DTC 78

DTC 78.:

Cooling Fan Circuit.:

CIRCUIT DESCRIPTION
Ignition voltage is supplied directly to the secondary cooling fan relay coil on CKT 41. The Powertrain Control Module (PCM) controls the relay by providing the ground path through CKT 473.

When the PCM is commanding a component "ON," the voltage of the control circuit should be "low" (near 0 volts). When the PCM is commanding the control circuit to a component "OFF," the voltage potential of the circuit should be high (near battery voltage). The primary function of the driver is to supply the ground for the component being controlled.

Each driver has a fault line which is monitored by the PCM. The PCM will monitor the voltage at the driver. If the fault detection circuit senses a voltage other than expected, the fault line status will change and the DTC will set.

DTC 78 WILL SET WHEN:
The PCM detects the wrong voltage potential for 26 seconds on CKT 473.

ACTION TAKEN (PCM WILL DEFAULT TO)
DTC 78 will be stored in the PCM memory but will not turn "ON" the Malfunction Indicator Lamp (MIL).

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

1. The Tech 1 (or equivalent Scan tool) will enable the circuit for 5 seconds.
2. This check can detect a partially shorted coil which would cause excessive current flow. If excessive current flow is detected, a circuit check will be performed to isolate the device from the wiring. Leave the circuit energized for 2 minutes to allow the coil to warm up. The coil may open (amps drop to "0") when warm.
3. The remaining circuit checks will identify a circuit problem that has caused an excessive current flow or inoperative relay. If a circuit check is done on a relay, it is important to identify and test the relay coil terminals ("H3" and "C1") to avoid improper diagnosis.

DIAGNOSTIC AIDS
Inspect the harness and all related connectors including the PCM connections. These may cause an intermittent. Check for damaged or pushed out terminals.