Powertrain Controls - ECM/PCM

Driver Circuit - Quad Driver "B":

CIRCUIT DESCRIPTION
The PCM controls several components through the use of a "Quad-Driver" module integrated into the PCM. Most devices, such as solenoids or relays, receive battery power through a fused circuit to the ignition, and turn "ON", when the circuit is completed to ground. The "Quad-Driver" consists of several electronic switching circuits that provide the ground connection to complete these circuits and turn the devices "ON".

Each "Quad-Driver" has a fault line that is monitored by the PCM. When the QDM switch for a particular circuit is "OPEN", the voltage at the PCM terminal for that circuit will be high (near battery voltage), and when the device is turned "ON" (circuit completed to ground), the voltage will drop to zero or near zero volts. When a device is commanded "OFF", the PCM expects the voltage in that circuit to be high, and when a device is commanded "ON", the PCM expects the voltage in that circuit to go low. The QDM fault detection circuit monitors the voltage in the sense line for each device controlled by the QDM. If the QDM fault detection circuit "sees" a voltage other than what it expects for a device in a particular "ON" or "OFF" state, the fault line status will change, and a Code will set. The fault line signal status can be displayed on a "Scan" tool.

Code 56 will set if the following conditions are present:
^ The engine is running.
^ The PCM detects an incorrect voltage level on a circuit connected to QDM "B".
^ the above conditions are met for 5 seconds.

TEST DESCRIPTION

Code 56 Diagnostic Chart (1 Of 3):

Numbers below refer to circled numbers on diagnostic charts.
1. The PCM does not know which controlled circuit caused the Code to set. This chart will go through each of the circuits to determine which is at fault. If QDM "B" related diagnostic trouble codes are present (EGR, shift solenoid, or TCC codes) use the applicable diagnostic charts for those codes first.
2. If QDM "B" related symptoms are present, the checks on page 3 of 3 of this chart should isolate is the cause.

QDM "B" SYMPTOMS
^ Improper shifting
^ TCC won't engage, or engagement is harsh.
^ Poor driveability due to EGR at all times.

Code 56 Diagnostic Chart (2 Of 3):

3. This test will determine which circuit is out of specifications.

Code 56 Diagnostic Chart (3 Of 3):

4. This test will determine if the problem is the circuit of the component.
5. As the factory installed PCM is protected with an internal circuit breaker, it is highly unlikely that the PCM needs to be replaced.