Powertrain Controls - ECM/PCM

DTC 28 Quad-Driver Module (QDM 2) Circuit:

Quad-Driver Module (QDM 2) Schematic:

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) controls most components with electronic switches which complete a ground circuit when turned "ON." These switches are arranged in groups of 4, called Quad-Driver Modules (QDMs) which can independently control up to 4 outputs (PCM terminals), although not all outputs are used. When an output is "ON," the terminal is grounded and its voltage normally will be low. When an output is "OFF," its terminal voltage normally will be high.

QDMs are fault protected. If a relay or solenoid coil is shorted, having very low resistance, or if the control side of the circuit is shorted to voltage, it would allow too much current into the QDM. The QDM senses this and the output turns "OFF" or its internal resistance increases to limit current flow and protect the QDM. The result is high output terminal voltage when it should be low. If the circuit from B+ or the component is open, or the control side of the circuit is shorted to ground, terminal voltage will be low, even when output is commanded "OFF." Either of these conditions is considered to be a QDM fault.

QDM 2 has a fault line to indicate the presence of a fault to the PCM's central processor. A scan tool displays the status of the fault line as "low" = OK, "High" = fault. In vehicles not equipped with cruise control the fault line status will raad "High" when in P, R, N with an automatic transmission. Vehicles with manual transmissions and no cruise control will read "high" until the PCM receives vehicle speed.

DIAGNOSTIC TROUBLE CODE (DTC) 28 WILL SET WHEN
^ QDM 2 fault line:
- Is "high" for 20 seconds or more.
- Battery voltage is greater than 10.5 volts.

ACTION TAKEN (PCM WILL DEFAULT TO)
The cooling fan will enable.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. This step checks the coolant fan circuit.
2. This step checks the A/C clutch circuit (if equipped).

DIAGNOSTIC AIDS
^ A Tech 1 scan tool has the ability to command the coolant fan and A/C clutch relays "ON" and "OFF."
^ A faulty connection, or an open CKT 335 or 459 can result in a DTC 28.