Earlier rear engined VW models (Beetles, split screen / T1 buses and bay window / T2 / T2a / T2b buses didn’t have rev counters (also known as tachometers or tachos) as standard. They were available as an option in some models, and became standard in some higher spec, later model T25 / T3 / Vanaon models. All of the VW rev counters in these models were electronic, made by VDO, and driven from the coil negative terminal (petrol / gasoline models), or the alternator ‘W’ terminal in diesel models.
How all the types of rev counter and rev counter signal involved in Subaru powered VW conversions work:
All designs of rev counters (also known as a tachos or tachometers) used in rear engined VW models were electronic – i.e. they never used mechanical rev counters as some other makes. They were driven by either:
1. The coil negative terminal , where the points connect. This was used in petrol / gasoline models.
2. A dedicated terminal on the alternator (usually marked ‘W’). This was always used on VW diesels (some other makes also used the alternator W terminal to drive the rev counter on petrol models too, but not VW).
……….as these were the only sources of electrical engine speed information.
Electronic rev counter technology of the time (typically discrete analogue, or in VDO’s case, very early bespoke IC’s (from about 1968 onwards) meant they only worked with the one type of signal that they were intended for. I.e. you couldn’t swap a diesel rev counter for a petrol one or vice versa. This includes the types used all rear engined VW models as standard, and many available as aftermarket accessory rev counters until around the mid 1990’s. From the mid 1990’s onwards (so aftermarket only for rear engined VW’s, or at least the models not built in South Africa of South America), newer designs became more flexible in terms of what input signals they could accept. Some had multiple input input connections, so you had to select the right one, but better designs (typically later ones, using newer technology) could accept multiple input signal types on the same input – automatic filtering out the any unnecessary / harmful parts of the final, leaving just what is needed.
Rev counter signal types:
- The signal from the coil negative terminal on all rear engined VW petrol models is a 12V square wave with a massive superimposed spike each time the points close. This spike is typically many hundreds of volts.
- The signal from the alternator ‘W’ terminal on the diesel model is a sine wave of variable frequency and voltage.
In both cases the frequency is the important part of the signal in terms of what the rev counter needs to know, as it represents engine speed. With the alternator W signal terminal, the other information the signal includes (the voltage of the signal increasing as rpm increases, and the fact that the signal is an AC sine wave is filtered out or just ignored by the signal conditioning circuitry in the rev counter, which is typically only looking for each time the AC voltage crosses zero – i.e. using a zero crossing detector circuit to isolate the frequency information. With the coil negative terminal signal in petrols, there are two approaches. In both, the high voltage spike has to be removed, so it does not damage the other circuit components. With one method, the whole signal is scaled down. This reduces the 12V square wave part of the signal to being insignificant, and measures the frequency of the scaled down voltage spike to drive the rev counter. This results in a circuit which cannot be driven by a 12V square wave, as is the case with the VW petrol rev counters. The second method removes the high voltage spike (typically by using TVS diodes, etc), and using the resulting ‘clean’ 12V square wave to drive the rev counter, so typically works from other 12V square wave sources such as dedicated ECU outputs.
On pretty much all engine management systems prior to the introduction of network systems such as CAN bus, both petrol and electronically managed diesel, the rev counter was typically driven directly by a dedicated output from the ECU. This covers all of the pre CAN bus Subaru models. With this system the signal is a clean 12V square wave (i.e. no superimposed spike of hundreds of volts). This is not compatible with of the rev counter types used in rear engined VW’s or the aftermarket VDO models which were clamped into the panel with two nuts and a U shaped bracket, as they scale the whole signal down so require the presence of the high voltage spike on the signal.
Later VDO aftermarket models (from around the mid 1990’s onwards) were clamped into the panel with a large plastic nut which screws onto a thread around the rear case outer diameter are more versatile in terms of what input signals they accept. They typically work from a 12V square wave, a 12V square wave with superimposed high voltage spikes or an alternator W terminal.
What rev counter should I buy to fit into my Subaru powered VW?
If there is an original VW rev counter for your model, it will most likely look better, maybe fit better, and maybe be better quality than many of the aftermarket options. If it was originally intended for a petrol application, it won’t work from with the signal from a Subaru ECU, but this can very easily be solved by fitting one of our rev counter interfaces between the ECU and rev counter (no modifications ot the rev counter of the ECU required).
If there is no original VW rev counter for your application, or they’re too expensive, or if you just prefer an aftermarket one, you should used one intended for a petrol application. You have two choices of how to drive it from the dedicated Subaru ECU signal. For the installation to be as simple as possible, make sure the model you choose is compatible with a 12V square wave signal. This should be clear in it’s documentation (but often isn’t). In general, this excludes many older designs from VDO and others makes from up to the mid 1990’s. Many later designs from the early 2000’s onwards will work from many different signal specs including a 12V square wave.
See the FAQ below for a lot more detailed information on rev counters and using them in Subaru powered VW conversions:
For a couple of reasons:
Firstly, all Subaru engines widely used in VW conversions use multiple coils. Typically these are in a ‘wasted spark’ arrangement, where each coil fires 2 cylinder simultaneously, but some models are ‘coil on plug’ (many turbos and the EZ30 flat six), where there is one coil per cylinder. With both of these systems there is no one signal that represents engine speed. There are multiple signals, all running at a factor of engine speed (i.e. the engine speed divided by the number of coils).
Secondly, although the multiple signals mentioned above are accessible with some Subaru Phase I ‘wasted spark’ EJ series engines, and can be combined, they are not a accessible on others (Phase II ‘wasted spark, and all ‘coil on plug’).
This means there is not one approach which enables the signals to be combined to give the high voltage engine speed signal that the VW petrol rev counters require across all the engines. Also, the more coils you have, the more of a mess having to add diodes to combine their signals together becomes, as there is no particularly neat way to do it.
Many years ago we used to combine the coil signals using diodes on the Phase I ‘wasted spark’ models, but this was never very satisfactory, as one method which suits all of the engines is much more desirable.
In theory, nothing, as long as the circuit modification has been well designed. However, in practice, the circuit is not accessible without removing the rev counter from the dash, and doing so means the very big risk of disturbing the very fragile, and long obsolete blue flexible blue circuit board.
For a while many years ago we used to bypass the VW rev counter driver circuit via a method which is reversible (in case anyone wants to re-use it later), replacing all of it’s functionality with a more modern circuit. While this worked fine, it also meant disturbing the dreaded flexible blue circuit board, which we do not consider worth the risk. Hence switching to making rev counter interfaces, which are a ‘plug and play’ solution.
No. The VW rev counter is designed to run from a variable frequecy / variable voltage sine wave, from the diesel alternator W terminal. We could make a rev counter interface which gives out the this signal, however there would still be two problems. Firstly, diesel rev counters driven by the alternator W terminal are calibrated to measure alternator speed, but display engine speed (they pretty much never the same due to the pulley drive ratio). Therefore if you just feed one with a signal at engine speed, it will not read anything like right. The rev counter has easily adjusted calibration, but setting it is not easy, especially if you don’t have an accurate method of displaying the actual engine speed. Secondly, the face of a diesel rev counter shows a far lower rev range than a petrol, making it unsuitable for a far higher revving Subaru engine, and non ideal for even a low revving VW flat 4 or probably some more modern diesels. Combine these two, and it isn’t really worth making a rev counter interface for the diesel rev counters.
Despite the display only showing a maximum of 6000 rpm, the gauge mechanism continues to move linearly to about where 7000 rpm would be if the gauge face went that high. Yes, this means you go ‘off the scale’, and the needle goes over the fuel gauge, but it is easy to still read what the gauge would be displaying if the markings continued past 6000 rpm.
The above is obviously not ideal. But it means that if you have an instrument cluster with the increasingly sought after (and therefore valuable) petrol rev counter, in which everything works, you can use it without resorting to the very big risk of disturbing the very fragile, and long obsolete blue flexible circuit board.
If you want a perfectly factory fitted looking 7000 rpm display rev counter, then you need to get one from the right spec of MkII Golf, Rabbit or Jetta, and accept the risk of disturbing the dreaded blue circuit board to install it. Finding the right one is not straight forward though, mainly because for some reason VW dual sourced the instrument clusters for the Mk II Golf / Rabbit / Jetta from both VDO and MotoMeter. Although I believe they are interchangeable in their intended applications as a whole instrument cluster assembly, the components within them, such as the rev counter mechanism, are not. VDO also used more than one design. Only the rev counter from one type of VDO Mk II instrument cluster will fit. It is the one with curved temperature and fuel gauges, positioned identically to those in the T25 / T3 / Vanagon rev counter dashes. Here you can see that the gauges are all identical apart from the rev counter facia graphics:
Golf-Rabbit-Jetta VDO 7k rpm T25-T3-Vanagon 6k rpm
The rev counter mechanism, face and temp and fuel gauges are a direct fit into the T25 / T3 / Vanagon dash, but the plastic moulding which holds them in place is not interchangeable. You need the bus one. These were available until recently, but may now be obsolete. The housings can be seen below, with the white one being Golf / Rabbit /Jetta (VDO part numbers 88 471 307 (upper) and 88 47 250 (lower)), and the black one being T25 / T3 / Vanagon (VDO part numbers 88 471309 (upper) and 88 471310 (lower)). Only the parts where the gauges fit are the same – every other detail is incompatible:
Golf-Rabbit-Jetta (white) T25-T3-Vanagon (black), rear
Golf-Rabbit-Jetta (white) T25-T3-Vanagon (black), front
In theory, yes, but to do it properly, you need to change the dreaded (incredibly fragile) blue flexible circuit board for the right one (you cannot use the flexible circuit board from a model with a clock or a blank in place of the rev counter an any kind of acceptably neat way). They are all obsolete unless VW Classic Parts have persuaded VDO to reproduce them recently. Presumably they no longer have the tooling, as some of the Golf ones are available again, yet the bus ones have been obsolete for many years. You also need the two interlocking plastic mouldings which mount the rev counter, fuel and temperature gauge mechanisms into the bus instrument cluster. These were available until recently, but may now be obsolete. The housings can be seen below, with the white one being Golf / Rabbit /Jetta (VDO part numbers 88 471 307 (upper) and 88 47 250 (lower)), and the black one being T25 / T3 / Vanagon (VDO part numbers 88 471309 (upper) and 88 471310 (lower)). Only the parts where the gauges fit are the same – every other detail is incompatible:
- Golf-Rabbit-Jetta (white) T25-T3-Vanagon (black), front
- Golf-Rabbit-Jetta (white) T25-T3-Vanagon (black), rear
Here you can see the basis spec version of this part for T25 / T3 / Vanagon models with no click or rev counter, and why it doesn’t lend itself to fitting a VW rev counter:
T25-T3-Vanagon, no rev counter or clock
If you know that the signal you have is a clean 12V square wave, (e.g. a dedicated output from an ECU), try it*. It shouldn’t damage a rev counter designed to work from either the coil negative terminal or an alternator W terminal, but may well not drive them either. Typically you’ll see the needle twitch very slightly when turning the ignition on, but it won’t move in response to engine revs.
* At you own risk! If you chose to do so and kill a rev counter or ECU which you do not have the correct documentation to understand how to connect correctly, that’s your choice.
Yes, if the ignition / management system you’re using gives out a clean 12V square wave (i.e. no high voltage spike) like an engine management ECU, but your rev counter will only work with the high voltage spike. Note the voltage of the spike is limited to 400V, as that is plenty for the intended application, and prevents the voltage potentially being far higher than it would be from a coil and points.
You will do if the signal is from the coil negative terminal or from rev counter interface. 400V (our rev counter interfaces) or up to about 1000V (coil negative terminal, especially with electronic ignition) are present there for an incredibly short time (typically around 1ms) every time a cylinder fires. This is 100% normal, and the VW petrol rev counters can’t work without it. It’s very low energy – not remotely like the powerful shock from the high tension side of an ignition system (i.e. the HT leads – especially on a Subaru rather than a VW engine), but you still know if you touch it.