Home » GM 6.5 » GM/Detroit 6.5L – Engine Management – TRANSMISSION CONTROL – TRANSMISSION OUTPUT SPEED SENSOR AND VEHICLESPEED SENSOR (VSS)

GM/Detroit 6.5L – Engine Management – TRANSMISSION CONTROL – TRANSMISSION OUTPUT SPEED SENSOR AND VEHICLESPEED SENSOR (VSS)

Description

The transmission output speed/vehicle speed sensor is mounted on the driver’s side of the transmission case, near the rear (figure 5-56). The sensor consists of a permanent magnet surrounded by a coil of wire. It reads the rotation speed of a 40-toothed ring mounted to the rear carrier assembly on 4L80E models or to the output shaft on 4L60E models.

The sensor maintains a slight air gap (.045-.109 in.) between itself and the case. The sensor produces an AC signal voltage whose frequency varies with transmission speed:
• Low speed = low voltage signal frequency (.5 volts at 100 rpm)
• High speed = high voltage signal frequency (100 volts at 8,000 rpm)

2WD/4WD Differences
The sending of vehicle speed information to the PCM
differs slightly between two-wheel-drive and four-wheel-
drive configurations.
• Two-wheel-drive systems use either a transmission output speed sensor or a VSS.
The sensors perform essentially the same function. Each sensor’s signal is used by the buffer/DRAC module to inform the PCM of vehicle speed.
• Four-wheel-drive systems use both a transmission output speed sensor and a VSS sensor. The VSS is located on the transfer case.

Information from the transmission output and/or VSS sensors is used by the PCM for TCC application and release, line pressure control, shift timing control, and torque management control.

VSS Buffer Module

The VSS signal is sent to a Vehicle Speed Sensor (VSS) Buffer Module (figure 5-57). The module acts as a signal interpreter. It changes the AC voltage of incoming sensor signals so they can be used by the PCM. The module outputs a 4000-pulse-per-mile signal to the PCM. The buffer also emits signals used by other functions such as the Antilock Brake System (ABS) and speedometer/odom­eter.

The VSS buffer module is also known as a Digital Ratio Adapter Controller (DRAC) Module. The buffer/DRAC module allows for easier updating of system software if vehicle tire size and/or axle ratio are changed, which is a more common occurrence on trucks.

Circuit Operation
The VSS buffer/DRAC module receives a 12 volt ignition signal on CKT 250 through the ignition switch. Ground for the module is provided on CKT 450 (figure 5-58).

The module receives the output speed /vehicle speed sensor signal on CKT 821. This signal ranges from .5 volt at low transmission speeds (100 rpm) to 100 volts at high speed (8,000 rpm). The module also provides a ground for the sensor on CKT 822.The module supplies transmission output and vehicle speed information to the following components on the following circuits:
• To the PCM on CKT 437 and CKT 1586 or CKT 834
• To the speedometer on CKT 824
• To the antilock brake module on CKT 696

DTC 16 — “Vehicle Speed Signal Buffer Fault”
The VSS buffer module sends a vehicle speed signal to the PCM on CKT 834. The PCM uses this to control engine operating functions. DTC 16 identifies an open or short to ground in CKT 834. It sets when the following conditions exist for at least 2 seconds:
• Vehicle speed greater than 20 mph
• Engine speed between 1000 and 4400 rpm
• CKT 834 open or shorted to ground

DTC 16 will result in loss of cruise control or fuel cutoff.

DTC 24 — “Vehicle Speed Sensor Signal Low”
DTC 24 also when there is no vehicle speed detected by the PCM at start off. DTC 24 sets when the following conditions exist for at least 3 seconds:
• There are no stored DTCs 21 (APP 1 Circuit High), 22 (APP 1 Circuit Low), 28 (Trans. Range Pressure
Switch Assembly Fault), 33 (EGR/Baro Sensor Circuit High) or 34 (Stepper Motor Circuit Fault)
• The vehicle is in any gear range other than Park or Neutral
• Voltage on CKT 437 (output speed signal to the PCM) is constant
• Transmission output speed is less than 200 RPM
• Manifold absolute pressure is greater than 40 kPa
• Throttle position is between 10% and 100%

If DTC 24 sets, the vehicle defaults to a singel gear range with no upshifts or downshifts.

DTC 72 — “Vehicle Speed Sensor (VSS) Circuit Loss (Trans. Output Speed Signal)
DTC 72 can set under two sets of conditions:

When the vehicle is in Park or Neutral:
• There is no DTC 28
• Transmission output speed change is greater than 1000 rpm
• Engine speed is greater than 200 rpm
• All conditions exist for at least 2 seconds

When vehicle is in a gear other than Park or Neutral:
• There is no DTC 28 stored
• Transmission output speed change is greater than 2050 rpm
• Engine speed is greater than 200 rpm
• All conditions exist for 2 seconds

 

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