Interfacing the Tachometer

March 2005

Analysis

There are a few approaches that can be taken to interface to the tachometer:

• Mechanically attach the camshaft position sensor (CMP) to the new electric drive. This singal feeds the ECU for an engine signal, which in turn is sent to the tachometer.
• Simulate the camshaft position sensor (CMP).
• Bypass the ECU and signal the tachometer directly.

The Camshaft Position Sensor

The Camshaft Position Sensor is located under the timing belt cover under the top, camshaft pulley. It is a magnetic sensor which reads gaps in the camshaft pulley. These gaps are not evenly spaced. The sensor is know as G28 on page 97-68 and connected through connector T3 (3 pin: white, yellow and black) which feeds the Motronic ECM which drives the tachometer in the instrument panel.

Camshaft Position Sensor, and showing how it sensor is interfaced with the pulley.

RPM Signal to Tachometer

The RPM signal is fed through pin 11 on the T32a (blue) connector on the back of the tachometer (page 97-73). The wire colour is green/brown.

The tachometer connector, with RPM signal wire identified

Simulating RPM Signal

The easiest way to drive the tachometer is directly. To test this out, the wire to the tachometer was cut and a signal generator was connected. The signal sent to the unit was a square wave 5 volt signal at 1kHz. This worked. It produced a reading on the tachometer of about 450RPM.

After checking some measurements, 4000RPM requires a signal of about 126Hz.

The signal generator sending a signal to the tach. Notice the tach (lower right on instrument panel) reads just below 4000 RPM)

The Siemens Controller

Well it turns out that the Siemens controller sends a tach signal, pulsed from -12V to +12V. It also turns out that a scaling factor can be programmed to adjust the signal. And nice enough, the input tach sensor for the New Beetle accepts this voltage without modification.