Schematic Reference: Engine Controls Schematics

Voltage is applied to the Engine Speed Output circuit by the PCM. The PCM creates the Vehicle Speed Output signal by rapidly grounding this circuit via an internal switch called a driver. The driver operates at the same rate as the Low Resolution signal input. The other components on this circuit recognize the voltage being pulled to ground as an indication of engine speed.

The primary function of the driver is to supply the ground for the component being controlled. Each driver has a fault line which is monitored by the PCM. When the PCM is commanding a component ON, the voltage of the control circuit should be low (near 0 volts). When the PCM is commanding the control circuit to a component OFF, the voltage potential of the circuit should be high (near battery voltage). If the fault detection circuit senses a voltage other than what is expected, the fault line status will change causing the DTC to set.

Using Freeze Frame and/or Failure Records data may aid in locating an intermittent condition. If the DTC cannot be duplicated, the information included in the Freeze Frame and/or Failure Records data can be useful in determining how many miles since the DTC set. The Fail Counter and Pass Counter can also be used to determine how many ignition cycles the diagnostic reported a pass and/or a fail. Operate vehicle within the same freeze frame conditions (RPM, load, vehicle speed, temperature etc.) that were noted. This will isolate when the DTC failed. For intermittents, refer to Symptoms .

Number(s) below refer to step numbers on the diagnostic table.

3. Further circuit diagnosis may require I/P removal. The circuit from the PCM to the tachometer must be checked for being open, shorted to ground or voltage. If no trouble is found, follow the appropriate I/P diagnostic procedure.