DESCRIPTION
Note. This article contains test charts that are part of General Motors Computerized Engine Controls. Only those charts required to test electric cooling fans are included. Other diagnostic codes may appear while performing electric cooling fan diagnosis. For complete information on General Motors Computerized Engine Control systems see ENGINE PERFORMANCE section.
All FWD and some RWD vehicles use an electric cooling fan. The electric cooling fan is used for engine and A/C condenser cooling and operates when A/C is on and if engine overheats.
OPERATION
All electric cooling fans operate when engine coolant temperature exceeds a certain value. The ECM completes ground path for the cooling fan relay. Relay contacts then close and complete circuit between fusible link and fan motor. When the engine cools, the switch opens and fan stops. If coolant sensor fails, the ECM will command constant fan operation.
A/C-equipped vehicles have a separate signal line to the ECM for fan control. When A/C control switch is on, and A/C system low pressure switch is closed, ECM receives a signal on this line and turns cooling fan on. The compressor clutch does not have to engage for ECM to turn fan on.
On some models, when engine is off, ECM may turn on the fan relay and run fan for up to 7 minutes. This occurs if hot conditions were present while engine was running. Hot conditions are based on Manifold Absolute Temperature (MAT) signal to ECM, coolant temperature and time from start.
TROUBLE SHOOTING
See trouble shooting charts for cooling fan diagnosis.
CHART C-12, COOLING FAN (VIN 8)
Cooling fans are controlled by ECM based on various inputs. Battery voltage is supplied to fan relay terminal "E". Ignition voltage is supplied to relay terminal "D". Grounding circuit No. 409 (relay terminal "F") will energize the primary cooling fan relay and supply battery voltage to primary cooling fan motor. Grounding circuit No. 473 (relay terminal "F") will energize the secondary cooling fan relay and supply battery voltage to secondary fan motor.
A/C cooling fan switch, mounted in the A/C high pressure line, will open when head pressure exceeds 240 psi (17 kg/cm 2 ). This input causes the ECM to ground circuits No. 473 or No. 409. If Code 14 or 15 is present or ECM is in the back-up mode, ECM will turn on cooling fan.
Diagnostic Aids
If an overheating condition is suspected, verify if this is due to actual boil-over. If gauge or light indicates an overheat condition, and no boilover is in evidence, inspect the gauge/light circuit for malfunction.
If vehicle is overheating and the gauge or light indicates the same, but cooling fan is not coming on, check coolant sensor temperature using a "Scan" tester. Sensor may have shifted calibration and should be replaced. If engine is overheating and cooling fan is on, check cooling system.
The ECM will turn on primary (Fan 1) cooling fan at 226°F (108°C) and turn off at 221°F (108°C). The ECM will turn on the secondary (Fan 2) cooling fan at 235°F (113°C) and turn off at 226°F (108°C).
Scheme 1
Scheme 2
Scheme 3
CHART C-10, A/C CLUTCH CONTROL
A/C clutch control is ECM-controlled to delay A/C clutch engagement after A/C is turned on, allowing ECM to adjust engine RPM before A/C clutch engages.
ECM will engage A/C clutch any time A/C has been requested unless any of the following conditions exist
- High coolant temperature.
- Wide open throttle.
- High engine oil temperature.
- High engine RPM.
ECM can determine A/C request by sending a voltage signal to the A/C control head. When A/C control switch is closed, the A/C request voltage signal is grounded.
When an A/C request has been detected by ECM, ECM will ground A/C clutch control relay drive circuit, energizing compressor clutch portion of the circuit. If system is properly charged, circuit will be completed to A/C compressor clutch.
When A/C request has been detected by ECM, cooling fan(s) will be turned on unless vehicle speed is too high.
Note. Test numbers refer to test numbers on diagnostic charts.
Scheme 4
Scheme 5
Scheme 6
Scheme 7
- Check A/C fuse and fuse circuit power distribution.
- This checks ECM's ability to control A/C clutch control relay.
- This determines if ECM is capable of detecting the A/C request signal being grounded.
- Before replacing A/C control components, refer to «A/C-HEATER SYSTEM - AUTOMATIC»(/chevrolet/corvette/c4-1984-1998/remont/automatic-hvac-system/#ac-heater-system-automatic) article in this section for further A/C diagnostics.
See also:
• A/C-HEATER SYSTEM - AUTOMATIC