Chart C-12 Engine Cooling Fan Circuit Diagnosis
Chart C-12A - Two Speed Coolant Fan Check:
Wiring Diagram for Two Speed Coolant Fan:
CHART C-12A TWO-SPEED COOLANT FAN CHECK
Circuit Description:
The coolant fan is energized through a low speed, high speed and if equipped with heavy duty cooling a pusher fan relay.
Power for the fan(s) comes through the fusible link to terminal "1" on all relays. The relays are energized when current flows to ground through the activation of the A/C pressure, cool temperature switches, and/or the ECM.
Low Speed Relay - The low speed relay is energized by the ECM or the A/C pressure switch. The ECM energizes the relay through terminal "D2" when the coolant temperature reaches 98 degrees C (208 degrees F) and vehicle speed is below 45 mph. The low speed relay is also energized through the A/C pressure switch through terminal "B" when refrigerant pressure reaches 150 psi (1034 kPa).
High Speed Relay - The high speed relay is energized by the A/C high pressure and coolant override switches. If the A/C refrigerant pressure reaches 275 psi (1896 kPa) or the coolant temperature reaches 108 degrees C (226 degrees F) the high speed fan relay is energized. The ECM has no control of the high speed fan relay.
Pusher Fan Relay - The pusher fan is installed as part of the heavy duty cooling package and is turned "ON" anytime the high speed fan is running. When the A/C pressure reaches 275 psi (1896 kPa) or the coolant temperature reaches 108 degrees C (226 degrees F) both fans are energized. The ECM has no control of the pusher fan relay.
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
1. Grounding the diagnostic test terminal should cause the ECM to ground CKT 535 and the fan should run in low speed.
2. Grounding the temperature switch harness terminal will check CKT 536 and will also check the high and if equipped, pusher fan (VO8) relay.
3. Checks CKT 533 between the fan control relay terminal 4 and the motor. If the fan does not operate, CKT 533 is open,
4. This step checks to see if the temperature switch is grounding and is grounded when the light comes "ON". The switch should close at 108 degrees C (226 degrees F).
5. If the vehicle is equipped with A/C, the following steps will check the high pressure switch and related wiring from the switch to the fan control relay. If poor A/C performance is noted, the A/C pressure switch should be checked. The low speed fan should come "ON" if high pressure exceeds 150 psi (1036 kPa).
Chart C-12B - Two Speed Coolant Fan Check, Fan Runs At All Times:
CHART C-12B FAN(S) "ON" AT ALL TIMES TWO-SPEED COOLANT FAN
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
1. Check to see if CKT 535 is shorted to ground which would keep the relay grounded at all times.
2. Check to see if CKT 536 is shorted to ground. A light indicates the wire is shorted to ground and the following steps will isolate the short.
3. If the test light is "OFF" after disconnecting, the ECM is shorted internally. Before replacing the ECM, be sure and check the resistance value of low speed side of the fan control relay. Replace if resistance is less than 20 ohms. Also be sure that circuit 535 is not shorted to B + , and check the resistance of the canister purge solenoid and replace if under 20 ohms.
Chart C-12C - No Low Speed on Two Speed Fan:
CHART C-12C NO LOW SPEED FAN WITH TWO SPEED FAN
Test Description: Numbers below refer to circled numbers on the diagnostic chart.
1. Check for battery + at relay harness connector.
2. Jumpering terminals "1" to "4" bypasses the relay which should cause the fan to run if fan motor and wiring to the motor are good.
3. Grounding the test terminal should cause the ECM to ground CKT 535. At this point, the test light should light if the ECM is good and CKT 535 isn't open.
4. This checks for battery + and ground to the fan motor. A test light "ON" at this point indicates a faulty fan motor connection or motor.