DTC 312

FB. DTC 3-1-2
3-1-2 RPM signal faulty

Principle of operation

The RPM signal is supplied by a sensor on the transmission, which measures the input shaft speed after the torque converter. The signal is used to compare the transmission speed with the vehicle speed, enabling the gearshifting times to be computed. The engine and vehicle speed signals influence the line pressure, control of the lock-up function and torque reduction, resulting in smooth gearshifting.

Fault description

Consisting of a continuous stream of pulses, the RPM signal varies more rapidly than the system can accommodate. The fault may occur it the signal is subjected to interference from other leads or if a loose contact causes an instantaneous open-circuit. The MIL on the combined instrument lights.





Circuit description

The circuit consists of the TCM, RPM sensor and associated wiring and connectors.





1
Measuring resistance of RPM sensor and leads

- Switch off ignition.
- Connect break-out box to wiring (30-pin connector).

Connect ohmmeter between # 16 (NCO) and # 17 (NCOG).

- If instrument indicates 400-800 Ohm, sensor and leads are OK.
In this case, DTC is due to intermittent fault. Check wiring and connections for open-circuit. Check leads and connections as described in Testing and Inspection, General Tests Component Tests and General Diagnostics then return to Testing and Inspection. Diagnostic Procedures

- If instrument indicates other value, proceed to next step.





2
Measuring resistance of RPM sensor

- Switch off ignition.
- Measure at connector D on transmission (sensor side).

Connect ohmmeter between terminal 1 (NCO) and 2 (NCOG).

- If instrument indicates 400-800 Ohm, sensor is OK.
Check leads for short-circuit to ground, also check ground terminal connection as described in Testing and Inspection, General Tests Component Tests and General Diagnostics then return to Testing and Inspection. Diagnostic Procedures

- If instrument indicates other value, sensor is faulty. Replace RPM sensor.

If readings are correct, check that no leads (e.g. ignition leads, car phone etc.) are run or other systems (radio etc.) installed so as to cause interference.