VW Diesel Tachometer

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Introduction:

Ever since I've owned my diesel Caddy, I really wished it had a tachometer. The one option that is available on later model VW diesels is the "W-terminal" alternator. This alternator has an extra terminal (labeled "W") that allows an RPM-proportional signal that can drive a dash-mounted tachometer. It is common on the turbo diesel models and requires a full gauge cluster swap to have the tach replace the stock clock. The "W-terminal" also is used to drive the "upshift" indicator that is apparently used on certain '82 and later models.

Unfortunately, my original '81 alternator is not the "W" type and it is still working fine, so I don't really want to replace it, same with the instrument cluster. The alternator is driven off the engine by a v-belt, so the readings provided by the tachometer are probably not really accurate. In any event it would have to be calibrated in some manner (I guess by a mechanical tach?). So, I decided it was best to design a fully digital tachometer system to avoid all these problems.

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Design:

One of my old engineering projects I worked on at NASA involved using optical shaft encoders to pick up shaft speed and rotation for use in position sensors. These encoders use a series of concentric circles of alternating light and dark marks to detect motion. For speed only, one set of marks is sufficient.

Tachometer pickup wheel

In the case of the VW diesel, the injector pump and cam shaft are driven by a toothed belt off the crankshaft. The crank shaft sprocket is 1/2 the diameter of the rest of the sprockets, so they spin at 1/2 the engine RPM. The crankshaft sprocket is not easy to get to, but the one on the fuel injector pump is accessible by an existing hole in the timing belt cover, at least on my A1/1.6D engine. Since the injector pump sprocket is turning 1/2 speed of the engine, it is necessary to have the pickup wheel make two on-off cycles per revolution.

Pickup wheel on sprocket

I fabricated the pickup wheel from a sheet of stainless steel sheet metal. I sized it to fit inside the sprocket on the fuel injection pump, this made the outer diameter 118mm with a 43mm hole cut out of the center. I masked off two opposite quadrants and spray painted the remainder with flat black enamel paint. The wheel is attached to the sprocket on the fuel injection pump with a flexible adhesive. In the above picture, you can see one black (non-reflective) quadrant just below and to the left of the retaining nut on the injection pump pulley. The two reflective quadrants are visible just under the A/C hose that runs across the middle of the picture.

Photo sensor installation

Conveniently, there is a rubber plug in the timing belt cover at just the right place to allow the photo-sensor to "look at" the bottom of the injector pump sprocket. I trimmed the mounting tabs on the sensor until it was 3-5mm away from the surface of the pickup wheel. I cut the center of the plug and then glued the sensor into the plug. The entire plug/sensor may easily be removed from the timing belt cover if needed.

Tachometer readout

Once the sensor was in place, I created a mounting hole for the digital meter in the dash. To the left is the stock emergency flasher switch, to the right is the fog light switch I added. I may end up tilting the display upwards a bit for better visibility. Here, you can see it registering 779 RPM at idle. The nice thing about this digital tach setup is that it is precise and no calibration is required, the accuracy is built into the design. This tachometer also reads out to the individual RPM, not just 100's like run-of-the-mill digital tachs.

The wiring for this design turned out to be deceptively simple. The photosensor has 4 terminals, and they were different than the Omron data book I had. The book describes them a Anode, K(c)athode, Emitter, and Collector, but the sensor had +,-,L and Out. After some trial and error, I found that the internal LED was connected internally to the + terminal and I had to add a current-limiting resistor between the L and - terminals. I used a 620 ohm resistor which limits the current to about 25mA. Likewise, the Out terminal is an open collector output and by tying it to + through a load resistor (620 ohms) gave me a nice 0-12V swing on the output and again about 25mA of current. I soldered the resistors directly to the terminals on the sensor, then ran 3 wires for +12V, ground and output through the firewall to the panel meter. I spliced into the radio power for +12V and ground (switched w/ ignition) and hooked the power and signal to the meter. I used 1/2 watt resistors for this application, and selected a value to limit the current to above value. From the photosensor data sheet, resistance values up to about 1K ohms should produce acceptable current values, don't go any lower than 600 ohms, though.

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Parts List:

Below are the parts I used in this project. I ordered the tachometer directly from Acculex (see link below) and the photo-microsensor was ordered from Digikey along with a very informative Omron Application Guide (highly recommended). The pickup wheel material was purchased at a local hardware store and the other parts were out of my well stocked parts box (but are readily available):

Acculex DP-680-RATE Rate Meter/Tachometer - $90
Omron EE-SB5 Reflective photomicrosensor  -   5-12
(I used the EE-SB5V but others will work)
Optical pickup wheel, s/s sheet metal     -   5
Misc. wire and resistors                  -   3
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Total                                     - $88-95

If you would be interested in making your own photo-sensor, you may be interested in the following web page:

Basically, you are looking for a device that will turn on and off with either a change in reflectivity or by physically interrupting the optical path with some sort of slotted wheel. Usually, this consists of a light emitting diode (LED) that provides the illumination, and a phototransistor that senses the presence or absence of that illumination, To avoid problems with stray light sources, infrared is preferred over visible light, and the sensitivity or the photo transistor should be closely matched to the output of the LED. LEDs tend to have a rather narrow output wavelength and also beam width (think of them as tiny lasers), It is important to collect and focus the light out of the source, onto the target and then back to the sensor. While it is possible to make your own photo-sensor, I felt for $12 (actually only $8.50 when I bought mine) my time was better spent on the rest of the system than fooling with more discrete components, etc.

A key feature of these optical sensors is the use of infrared LEDs and photo transistors. This both prevents interference from stray external light sources and by having the two components match for a specific frequency of radiation, they are even more immune to interference. Because of this, you won't see any visible light coming from the LED when its operating.

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Theory of Operation:

After designing and installing the digital tachometer described above, and publishing this project on the world wide web, I had a few readers ask about applying this design to an analog tachometer. In order to see what changes are required for an analog tachometer, it is necessary to understand a bit about how they work and compare that to how the digital tachometer (or more correctly rate meter) that I already had works.

Any rate meter simply measures the rate at which some event occurs. Usually this is done by counting the events (contact closures, electrical pulses, etc.) for a given period of time (known as the integration interval) and then simply dividing the number of events by the time to get a rate. In my case, I had a rate of 1 pulse per engine revolution and by using a rate meter calibrated in events per minute, I could display revolutions per minute.

In a spark ignition engine, tachometers typically use a somewhat different mechanism to obtain engine speed. Since the ignition is firing the spark plugs, there is a handy signal source present off of the ignition coil primary (in older vehicles at least) that is in the form of a square wave (0-12V). If you know how many cylinders and assuming a 4-stroke engine, which takes two revolutions to complete a full cycle, you can calculate the relationship of ignition pulses to revolutions as follows:

PulsesPerRevolution = NumberOfCylinders / 2

After market tachometers are often equipped with a selector switch on the back to choose 4-6-8 cylinder operation. This simply scales the input based upon the above formula. So, in a typical 4-cylinder VW gasoline engine, you'll have 2 ignition pulses per revolution. So the first modification to my digital tachometer circuit is to up the input pulse rate from 1 to 2 (or more) pulses per revolution.

Unlike the digital rate meter described above, the analog tachometer is usually built up out of an electro-mechanical current meter and some sort of input conditioning circuitry. Instead of integrating the input signal over discrete time intervals, the mass of the meter movement itself is used to continuously integrate the input pulses. Thus, if the meter takes 10mA to swing full scale, if you were to switch the current between 0 and 10mA at a fast enough rate, the meter needle would read out the average of the on and off cycles. In order for this to work, it is necessary to condition the input signal to provide a repeatable output signal given an possibly varying input signal.

The input circuity is used to send calibrated pulses to the meter, usually done with what is known as a one-shot timer. For purposes of discussion, assume the one-shot timer send out a 1mS wide pulse for every input pulse (of varying duration) and the current is calibrated to the 10mA needed by the meter. If the input pulses come at the rate of 10/second (or every 100 mS), the meter will "see" an average current of 1/100 * 10mA or 1% of full scale. If the input speed is upped to 200/second (5mS), then the meter will "see" 1/5 * 10mA or 20% of full scale. Upping the input frequency to 1000/sec will give a full 100% reading on the meter.

However, I'm not very interested in designing a tachometer from scratch (been there, done that, bought the T-shirt) I just want design a circuit to drive one. One subtle and not so obvious difference between a digital rate meter and an analog tachometer is in the input. Since the tachometer is designed to be driven off of the ignition primary of the engine, it has a very strong input signal (i.e. low impedance) and the load of the tachometer on the ignition is trivial. A digital rate meter on the other hand is most likely designed to operate with high impedance signals and thus probably has a suitably high input impedance (10 MOhms is typical). So, simply hooking up the output of a photo sensor to any run of the mill automotive tachometer may not be successful.

In my case, I was originally using an internally amplified photo sensor and while it easily drove my digital rate meter, it had some trouble with the large tuneup tachometer I had at home. The tach would invariable read about 2/3 the value of the digital meter. It turns out the relatively low input impedance of the analog tach was dragging down the output of the sensor, which caused the RPM reading to fall. While I was only operating the photo sensor at about 1/2 its rated current and probably could have pushed it a bit harder, I felt that design was too marginal for comfort, possibly shortening the life of the component in the elevated temperatures under the hood.

So, I set about making more extensive modifications to the design as described below:


Analog Tachometer Design Modifications:

So, while a digital tachometer is really neat, there are a few drawbacks:

  1. Digital readouts are harder to interpret quickly while driving, i.e. finding exact shift points
  2. This readout module is not back-lit, making it invisible in the dark, I could not find a good way to backlight the display.
  3. It is a bit pricey, over 80% of the cost of this project

Anyway, to accommodate an analog tachometer, designed to operate off a 4-cylinder gasoline engine ignition system, a very simple change is required. By simply laying out 4 dark and 4 light segments (instead of 2) you'll make a wheel that produces 2 on-off cycles per revolution, exactly duplicating a 4-cylinder spark ignition engine. That's the easy part.

2-pulse/rev. pickup wheel

If you are lucky, and have a good high impedance analog tachometer, you might be able to construct a circuit as shown below and have it work. Note the use of the amplified EE-SB5V photo sensor. Dropping the values of the two resistors to about 500 ohms would push the input and output currents up and hopefully drive a decent tachometer.

Here's a typical schematic of how such an EE-SB5V setup might look.

This was drawn by Job Oberio

For a more universal solution, I decided to take a slightly different approach. Since the added cost of the internal amplifier in the EE-SB5V was both substantial and inadequate for this application, I instead chose the lower cost EE-SB5 (non-amplified sensor). Below is a sketch of what the revised circuit looks like:

Analog tachometer schematic

(click to download a larger version)

So, the solution was to add a second stage of current gain similar to the EE-SB5V, but at less cost and without adding it in the sensor itself. So, I returned to another old design solution and the venerable 555 timer. You'll notice it is the same chip used in the previous analog tachometer circuit, but in this case I have it wired up as a Schmitt trigger to both clean and boost the signal up into the 100mA range.

Adding this additional circuitry requires the addition of a small electronics box to the system, but this box will provide a nice place to connect things up at. The bipolar Schmitt trigger can drive about 200mA loads and has no trouble with the large shop tachometer I'm testing with. The signal is a nice clean square wave and seems to work at both the lower and upper RPM ranges. And this circuit allows an un-altered analog tachometer off of a gasoline (spark ignition) engine to work off of a diesel engine.


Other Applications (i.e. electric vehicles):

A number of these tachometer drive kits have been successfully adapted to work on other than VW diesel vehicles. Among those applications have been on vehicles converted to electiric operation and to marine and stationary diesel engine applications. All those applications are similar in that there is no ignition type signal available to drive the tachometer. As far as the tachometer signal generator is concerned, it does not care what sort of moving equipment it is connected to. If it rotates, it can generate a signal proporional to the RPM and feed that to drive an analog tachometer. In these applications, a custom pickup wheel and sensor mounting bracket would of course need to be fabricated by the end user.


Tachometer Kits:

Since there is enough interest in this circuit, I am offering this as a simple bolt-on kit (actually no bolts would even be required).

Kit options include:

Order a parts kit for US delivery Order a parts kit for International delivery
- Include phone number in "Note To Seller" field for customs form

Order a parts kit for US delivery Order a parts kit for International delivery
- Include phone number in "Note To Seller" field for customs form

Here are some options for tachometers:

Various sizes are available:

One problem with replacing the clock with a tachometer is that you lose the clock.

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Tachometer Kit Installation Instructions:

(1)Pickup wheel on injector sprocket

(2a)Measure the cover to wheel distance

(2b)Measure the photo sensor projection

(2c)Photo sensor installed in cover

(3)Wiring connected to sensor

(4)Power and ground wire connections

(5) Electronics module mounting

After a few years of use, I found a totally unexpected benefit of this tachometer design. I decided to install a cruise control on my VW diesel and guess what, it needs an engine RPM signal to operate - BINGO!

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OEM Tachometer Installation:

If you currently have the two gauge cluster with the analog clock and you are planning to use an in-dash tachometer in your VW diesel, you have a few options:

So, your first step is to locate a suitable tachometer to use for the conversion. It is best to obtain an entire instrument cluster if possible. They can often be found on eBay. For an A1/Mk1 VW, use one from from an '84 GTi model. For other model VWs, you would want to use one from a similar model to ensure proper fitment in the in-dash instrument cluster and for other makes of vehicles, find a compatible model vehicle to obtain a tachometer from. Other applications are for vehicles which have been converted from gasoline to diesel engines and even for vehicles converted to for example electric drive.

Disassemble the cluster by removing the lamps (save them for spares) and then the flexible circuit board. Remove the tachometer and the fuel and water temp gauges that go with it. One advantage of the getting the entire cluster is that you get a new housing without the hole drilled to access the analog clock stem. Another advantage is that you are getting a whole boat load of spare parts, light bulbs, odometer gears, etc.

Here's a good writeup on removing the instrument cluster if you need some pointers:
//www.4130-products.com/step/remove/
I do mine a bit different:
- I reach up underneath and unscrew the speedo cable (assuming you have a screw-on connection)
- Don't forget the pull-out headlight switch knob, there is a button underneath the switch to release the knob and shaft (I like to insert the shaft back in and gently push the switch to the off position w/o pushing the shaft back into the switch)
- Then if the cluster won't come out easily, some trimming of the back opening of the dash cutout can make it much easier. My '82 came out very easy, but my '81 had a lot of flashing left from the molding process and I had to trim the opening a fair amount to get it out (none of the trimming is visible with the cluster in place).

In any event, you'll probably want to use your old speedometer, the old wiring and replace the clock side of things with the tachometer. The fuel and water temp gauges may be located differently, but should still work. The only modification needed is to carefully trim back the "tachometer" "W-terminal" connection from the tach, tape it off then connect a separate wire to the terminal on the tach which in turn gets connected to the tachometer pickup signal.

Shown below is a VW A1 diesel instrument cluster. Other models may look different but the basic idea is to pull the cluster out, remove the clock and install the tachometer in its place. As far as the wiring changes, compare the clock and tachometer wiring and identify the power and ground connections on both. Then the extra wire that the tachometer has is the one that you replace with the output of the tachometer signal generator. Now on to the A1 details...

(1)Stock cluster removed

(2)Clock module removed

(3)Diesel bezel top, gas bexzel bottom

(4)Transferring indicator light legend

(5)Install tach, add new RPM connection

(6)Installation of the new cluster

(7)Its TACHO time!

NOTE:

You can follow the above proceedure to swap a tachometer for a clock in a gas engined VW as well. Just ignore all the wiring changes I had to do for the diesel conversion. Also if you are swapping a diesel tach into a diesel cluster, then again, you don't have to make any wiring changes, either, since the W-terminal RPM signal is already present at the tachometer. Its only when swapping a gas tach into a diesel that you need to connect the RPM signal generator output to the tachometer input.
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Analog Parts List:

Below are the parts I used in this project. I ordered the tachometer directly from Acculex (see link below) and the photo-microsensor was ordered from Digikey along with a very informative Omron Application Guide (highly recommended). The pickup wheel material was purchased at a local hardware store and the other parts were out of my well stocked parts box (but are readily available):

Omron EE-SB5 Reflective photomicrosensor
Optical pickup wheel, s/s sheet metal
Connectors, wire, project box, circuit board and components:
  1 x 620-750ohm resistor (this means any resistor between 620 and 750ohms will work)
  1 x 33Kohm resistor
  2 x 100Kohm resistors
  2 x 0.01uF capacitors
  555 Bipolar timer
  May also include a 2nd 620-750 ohm resistor and LED if requested
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Parts Kit Information:

First check the parts kit contents (see above) and familiarize yourself with the parts. If not familiar with resistor and capacitor codes, refer to the following for information:

Some of the components may vary slightly from the schematic, for example the 620 ohm resistors are non-critical and anything up to about 750 ohms will work fine. Your kit should include:

Parts Kit Assembly:

If you purchased the parts kit, you'll need to assemble and test the electronics module.

Analog tachometer schematic

Follow the circuit schematic above for connections and also you can use the parts layout information that is included with the parts kit. You may notice some minor changes between the schematic and the parts layout diagram and photo below. The schematic was drawn up based on laying parts out so they look good on the schematic. The parts layout and photo below are done in a way that makes the wiring and soldering easier with the PCB that is supplied with the parts kit. SO if you want to follolw the schematic to a "tee", feel free to do so. If you want to follow the parts diagram and photo, feel free to do so. There are notes in the instructions below mentioning where differences may be found:

Component Layout

Parts Kit Troubleshooting:

Once everything is connected, its time to test the circuit.

If the LED blinks with an input signal, you are set, it works!

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[Last updated: 26.March.2021]