DTC 128

DTC 128 Chart:

A/C Compressor Control Circuit:

CIRCUIT DESCRIPTION
The Powertrain Control Module (PCM) has the ability to monitor A/C refrigerant pressure and temperature. A/C refrigerant pressure is monitored though the use of high side and low side pressure switches. The high side pressure switch is a normally closed switch that is grounded through CKT 251 and monitored by the PCM on CKT 604. When the A/C high pressure line exceeds 430 psi the switch opens and the PCM disables the A/C compressor clutch.
The A/C high side temperature is monitored through the use of a thermistor to control the signal voltage to the PCM. The PCM applies 5 volts on CKT 731 to the sensor. When the sensor is cold, its resistance is high; therefore, the PCM will see a high signal voltage. As the sensor warms, its resistance becomes less and the signal voltage is pulled low through the sensor ground, CKT 808 If the PCM sees A/C high side temperature in excess of 87°C (189°F) the A/C compressor clutch is disabled.
If the PCM detects overtemperature and overpressure simultaneously the DTC 128 is set and A/C compressor clutch is disabled. Under extremely hot ambient conditions a rapid cycling situation could occur as this DTC is set and cleared. To protect the A/C system, after a DTC 128 is set, the PCM lowers the high temperature threshold for A/C comoressor clutch cutoff.

DTC 128 WILL SET WHEN
- DTC 121 not set.
- A/C disabled due to high refrigerant pressure.
- A/C high side temperature greater than 87°C (189°F) on first occurrence of DTC 128 and as low as 76°C (169°F) if DTC 128 has set 15 times in 30 seconds.

ACTION TAKEN (PCM WILL DEFAULT TO)
- Disable A/C compressor clutch.
- DTC 128 does not illuminate the Malfunction Indicator Lamp (MIL) (Service Engine Soon).

DTC 128 WILL CLEAR WHEN
The fault no longer exists.

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

1. Checks for current fault.
2. Checks for a shorted A/C high side pressure switch.
3. Checks for a short to ground in CKT 604 or faulty PCM.

DIAGNOSTIC AIDS
With the engine running, A/C "ON," the A/C high pressure switch status should be "NORMAL" when viewed on the Tech 1. If the status is "HI" and no DTC 128 is set, refer to Heating and Air Conditioning.