While this description is actually for the 95 Bonneville 3800 SC computer, most GM PCM's follow this general rule of thumb concerning the way the outputs of the PCM are monitored and operated.
QDM #4 DTC P1670
The Powertrain Control Module (PCM) is used to control several components such as the A/C Relay, Malfunction Indicator Lamp (MIL), Low Speed Cooling Fan Relay and the High Speed Cooling Fan Relay. 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 volts). 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 display on the Tech 1 scan tool. The PCM will compare the voltage at the QDM based on accepted values of the fault line. If the QDM #4 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 DTC P1670 will set.
DIAGNOSTIC TROUBLE CODE (DTC) P167O WILL SET WHEN
^ The engine is running.
^ The PCM detects an improper voltage level on a circuit connected to QDM #4.
^ The above conditions met for 5 seconds.
ACTION TAKEN (PCM WILL DEFAULT TO)
DTC P1670 does not illuminate the MIL. DTC P1670 could ground both fan relay outputs and disable A/C clutch relay operation depending on calibration.
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 DTC reoccurs with no apparent connector problem, replace PCM.
As you can see, both fan relays as well as the A/C clutch relay and Check Engine light are monitored by the QDM4 circuit. If any one of these circuits have a problem, it could cause the PCM to go into default mode and fix the state of the outputs on the affected QDM circuit. While your particular GM PCM might differ in operation and actions taken if an error is encountered, the results could be similar. If you are experiencing any kind of electrical problems or trouble codes concerning the QDM's, this info should help.
power corrupts. absolute power corrupts absolutely.
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