DTC 56

Quad Driver Circuit:

Circuit Description:
The PCM is used to control several components such as those illustrated above. The PCM controls these devices through the use of a Quad-Driver Module (QDM). When the PCM is commanding a component "ON," the voltage potential of the output circuit will be "low" (near 0 volt). When the PCM is commanding the output circuit to a component "OFF," the voltage potential of the circuit will be "high." (Near battery voltage.) The primary function of the QDM is to supply the ground for the component being controlled.

Each QDM has a fault line which is monitored by the PCM. The fault line signal is available on the data stream for "Scan" tool test equipment display on Tech 1. The PCM will compare the voltage at the QDM based on accepted values of the fault line. If the QDM "B" fault detection circuit senses a voltage other than the accepted value, the fault line will go from a "low" signal on the data stream to a "high" signal and Code 56 will set.

Code 56 will set if:

^ The engine is running.
^ The PCM detects an improper voltage level on a circuit connected to QDM "B".
^ The above conditions met for 5 seconds.

Code 56 Diagnostic Chart:

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

1. The PCM does not know which controlled circuit caused the Code 56. This chart will go through each of the circuits to determine which is at fault. If QDM related codes are present, use the applicable code chart first.

2. If QDM "13" related symptoms are present, the checks on Page 3 of 3 of this chart should isolate the cause of the fault.

QDM "B" symptoms:

^ Improper shifting.
^ TCC won't apply or TCC harsh engagement.
^ Poor driveability due to EGR staying "ON" all the time.

Code 56 Diagnostic Chart:

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

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


Diagnostic Aids:
Monitor the voltage at each terminal shown above while moving related harness connectors, including PCM harness. If the failure is induced, the voltage will change. This may help locate an intermittent condition. Check for bent pins at PCM and PCM connector terminals. If code reoccurs with no apparent connector problem, replace PCM.

Code 56 Diagnostic Chart:

Test Description:
Number(s) below refer to circled number(s) on the diagnostic chart.

4. This test will determine if the problem is the circuit or 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.