Other European Tachometers
Vintage European Tachometers of the 1980s
This page is for European Tachometers such as the Veglia-Borletti tachometer out of a 1987 Lamborghini Jalpa car.
This page will be updated should I get a chance to work on any other similar vintage European tachometers in the future.
At this point this information is based on a Jaeger branded tachometer from a 1987 Lamborghini Jalpa, although the tachometer has a February of 1989 date code on the back
Product: Lambo Tach Signal Inverter
Positive to Negative Tachometer Signal Adapter
Some European vehicle OEM ignition systems generate tachometer signals with negative voltage pulses. If you convert the ignition system to one that generates tachometer signals with positive voltage pulses, your tachometer will not work. The AccuTach Co. Lambo Tach Signal Inverter is an adapter that converts positive voltage tachometer signal pulses into negative voltage tachometer signal pulses compatible with your tachometer.
This product has been tested with a tachometer from a car with a Bosch 0227300002 ignition module. Other Bosch 022730000x family ignition modules may also generate negative voltage tachometer signal pulses. Prior to buying this product, please ensure that you verify that the OEM Bosch ignition module does, in fact, generate negative voltage tachometer signals.
Please read the Install Guide to make sure you are comfortable with the installation prior to purchasing this product.
Right-click and Download or Save As here to download the Install Guide.
One interesting thing I learned during the reverse engineering process is that the tachometer face is electrically connected to the tachometer input signal due to a screw that holds the face to the end of the input signal standoff post.
The input circuit makes it obvious why the tachometer does not work with the MSD ignition or with my bench setup with 0 to 12V calibration pulses. The SAK 215 integrated circuit is triggered by a positive pulse on its input. But the Bosch ignition module puts out a negative voltage pulse, so a transistor was added to invert that negative voltage system. With no signal on the tachometer input, R2 pulls the voltage on the base of the transistor up, which turns Q1 on. When Q1 is on, the input to the SAK215 is pulled to ground. To generate a positive input to the SAK215, the transistor base must be pulled to about 0V to turn it off. With the transistor off, R3 pulls the SAK215 input up to its regulated 7.4 to 8.2V, triggering the tachometer.
To get the base of the transistor down to about 0V, the input of the tachometer needs to be pulled down to at least -15V, worst case to at least -17.5V. This is due to the voltage divider formed by R1 and R2 from the SAK215 regulated voltage.
This explains why the 0 to 12V signal from the MSD ignition system and my bench setup did not trigger the tachometer.
When I set my function generator to it’s maximum amplitude of 15V and minimized the offset, the signal was 0 to -15V and my bench setup was able to trigger the tachometer successfully. When the tachometer is successfully triggered, the SAK215 drives current through the ammeter. The higher the tachometer signal frequency, the higher the average current goes through the ammeter.
It is interesting to note that the ammeter is a 90 degree ammeter but the needle on the face travels a lot more than 180 degrees. This is accomplished by a pair of gears that allow the meter to drive the needle with longer travel.
You can download a PDF of the document used to generate this page here.
While this car has the Bosch 0227300002 ignition module, it is likely that similar vintage vehicles with Bosch 022730000x modules will have tachometers with similar negative trigger voltage tachometers such as this one. This could possibly include some vehicles from Porsche, Maserati, Ferrari, Mercedes, BMW, Alfa Romeo, Jaguar and maybe others.
The tachometer can be modified to work with a positive pulse tachometer signal, but AccuTach Co. has developed a positive-to-negative tachometer adapter that will make tachometers like this one work with modern ignition systems. See the top of this page for more details.