Subaru VSS to Alpha-Pro

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Jim White
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Joined: Wed Aug 16, 2017 5:59 pm

Subaru VSS to Alpha-Pro

Post by Jim White » Fri Sep 29, 2017 3:53 pm

I connected my "new" 2013 Subaru Outback SUV's VSS signal to my (ribbon cable) Alpha-Pro odometer box and all worked fine at slow speeds but at higher speeds the signal is apparently way too fast for the Alpha-Pro. I've looked at some of the taxi meter connectors and cruise control connectors but the only one I tried (supposedly universal) that was supposed to divide "any" VSS signal by 4 either didn't even detect the Subaru VSS or didn't provide a signal the Alpha-Pro could use. I suspect the supposedly universal VSS device wasn't detecting the Subaru signal because (IIRC from somewhere) the newer Subaru VSS signal varies both in voltage and frequency. I've not verified that yet but will if I don't find some ready-made or other simple solution.

How are other folks getting odometer pulses on the "newer" Subaru's? There is no info on it that I could find at the Small Systems site.

Thanks
Jim White

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John Fishbeck
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Re: Subaru VSS to Alpha-Pro

Post by John Fishbeck » Sun Oct 15, 2017 4:28 pm

Jim,

I think you may be out of luck on this one. Are you certain you have an analog VSS signal? Subarus, from 2008 on, and like most other manufacturers, has transitioned to a CANBUS electrical system. For most cars there is no analog pulse VSS signal anymore (and I do know what you're talking about, as I use the VSS signal on my 2006, pre-CANBU Impreza 2.5i to drive my Timewise 798A. Works like a champ). Instead of analog signaling, devices communicate via the transmission of data packets over a dedicated bus network system. Analog VSS is no more in many cases.

That said, perhaps I'm mistaken regarding your 2013 Outback. Perhaps your Outback has a dedicated, separate VSS signal feed? But I suggest you do a bit of research here - the reason things aren't working for you may be because of this issue.

There are CANBUS-to-VSS signaling adapters out there, but they are manufacturer specific. Have no idea of availability for your application. You'll need to Google.

Other solution is the old school magnets and sensor.

Good luck!

Regards,
John
John Fishbeck

wgl
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Re: Subaru VSS to Alpha-Pro

Post by wgl » Tue Oct 17, 2017 10:48 pm

I have experimented with universal diagnostic port interfaces that extract VSS signals from CAN bus (and some earlier ISO) data streams to operate Timewise and ALFA (and presumably other) rally odometers.

On most cars the OBD-II signals give good enough results for such activities as competing in, or running course opening on a stage rally, or competing in a trapped TSD rally. However, there is just enough dithering (compared to a directly pirated VSS signal, or signal from a dedicated wheel sensor) that they are not yet competitive at the very highest level of straightforward TSD rallying.

The convenience of these interfaces is tantalizing. The OBD-II port contains both a full time battery +12 VDC feed, and a chassis ground. This makes it a simple "plug and play" option to operate a rally odometer in nearly any car or light truck.

There are reasonably priced commercially produced VSS signal conditioning/frequency divider modules. The ones I use accept a variety of input wave forms. They output a stable 5 volt "square" wave. Frequency divide ratios of 1:1, 2:1, 4:1, 8:1, and 16:1, as well as input sensitivity are selected by moving circuit board "jumpers".

If you have access to service literature for your car you may be able to determine the location and type of speed sensor, as well as possible tap-in points. This is not always a simple exercise.

Good luck,

Greg Lester

Jim White
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Re: Subaru VSS to Alpha-Pro

Post by Jim White » Wed Oct 18, 2017 9:06 am

Thanks guys. More input welcome. According to the Subaru Outback documentation I've been able to see, in gear and running at idle the VSS signal below the instrument cluster should be outputting a +5 V square wave of about 560 pulses per second. That very much seems to square with what I see the Alpha-Pro handling as the signal. And up to maybe 20 MPH the Alpha-Pro seems to be reasonable though I've not been scientific about checking it since all bets are off at higher speeds.

The other gotcha I've seen with the VSS signal is that it ticks 1 to 3 times when the gearshift lever is moved. Why the variance I don't know but most of the time it's only 1 tick. Since my car has the CVT that's not much of an issue since shifting is only needed for changes in direction and, in part, that can be handled by making sure the Alpha-Pro forward/reverse is always flipped before shifting the car for forward or reverse.

I've looked at the "universal" OBD-II to VSS ads and contemplated purchasing one but at $200 and from my study of the OBD signals my expectation would be that, as Greg notes, "dithering" would be an unavoidable problem that likely would be ignorable for taxi's but not so much for rallying. Unfortunately I also tried what was purported to be a universal divide by 4 VSS signal divider though when I got to the retailer to pick it up I was informed that they'd by then decided that "universal" really meant Chrysler. In any event it was duly hooked up and passed no signal whatsoever to the Alpha-Pro so the retailer took it back and gave me my money back. Luckily I also had an old EZ-Pulse mounted on a plastic wheel cover and was able to cut a disk from that wheel cover and with judicious holes and rubber padded spacing blocks wire tied it to the 17" aluminum wheels and that ran POR with no issues whatsoever.

I've not yet gotten around to viewing the signal on an oscilloscope but I hope to soon. One of my local maker space friends has an Nscope (nscope.org) which allows making a PC display an oscilloscope signal so I intend to get that going and give it a try. The maker space also has other portable oscilloscopes though no 12 volt ones specifically for auto use but I expect an inverter will handle that if the Nscope doesn't work out.

I'll keep you posted on what I find and finally work out.
Jim White

Jim White
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Re: Subaru VSS to Alpha-Pro

Post by Jim White » Wed Oct 18, 2017 12:27 pm

nScope seemed to work a treat! I've attached an image. Bummer though, unless I've got the wrong signal wire, which I will double check (pin 6 on connector B54), the signal I've attached to is clearly not a "VSS" signal and certainly not a square wave or +5V. While it is rock steady at a constant speed on level ground it actually turns out to be engine speed dependent but only causes a trace when the vehicle is in motion (or the shift lever moves and induces a bunch of apparent noise often well in excess of the 0.5 V normal signal range).

I'll study some more but it may be that mounting magnets (or an EZ-Pulse) are the only solutions (or return to older Subaru models).
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Jim White

Jim White
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Re: Subaru VSS to Alpha-Pro

Post by Jim White » Wed Oct 18, 2017 5:06 pm

It looks like the Subaru Secondary Speed Sensor is closely tied to vehicle speed while the Primary one isn't so much. I've attached an image to this response that shows the nScope result at about 35 MPH. Not exactly the square wave that the Subaru documentation shows but close enough. Unfortunately, as I'll attach to a subsequent message, at lower speeds the voltage drops and its range changes to include going negative and maybe even with less total range. More playing with that tonight after dark when traffic is lighter and I can see my computer screen without the sunshine washing it out.

Yes, I just borrowed the nScope last night and I've decided that I like it a lot. Super when you stay within the +/-5 V range. I can also vouch for its shutting off and requiring a reset at over voltages too since the noise from one gear shift lever move shut it right off!
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About 35 MPH.
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Jim White

Jim White
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Re: Subaru VSS to Alpha-Pro

Post by Jim White » Wed Oct 18, 2017 5:12 pm

About 7 MPH (see prior message for discussion). Dropping into negative volts and decreasing the signal amplitude adds to the difficulty of reprocessing the signal so it's suitable for the Alpha-Pro. (So the solution may be magnets.)
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About 7 MPH.
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Jim White

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TeamHarco
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Re: Subaru VSS to Alpha-Pro

Post by TeamHarco » Thu Oct 19, 2017 2:02 pm

Not to be a jerk, though I play one on TV...

What is the objection to placing magnets on an axle shaft and mounting a sender to read the pulses?

This could have been accomplished in far less time, and bother, than you have gone through in your search for a usable VSS signal from the CAN bus.

I'm sure the convenience, should it actually have worked out, would be the reward. But having a secondary input is always a good thing. You could have had the reliable and simple input as mentioned above, while working out the details of the VSS hook-up. Just my 0.02.
Scott
Team Harco Motorsports
"Win on Sunday, Sleep on Monday"

Jim White
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Re: Subaru VSS to Alpha-Pro

Post by Jim White » Fri Oct 20, 2017 10:10 am

TeamHarco wrote:Not to be a jerk, though I play one on TV...
Scott, what TV show? I'd just like to check and see if you act it any better than you do it in person:-)

A) I don't happen to have the magnets or a bracket or ... on hand (and I'd have to get my hands dirty).
B) I enjoy playing with electronics.
C) The ease of clipping a connector onto a VSS wire for the last 3 or 4 Subaru's and having the odometer just work has been a real treat.
D) I already have a functional EZ-Pulse that I can finagle onto a wheel to provide odo pulses should I not get direct car electronics working.
E) I know a rallyiest who had magnets (maybe not quite well mounted?) who had rallied for a couple years with them before rounding a tight corner at speed and encountering a surprise pothole that neatly got just the right flex to strip all 4 magnets off.
F) An ODB plug in would be a real boon (and suit taxi, nav systems, highway maintenance vehicles, occasional cop cars, etc., witness all the after-market devices). But I don't believe ODB or CAN have a suitable signal (bummer).
G) I've had "factory trained" mechanics modify prior device installations where undoing the nuts and swapping the (mis)reinstalled parts after a repair was deemed too difficult so they simply took a hacksaw to the long side to gain the clearance needed.

But, yes, magnets are still an option.
Jim White

wgl
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Re: Subaru VSS to Alpha-Pro

Post by wgl » Sat Oct 21, 2017 9:47 am

If you don't want to mount magnets I can supply probes that sense ferrous "targets" (wheel studs, bolt heads, etc) directly. Slightly more expensive than the probes that I make (that require you to mount permanent magnets), but potentially more reliable.

Greg Lester

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