Circuit Description
Caution: Refer to Electric Coolant Fan Caution in the Preface section.
The engine cooling fan circuit operates the main cooling fan and the auxiliary cooling fan. The cooling fans are controlled by the engine control module (ECM) based on inputs from the engine coolant temperature (ECT) sensor and the air conditioning pressure (ACP) sensor. The ECM controls the low speed cooling fan operation by internally grounding the ECM connector terminal K28. This energizes the low speed cooling fan relay and operates the main cooling fan at low speed. The ECM controls the high speed cooling fan operation by internally grounding the ECM connector terminal K12. This energizes the high speed cooling fan relay resulting in high speed fan operation as the cooling fans are now connected in a parallel circuit.
Diagnostic Aids
| • | If the owner complained of an overheating condition, it must be determined if the complaint was due to an actual boil over, or the engine coolant temperature gage indicated overheating. If the engine is overheating and the cooling fans are ON, the cooling system should be inspected. |
| • | If the engine fuse block fuses EF3 or EF10 become open immediately after installation, inspect for a short to ground in the wiring of the appropriate circuit. If the fuses become open when the cooling fans are to be turned ON by the ECM, suspect a faulty cooling fan motor. |
| • | The ECM will turn the cooling fans ON at low speed when the coolant temperature is 96°C (205°F). The ECM will turn the cooling fans OFF when the coolant temperature is 90°C (194°F). |
| • | The ECM will turn the cooling fans ON at high speed when the coolant temperature is 100°C (212°F). The ECM will change the cooling fans from high speed to low speed when the coolant temperature is 94°C (201°F). |
| • | The ECM will turn the cooling fans ON at low speed when the A/C system is ON. The ECM will change the cooling fans from low speed to high speed when the high side A/C pressure is 1 859 kPa (269 psi) then return to low speed when the high side A/C pressure is 1 449 kPa (210 psi). |
| • | The cooling fan circuit can be inspected quickly by disconnecting the ECM connector and grounding the connector terminal K28. This should create low speed cooling fan operation with the ignition ON. By grounding the ECM connector terminals K12 and turn the ignition ON, high speed cooling fan operation should be achieved. |
Step | Action | Values | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Diagnostic System Check - Vehicle? | -- | Go to Step 2 | |||||
2 |
Are the fuses OK? | -- | Go to Step 3 | Go to Diagnostic Aids | ||||
3 |
Does the cooling fan run at low speed? | -- | Go to Step 4 | Go to Step 8 | ||||
4 | The cooling fans should run at high speed when the coolant temperature reaches 100°C (212°F). Do the cooling fans run at high speed? | -- | Go to Step 5 | Go to Step 19 | ||||
5 |
Do the cooling fans run at low speed? | -- | Go to Step 7 | Go to Step 6 | ||||
6 |
Did you complete the repair? | -- | System OK | -- | ||||
7 |
Do the cooling fans run at high speed? | -- | System OK | -- | ||||
8 |
Did the test light illuminate? | -- | Go to Step 9 | Go to Step 12 | ||||
9 | Connect a test light between terminal 1 of the main cooling fan connector and battery positive. Did the test light illuminate? | -- | Go to Step 11 | Go to Step 10 | ||||
10 | Repair the open circuit between terminal 1 of the main cooling fan connector and ground. Did you complete the repair? | -- | System OK | -- | ||||
11 | Inspect for damaged terminals in the main cooling fan connector and repair it or replace the main cooling fan. Did you complete the repair? | -- | System OK | -- | ||||
12 |
Is the test light ON for both cases? | -- | Go to Step 14 | Go to Step 13 | ||||
13 | Repair the power supply circuit.
Did you complete the repair? | -- | System OK | Go to Step 14 | ||||
14 |
Does the cooling fan run at low speed? | -- | Go to Step 15 | Go to Step 16 | ||||
15 | Replace the ECM. Did you complete the replacement? | -- | System OK | -- | ||||
16 |
Does the resistance measure the specified value? | 0 ohms | Go to Step 18 | Go to Step 17 | ||||
17 | Repair the open circuit. Did you complete the repair? | -- | System OK | -- | ||||
18 | Replace the low speed cooling fan relay. Did you complete the repair? | -- | System OK | -- | ||||
19 |
Did the test light illuminate? | -- | Go to Step 20 | Go to Step 24 | ||||
20 | Connect a test light between terminal 1 of the auxiliary cooling fan connector and battery positive. Did the test light illuminate? | -- | Go to Step 21 | Go to Step 23 | ||||
21 | Replace the auxiliary cooling fan. Did you complete the replacement? | -- | System OK | Go to Step 22 | ||||
22 | Repair an open circuit between the following terminals:
Did you complete the repair? | -- | System OK | Go to Step 11 | ||||
23 | Repair an open circuit between terminal 1 of the auxiliary cooling fan connector and ground. Did you complete the repair? | -- | System OK | -- | ||||
24 |
Is the test light ON for both cases? | -- | Go to Step 26 | Go to Step 25 | ||||
25 | Repair the open power supply circuit:
Did you complete the repair? | -- | System OK | Go to Step 26 | ||||
26 |
Do the cooling fans run at high speed? | -- | Go to Step 15 | Go to Step 27 | ||||
27 |
Does the resistance measure the specified value? | 0 ohms | Go to Step 29 | Go to Step 28 | ||||
28 | Repair the open circuit between the specific circuit wiring. Did you complete the repair? | -- | System OK | -- | ||||
29 | Replace the high speed cooling fan relay. Did you complete the replacement? | -- | System OK | Go to Step 15 |
