Drive by Wire Systems

No strings attached

Mark Lemay

Drive By Wire technology was first introduced on military aircraft about 20 years ago. On cars, it is replacing the conventional mechanical cable between the pedal and the throttle body. It consists of a sophisticated pedal position sensor (PPS) that closely tracks the position of the accelerator pedal and sends this information to the Engine Control Module (ECM). The advantages are many as we will learn in this article.

In previous systems, the driver input used to be handled by the TPS sensor. With DBW technology, this signal is sent from the Pedal Position Sensor. It may be referred to as the APP (Accelerator Pedal Position) sensor or PPS (Pedal Position Sensor).

APP System

Ford introduced this system on the 1996 Continental and Chevrolet introduced a similar system on the Corvette in 1997. GM’s original system required a separate control (TAC module), which acts like a throttle actuator control. It is still in use on many GM vehicles.

Mounted on the top of the accelerator pedal, the APP sensor (Illustration 1) relays the driver demand to the PCM and can be made of 2 or more potentiometers working with a 5 volt supply. To give the driver the normal pedal feel, anything from a spiral wound spring to a spring and cable assembly may be used.

The throttle body assembly (Illustration 2) contains the throttle drive motor, the throttle plate and the throttle position sensor. With this electronic throttle control system, there is better emissions control, improved engine speed governing and torque control, and finally less moving parts and other hardware.

Two systems configurations

You may run into two different configurations, one without a Throttle Driver Module where the PCM is the main brain and the next one with the use of Throttle Driver Module, where information interaction occurs between the TDM (throttle driver module) and the PCM according to the signals received from the sensors and then a command is sent to the throttle motor to open the throttle plate.

APP operation

On APP systems, all critical inputs and outputs signals are checked for proper operation and rationality to ensure that the readings make sense by comparing them with one another.  They are then compared to a look up table to ensure that the signals are rational.

Electronic throttle control is one of the most heavily monitored systems on the vehicle. For safety purposes, redundant path software and a program called security modeling are constantly in use as the PCM learns over time what the readings should be for a given APP setting (MAF, TPS, RPM, etc.)

If the readings do not correspond with learned data, the PCM will set codes. Among the different kind of failures, we can have anything from partial throttle operation to no throttle operation, or an engine that does not start. The PCM will set one or more codes to indicate the fault.

System check up and diagnosis

Here is a description of the main tests to carry on when a problem occurs:

•    Test the reference and ground circuits voltages first
•    Next test the sensor circuits
•    Use graphics to better view and compare data
•    Scope patterns might be a better way to monitor the circuit
•    Sensors have specific fault codes


Throttle Actuator

The throttle plate is driven by an electric motor and this actuator is a bi directional DC motor, spring loaded in both direction and defaults to about 7% open if a failure occurs.


TPS operation

The TPS sensors (multiple potentiometers) are mounted on the throttle body and are part of the throttle body assembly. They provide feedback to the PCM regarding the throttle position by tracking the throttle plate movement. One sensor reading is low while the other one is high, added  together they should equal 5 volts.

The above data (Illustration 3) shows the sensors readings, clearly on different tracks. The PCM is watching and comparing each signal to determine pedal position. Signal correlation is also used to detect faults. Note that the throttle drive motor is pulse width modulated and constantly moving. The TPS diagnosis is very similar to APP diagnosis.

If you suspect a Throttle Actuator malfunction, before proceeding with your diagnosis  make sure that there are no APP or TPS codes stored, and if there are address them first. Also check for TSB (technical service bulletins).

Also check the throttle bore for cleanliness and clean if necessary. To do so it is recommended to remove it from the vehicle.

Steps in diagnosis

Here is a list of the actions taken when a Diagnosis Trouble Code sets :

•    The control module illuminates the MIL if the diagnosis runs and fails
•    The control module records the operating conditions at the time the diagnosis fails and stores the information in the Freeze Frame and/or Failure Records.
•    The control module commands the TAC system to operate in the Reduced Engine Power mode.
•    A message center or an indicator displays Reduced Engine Power.
•    Under certain conditions the control module commands the engine OFF.

In the event of a failure the MIL and the Reduced Engine Power message will be displayed (illustration 4). The PCM will turn off the driver circuit to the throttle body. The throttle will stay on the stop and engine RPM will be approximately 1500 RPM.

Circuit check ups

Légende : Remove the connector form the throttle body. Hook a test light between the motor drive circuits and cycle the ignition key. The test light should flash as the TAC module drives the motor. If the test light flashes replace the throttle body assy. If the test light does not flash check power supplies and grounds. If everything test ok replace the TAC module.

Module Communications and Faults

Légende: On GM vehicles with a TAC module. Module to module communication is managed by the Class 2 or CAN bus. The PCM to TAC communication is handled by the UART bus and is not accessible to the scan tool directly. Communication with the TAC module is passed through the PCM to the scan tool. Feedback data from the TAC on throttle position and plate driver are processed by the PCM before going to the scan tool.

If the PCM looses communication with the TAC module, a Reduced Power Message will be displayed, and the vehicle may run (if it starts) with reduced power and no throttle response. The PCM will then store codes for the failure.

Word of advice

These systems present many diagnostic challenges to technicians. It is suggested that you attend a training class before trying to diagnose a drive By Wire system fault.