DTC P0726: ENGINE SPEED SENSOR CIRCUIT - PERFORMANCE Chevrolet Silverado 1500 HD

Note. For circuit identification, see CONNECTOR IDENTIFICATION and WIRING DIAGRAMS .

Circuit Description

The speed sensors are variable reluctance devices which convert mechanical motion to an AC voltage. Each sensor consists of a wire coil wrapped around a pole piece that is adjacent to a permanent magnet. These elements are contained in a housing which is mounted adjacent to a rotating ferrous member. Two signal wires extend from one end of the housing, and an exposed end of the pole piece is at the opposite end of the housing. The permanent magnet produces lines of flux around the pole piece. As a ferrous object (such as a gear tooth) approaches and passes through the gap at the end of the pole piece, an AC voltage pulse is induced in the wire coil. The Transmission Control Module (TCM) calculates the frequency of these AC pulses and converts it to a speed value. The AC voltage generated varies from 150 millivolts at low speed to 15 volts at high speed. The signal wires from the sensor are formed as twisted pairs to cancel magnetically induced fields. The cable is also shielded to protect from voltage-related fields. Noise from other sources is eliminated by using two-wire differential inputs at the TCM.

This is a type "B" DTC.

Conditions For Running The DTC

DTC will run under the following conditions

1. DTC P0716 and DTC P0717 are not active.
2. DTC P0727 is not active.
3. The components are powered, and ignition voltage is greater than 9 volts and less than 18 volts.
4. The input speed is greater than 800 RPM for 3 seconds.

Conditions For Setting The DTC

DTC will set under the following condition

1. DTC sets when the TCM detects a large unrealistic engine speed or if excessive noise is present in the engine speed sensor circuit.

Action Taken When The DTC Sets

TCM performs the following actions if DTC is set

1. DTC will be stored in the TCM history.
2. The TCM freezes shift adapts.
3. The TCM inhibits TCC engagement.
4. Illuminates the MIL when active.

Conditions For Clearing The DTC

DTC will clear under the following conditions

1. A scan tool can clear the DTC from the TCM history.
2. The TCM automatically clears the DTC from the TCM history if the vehicle completes 40 warm-up cycles without failure.

Test Description

The numbers listed below refer to step numbers in the diagnostic procedure

1. 2 - This step tests ignition voltage.
2. 3 - This step tests the wiring harness for opens or shorts.
3. 5 - This step tests engine speed sensor resistance.