PCM Controlled Tachometer Diagnosis

Circuit Description
The tachometer in the IPC receives engine RPM data via Class 2 serial data. The Tach Output circuit provides a direct input of engine RPM to the EBTCM. The EBTCM pulls up the Tach Output circuit to 12 volts. The PCM pulse width modulates this circuit to ground. The EBTCM uses this engine RPM signal for ABS and Traction Control calculations. The PCM grounds the circuit via an internal solid state device called a driver. The primary function of the driver is to supply the ground for the component being controlled.

Diagnostic Aids
Use the Connector Test Adapter kit J 35616-A for any test that requires probing the PCM harness connectors, electrical center fuse/relay cavities, component terminals or a component harness connector. Using this kit will prevent damage caused by improper probing of connector terminals.

If the problem is intermittent, refer to Intermittent Conditions for diagnosis. Intermittent Conditions

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
6. The actual frequency measured will vary with RPM. The presence of a frequency verifies that the PCM driver is functioning.
Typical values are approximately:
^ 23 Hz at 700 RPM
^ 46 Hz at 1400 RPM
^ 69 Hz at 2100 RPM