By Gene Markel
are very few all-mechanical controls left on late-model vehicles.
Almost all powertrain controls are electromechanical using a computer
as the controller. These include powertrain control of the accelerator
pedal with the latest being All Wheel Drive (AWD). The brakes and
damping are also electromechanical under electronic control. There are
peripheral systems that are all computer- controlled such as Electronic
Stability Control (ESC) and Adaptive Cruise Control (ACC).
electronic software programs are part of a controller or controllers.
AWD include three types of drive systems: Four Wheel Drive (4WD), AWD
and On-Demand AWD.
technician has to be more proficient in diagnosing system failures. He
or she has to be familiar with more than one manufacturer and be able
to understand their methods of diagnosis verses the dealership
technician who deals with one manufacturer.
scan tool and service information are the primary tools, but a thorough
knowledge of fundamental mechanical and electronic component operation
is also an important tool in achieving the right fix the first time.
This is a good reason to charge for diagnosing the vehicle’s systems.
scan tool that has the capability of operating a component can be a
necessity in diagnosing an operating error. Using the scan tool to
activate a component can eliminate a controller-to-component
communication problem. If the component does not activate, the ground
should be tested before replacing the component.
Light Truck 4WD
system has been in use since the 1940s. It is made up of a transfer
case and front differential and axle assembly. Most late-model
vehicles use a front differential that is mechanically engaged by a
switching mechanism that uses a solenoid or motor to engage the
differential and some systems have input to a controller.
is normally associated with rear-wheel-drive passenger cars and SUVs.
This system uses a transfer case, front differential and a power
divider that usually delivers 60 percent to the rear wheels and 40
percent to the front and is operational at all times. This type of
system is used by Cadillac CTS and SRX as AWD, Mercedes uses 4matic and
Subaru uses Symmetrical AWD to name a few.
On-Demand All-Wheel Drive
AWD is normally associated with a basic front-wheel-drive vehicle and
is made up of a Power Takeoff Unit (PTU), drive shaft and rear
Power Takeoff and Rear Differential
PTU contains a ring and pinion gear attached to the output shaft of a
transverse engine and transmission. The ring gear drives the pinion
that is connected to the driveshaft and rear differential. The rear
differential is actuated by a multi-disc wet clutch that can be engaged
by an electromagnet or hydraulic pressure. The clutch attached to the
rear differential unit of a Ford Edge or Chevrolet Traverse is engaged
by a stationary electromagnet in the same manner as the clutch on an
air conditioning compressor.
Haldex system on the 2010 Cadillac SRX uses a hydraulic piston to
actuate the wet clutch. A roller vane pump and check valve is used to
charge an accumulator that supplies hydraulic pressure to a solenoid
valve that meters pressure to the apply piston. Both the magnet and
apply piston are actuated by electronic control using Controller Area
Network (CAN) bus input to a controller that will send a command to the
actuator can communicate back to the controller the state of operation
of drive and this input is shared on the CAN bus. Actuation of the
drive is a result of software programming that utilizes input from the
ABS wheel speed, yaw and lateral acceleration sensors. The rear
differential electromechanical control input signal can be sent to the
powertrain or chassis controller.
Locking Differential Systems
the pre-computer electromechanical age, differentials used a
spring-loaded clutch to limit the differential as used in the
Positraction limited slip differential and the Detroit Locker that uses
a dog clutch as a differential. Current differentials can be locked
using an electromagnet or motor to engage a dog clutch or apply
pressure to a clutch pack.
operation uses sensor input, component feedback and output to
components for the synergistic operation of the vehicle.
should begin with a controller status that enables the technician to
monitor the status and communications between controllers on the CAN
bus. This makes it a necessity to have a scan tool that can communicate
with the controllers. This should be the starting point for scan tool
diagnostics. The interaction of powertrain, brake, chassis and body
controllers and systems will require functions to include output
control that will actuate a given component to diagnose for component
failures. The use of diagnostic trouble codes and data parameters are
not enough to solve a difficult component or system failure.
an electronic system, three key factors make the sensor system
operational. They are power, ground and signal. If there is a
high-resistance ground, the system may produce all kinds of problems
that may not produce a trouble code.
begin with sensors. There are two types of sensors most common to this
system: Two-wire and three-wire. Most three-wire sensors are digital
sensors producing a square wave signal. The scan tool can be used to
check reference voltage for the sensor and should range between 4.9 to
proper ground at the three-wire sensor, it should be under .050 volts.
Shorting the 5-volt reference to the signal wire should produce the
reference voltage at the scan tool.
checks the voltage supply circuit and the PCM ability to read the
signal. Shorting the signal to ground checks the ground circuit, and
the scan tool will display zero volts. Two-wire sensors used for speed
sensors are a magnetic flux inductive type and produce an analog
signal. The easiest way to test these sensors is to use scan tool
wheel-speed data and compare sensors. The sensor can be tested with an
Ohmmeter. If the sensor coil resistance is within specifications, the
most probable cause of a failure is the connector. Yaw and lateral
acceleration are two of the sensors that send a variety of signals to a
sensors must have a functioning power and ground to ensure the proper
operation of the sensor. Once again, scan tool data and specifications
are your best sources for diagnosing a sensor problem. It requires
sensor input to the controller for the software to activate a
component. This gets us back to the wiring and connections in the
system. Wiring and connectors can be difficult to diagnose. Having the
proper tools to disassemble connectors and repair them can be an
alternative to replacing a harness.