Part 1 of 2

Cooling Fans And Relays Control Circuit Diagram:

CIRCUIT DESCRIPTION
The promary and secondary cooling fans are computer controlled, based on inputs from the coolant temperature sensor, A/C control signals, vehicle speed sensor and A/C pressure sensor. Cooling fan relays get fused battery power all the time (except for circuit 250 to terminal #3 of secondary fan relay, which is hot when the ignition is "ON".) The relays become energized when the PCM completes the circuit to ground (circuits 335 and 473, for primary and secondary fan relays respectively.) The PCM energizes the relays when coolant temperature is greater than about 106°C (223°F), or when the A/C is requested and A/C high side pressure is greater than 1380 kPa (200 psi.) Once fan relay is turned "ON", it will stay on for a minimum of 30 seconds or until vehicle speed exceeds 70 mph (40 mph for secondary fan.)

TEST DESCRIPTION

Chart C-12, Cooling Fan Control Circuit Diagnosis (1 of 2):

Numbers below refer to circled numbers on diagnostic chart.
1. With the diagnostic terminal grounded, the PCM should energize the cooling fan relays and fans should run.
2. Checks for faulty A/C pressure sensor or circuit. If sensor signal indicates pressure above 300 psi (2069 kPa), or if circuit 802 is open, fans should run when ever A/C is requested.
3. With A/C clutch engaged and pressure sensor functioning correctly, fan should turn "ON" when pressure exceeds about 200 psi (1380 kPa.)
4. This test determines if the A/C pressure sensor is faulty, or if the PCM or wiring is the problem.

DIAGNOSTIC AIDS
If the complaint is an overheating problem, it must be determined if the engine is actually overheating, or if there is a false overheat indication from the temperature gauge/hot light. If the gauge or light indicates an overheating condition and the engine is not overheated, the gauge and circuit should be checked. This can be checked by comparing the temperature as indicated by a "SCAN" tool. If the engine is overheating and the gauge of light indicates overheating, but the cooling fans do not turn "ON", the coolant sensor may have shifted out-of-calibration and should be replaced. If the cooling fans work and the engine is overheating, the cooling system should be checked (radiator, thermostat, hoses and water pump.) Also check for blown head gasket(s).