. If the bus is water cooled (i.e. clip on speedo cable), for many years my sensors have had reverse polarity and over current protection built in, making them very hard to kill by connecting wrong even if you intentionally try. Prior to about 2010 they did not, so could be killed by incorrect connection. With no internal pull up, the pull up is in the ECU. Therefore to test the sensor you wither need to do so while it's plugged in to the engine management, or 'on the bench' with a suitable pull up to sink less than 20mA connected. Just watch the voltage on the signal wire as you rotate the speedo (or slowly turn the LH front wheel in a 2wd). You should see a square wave.
If the bus is air cooled (i.e. screw on speedo cable), and used one of my speed sensors, they do not have reverse polarity or output overload protection.

It could get more complex if you do not have one of my speed sensors, as some of them marketed as being for Subaru conversion are not really the right spec, such as the ones with clear encapsulation from Germany (which Brickwerks sell). The have selectable internal pull ups for different voltages, and what they claim is needed for Subaru applications is plain wrong. The internal pull up probably makes them easier to test though. They have an LED for some unknown reason, but I don't know whether it definitively shows whether the output has switched, or whether it is switched with a separate driver. It would be a nice thing to have on a sensor which is likely to fail (although not worth the extra cost / complexity in my opinion), but is totally unnecessary in a sensor with output overload and reverse polarity protection. With these features, they as reliable as OEM parts. They were almost certainly designed for non Subaru applications, probably to emulate the strange PNP output design of early Golf MFA sensors. Whoever it is that makes them tried to sell them to me 10+ years ago, seemingly without realising I make my own . As well as internal pull ups, their claims about the pulses per turn required for Subaru applications are also wrong. 
If the sensor has an unknown output spec, you'll have to work out whether it has an internal pull up or not by measuring. As you mention Arduino's, I imagine you'll know that you need to know what you're doing to do this? Pull too much current through a sensor without overload protection (such as by setting a meter to measure current rather than voltage), and you'll kill it instantly.

With a GPS speed sensor, file it in the nearest bin where it belongs, or put it on ebay if it works. They're an over complex solution looking for a problem in a vehicle where it is so easy to fit a proper speed sensor.

[totorro]

Thank you, that's a really detailed reply, much appreciated!

The ECU part number is incomplete (one digit missing), so I cant definitively tell which one you have. I suspect it is probably 22611 AH250? If so that would make it an EJ25 Legacy engine from MY02 or MY03 with EGR. Does that sound right?

Sorry, yes it should be 22611 AH250. An early 2000s Legacy sounds right.

Sounds like my best bet is to pull it out and run some tests on the bench. Being able to manually turn the speedo will be easier than trying to monitor it while moving (it's a syncro). Unfortunately I don't have a good photo of the sensor, but from memory it looks more like one from you than the Brickwerks one - no LEDs.

If I can confirm it's working (or not) then next step will be working out if it's wired up to the ECU correctly

With a GPS speed sensor, file it in the nearest bin where it belongs, or put it on ebay if it works. They're an over complex solution looking for a problem in a vehicle where it is so easy to fit a proper speed sensor.

It's still boxed and unused so maybe ebay is the best place for it. I agree, it seems like an extra unnecessary complication. The Speed Sensor FAQ on your site has also been a good read - thanks!

[totorro]

Had the chance today to pull out the instrument cluster and do some investigation. Confirmed that the VSS fitted is an RJES one, not sure what age.



Connected it up with a pull up resistor and with a screwdriver in the back of the speedo I could see it alternating between 0V and 12V on the output cable - so looks like it's all good. Thanks again for the info on how to test it!

Next step is working out what's going on with the wiring, if it's reaching the ECU, and if it's connected to the right pin...