Bench Testing BMW E30's Tachometer

In this article, I am going to show you how to bench test your E30's tachometer using a signal generator. Also, I will give you the specification for the tachometer signal to use.

1. E30 Instrument Cluster
2. 12V Power Supply - 12V Battery should work too
3. Function Generator that can generates square waves
4. Jumper Wires
5. Diode, if your function generator cannot adjust voltage offset
6. (optional) breadboard, for convenient

E30's instrument cluster basically takes a tachometer signal from ECU. Tach signal has the following characteristics:

1. It is a PWM signal with around 3.33% duty cycle - refer to the picture above for the output from an oscilloscope.
2. For bench testing purpose, you don't need to feed PWM signal. Regular square waves will work too.
3. Frequency depends on Engine RPM - Higher RPM, higher tach signal's frequency
4. I tested 2 tachs and found that the signal frequency is RPM/30 Hz. For example, 20Hz signal will move the tach needle to 600rpm, and 200Hz will be 6000rpm.
5. My clusters are all from 4-cylinder engines (BMW M42 and M40). They have different coding plugs though (7000rpm vs 8000rpm), yet show the same relationship with frequency.
6. It is possible that your cluster will have a different Freq/RPM relationship, especially for 6-cylinder models. However, it should be easy to work out the math.
7. In an old r3vlimiter post, the author suggested his signal frequency was about RPM/20 Hz. You can view the post here.
8. Vmax is 12V, but the tach will be happy with 10-14V.
9. Notice that I use Vmax, not Vpp (Peak-to-peak voltage), because the tach seems to care only voltage on the positive side.
10. I have tried feeding 12V Vpp signal (Vmin -6V to Vmax +6V). Tach does not seem to work, because it only sees +6V signal.
11. If I feed 20V Vpp signal (Vmin -10V to Vmax +10V), tach will move for a while. After a minute or two, it will shut down the circuit though. It doesn't like the -10V part.
12. To overcome this, you can adjust your voltage offset. So the signal will be from 0V to +12V.
13. Or if you have a cheap ass function generator like me, you can put a diode in and generate 24V square waves. Once the signal passes through the diode, it will become 0V to +12V signals with slight deformation.

If your SI battery is dead, your tachometer will not work. However, contrary to popular belief, you don't need to replace the battery. You only need to remove the dead batteries and the tachometer will work again. Dead batteries seem to draw electrical current and leave too little to operate the tach.

Also, your SI board and the coding plug must be connected to the cluster. This is the circuit which interprets incoming tach signal and then moves the needle.

1. Despite what many online sources claim, you only need to connect 4 pins on C1 connector (blue connector).
2. These are circled red in the wiring diagram above.
3. If you don't know how to count pin number, I have provided illustrations above.
4. You can solder wires to PCB, or use jumper wires to connect to pins on the back side.
5. Connect +12V power source to Pin 2 and 23.
6. Connect ground to Pin 20.
7. Connect your signal wire to Pin 7.
8. Connect your signal ground to Pin 20 to provide a common ground.
9. If you use alligator clips like I do, just clip the signal ground onto the power source ground's clip.

Once you're done, you should get something similar to the diagram I provided. At this point, change your frequency up and down, and the tach needle should move accordingly.