CM871 and CM876 Electronic Control Systems, ISX and ISM Engines – FAULT CODE 686

  Printable Version

Overview

ISX – Turbocharger 1 Speed Circuit SMALL | MEDIUM | LARGE

ISM – Turbocharger 1 Speed Circuit SMALL | MEDIUM | LARGE

Circuit Description

The turbocharger speed sensor is a variable reluctance speed sensor. It consists of a coil of wire and an iron core. The target on the turbocharger shaft is a ground flat in the center of the shaft. As the flat on the turbocharger shaft spins past the speed sensor, a signal is generated. The electronic control module (ECM) interprets this signal and converts it to a turbocharger speed reading.

Component Location

The turbocharger speed sensor is mounted in the center housing of the turbocharger. Refer to Procedure 100-002 for a detailed component location view.

Conditions for Running the Diagnostics

This diagnostic runs continuously when the engine is running.

Conditions for Setting the Fault Codes

The ECM detects that the turbocharger speed is above 120,000 rpm for more than one second.

Action Taken When the Fault Code is Active

• The ECM illuminates the amber CHECK ENGINE light immediately when the diagnostic runs and fails.
• The ECM will estimate the turbocharger speed.

Conditions for Clearing the Fault Code

The ECM will turn off the amber CHECK ENGINE light immediately after the diagnostic runs and passes.

Shop Talk

This fault code is logged when the ECM detects a turbocharger speed that is greater than the speed that is physically achievable by the turbocharger. Usually this indicates a faulty sensor or incorrect speed signals from the turbocharger speed sensor. This fault can be caused by failures in the following components:

• Turbocharger speed sensor.
• Engine harness.
• ECM power supply or grounds.
• Electronic vehicle accessories which emit excessive electromagnetic interference.

This fault becomes inactive any time the ECM detects a valid turbocharger speed. If the fault occurs intermittently:

• Look for causes of intermittent open circuit or short circuits in the turbocharger speed sensor circuit (including the speed sensor pigtail connector).
• Look for sources of electronic noise such as poorly installed vehicle electronic components.
• Be sure that the vehicle chassis grounds are in good condition.
• Be sure that the ECM power supply and ground circuits are in good condition and that this power supply is not shared with other vehicle electronics.

Refer to Troubleshooting Fault Code t05-686