Chart C-12A Coolant Fan Functional Check

Chart C-12A, Cooling Fan Check:

A/C Head Pressure Switch Circuit:

*** This information has been updated according to Technical Service Bulletin #416507, dated April 1994.

CIRCUIT DESCRIPTION
Power for the fan motors comes from a 40A maxifuse element to terminal "30" on the relays. The relays are energized when current flows to ground through the PCM Quad-Driver #4. The left fan has two speeds and the right fan is a single speed fan.

Low Speed (Fan 1) Relay - The PCM energizes the relay through terminal "WD11" when engine coolant temperature reaches 100°C (212°F) or when A/C is requested.

High Speed (Fan 2) Relay - The high speed relay is energized by the PCM if the A/C head pressure switch opens, indicating refrigerant pressure above 210 psi (1448 kPa), or engine coolant temperature reaches 108°C (226°F).

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Using the Tech 1 "Misc. Tests," Fan 1 control will cause the PCM to ground CKT 335 and the cooling fans should run at low speed.
2. Selecting Fan 2 with the Tech 1 allows control of CKT 473 and the high speed (Fan 2) relay.
3. Disconnecting the A/C head pressure switch harness connector will cause the PCM to energize the high speed (Fan 2) relay.

DIAGNOSTIC AIDS
An intermittent may be caused by a poor connection, rubbed through wire insulation, or a wire broken inside the insulation.

Check for:
^ Poor connection or damaged harness - Inspect PCM harness connectors for backed out terminals "WD1O" or "WD11", improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.
^ Intermittent test - If connections and harness check OK, a digital voltmeter connected from affected terminal to ground while moving related connectors and wiring harness. If the failure is induced, the voltage reading will change.
^ Mis-Scaled Engine Coolant Temperature (ECT) sensor - See "Engine Coolant Temperature Sensor Temperature vs. Resistance Values" table on DTC P0118 diagnostic chart. P0118
^ Basic cooling system problem - Refer to "COOLING SYSTEM"