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Vehicle Speed Signal (info)

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Site last updated on 15/08/08

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   There is a lot of information on vehicle speed sensors, so the best way to make it navigatable seems to be an ‘FAQ’ format (see below).
   Seen another or thinking of making a VSS? Be aware that the Subaru VSS gives out a particular waveform, and there are products out there which give a significantly different shaped waveform. As different ECU’s react quite differently to not having a VSS, and there can be multiple symptoms, it is a very good idea to use a VSS which outputs the correct waveform if you want to know it will work, as both of the RJES ones do.

 Please click the links below for more information:

1. What is a vehicle speed sensor?

2. Do I need a VSS on my Subaru powered VW?

3. What is the best way to generate a VSS for my VW conversion?

5. I’ve built my conversion without a VSS. How do I tell whether I need one?

6. Using the Subaru speedometer in your conversion?












What is a vehicle speed sensor?

   A vehicle speed sensor provides a signal to the vehicles electronic systems which is proportional to the road speed. Most modern cars have a VSS, and it’s signal is used by many different systems, including the speedometer, cruise control, automatic transmission controls, speed sensitive power steering, and engine management. When a VSS is used to drive the speedometer, it replaces the speedometer cable.
   The signals are generated in many different ways on different cars, but usually take the form of a square wave with a frequency proportional to speed. Subaru’s use a special transducer which screws into the fitting in the transaxle where the speedo cable used to go on earlier models. All MY90 --> Subaru’s have a VSS, even those with a speedo cable (these have a VSS built in to the insturmentation).
   Note that the acronym ‘VSS’ seems to be used for both Vehicle Speed Signal and Vehicle Speed Sensor.


Do I need a VSS on my Subaru powered VW?

   Some Subaru engine management ECU’s run quite happily with no VSS. There does not seen to be a definitive rule with which you can determine whether a Subaru ECU will require a VSS signal in order to run the engine correctly, other that ‘try it and see’. The later the engine, the more likely it is to need one. Usually pre MY95 engines don’t need one, and post MY99 ones do. In between it’s very difficult to tell. Therefore, it may be best to assume that you do not need one, and then fit one if you later find you do need one if your engine is MY95 - MY99. Please see for a list of ECU part numbers for which it is known whether they do or do not need a VSS.
  If you are installing Subaru cruise control on your VW you must have a VSS.



What is the best way to generate a VSS for my VW conversion?

   Our VSS units for Subaru ECU’s fit at the speedometer. Speedometer mounted options less prone to damage than CV joint mounted options, and are easier to install. We have two types available - one for all pre MY82 VW’s, and one for MY82 - MY90/92 T25’s. The former is a small unit which fits between the speedometer and cable, and the latter is solid state sensor which fits in the back of the speedometer, in the VW cruise control sensor location. See our Speed Sensors page for details.



I’ve built my conversion without a VSS. How do I tell whether I need one?

  A Subaru engine installation which needs a VSS signal, but does not have one, may show one, or a number of the following symptoms:

  • A rev limit much lower than it should be (typically 4400 rpm). If your conversion has this symptom, you almost certainly need a VSS in order to get the full rev range (typically 6250 rpm plus). The ECU thinks the engine is revving away in neutral, so limits the revs. The engine will run, and the VW will drive fairly normally with the lower rev limit, but you’ll notice when you have the full rev limit - it makes a huge difference.
  • The engine will shut off if you go from travelling at speed with a constant throttle position to idle suddenly (such as pulling up at a junction after driving down the motorway). It is not clear why this happens, but a VSS fixes it.
  • The engine may cut out when travelling at constant (high) speed if the throttle is not moved for more than about six minutes.



Using the Subaru speedometer in your conversion?

   If you want to install the Subaru speedometer in your conversion, then you will need the signal to exactly match the one from the donor Subaru. This presents an extra problem for VW conversions. Tyre size affects the signal accuracy, and your VW is unlikely to have exactly the same sized tyres as the Subaru from which your engine came. As any kind of sensor you can make easily will only give you a whole number of pulses per revolution ie. if you calculated that you needed 4.173 pulses per rev to correct your speedo reading, you can only make a sensor which gives 4 or 5. Subaru adjust the signal in the ECU software to account for different tyres on different models, but this approach is not available without being able to reprogram the ECU.

   The solution to this problem is to use an aftermarket VSS signal calibrator. This is an electronic device which effectively allows you to multiply the VSS signal frequency by a factor which you can adjust. They are intended to allow you to correct the reading on a vehicle with an electronic speedometer when you have changed the tyre size or final drive ratio (if the VSS is read from the gearbox). This is effectively the same situation you are in with a Subaru powered VW, except that you have swapped the rest of the vehicle as well as the tyres!
    The VSS calibrator we sell (to order only, as there is not a very big demand for them) is proven on Subaru powred VW’s, and is easily installed and set, ideally against a sat nav unit with a speed reading.

 Inventors of the Subaru - VW conversion bell housing using Subaru flywheel, clutch and starter

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