KF - Testing Notes

DESCRIPTION
The Powertrain Control Module (PCM) determines engine cooling fan requirements and controls the fan operation through the Low Fan Control (LFC) and High Fan Control (HFC) outputs. The HFC relay is a normally open relay. The LFC relay is normally closed.

Both Fans Off
The HFC output is off. The LFC output is on (grounded) which opens the normally closed relay.

Low Speed Fan On
The LFC output is turned off which closes the normally closed LFC relay. The HFC output is off, and B(+) is supplied to the LFC relay through the HFC relay.

High Speed Fan On
The HFC output is on which closes the HFC relay. With the HFC relay contacts closed, B(+) is not supplied to the LFC relay preventing both fan speeds from being activated at the same time.

Fan Control Diagram:

Dual Function A/C Pressure Switch (ACPSW)
Also known as Refrigerant Containment/Fan Function Switch, the medium pressure contacts of the ACPSW are used to turn on the high speed cooling fan when the A/C head pressure reaches a predetermined level.

Fan Control Diagram:

The circuit from the medium presure contacts is spliced into the HFC circuit between PCM and the HFC relay. When the A/C pressure reaches the predetermined level, the medium pressure contracts will close. This grounds the HFC circuit which closes the HFC relay and turns on the high speed fan.

NOTE
You should enter this Pinpoint Test only when you have been directed here from Diagnostic Routines,

REMEMBER
This Pinpoint Test is intended to diagnose only the following:

- Harness circuits: LFC, HFC, LFC Power-To-Fan, HFC Power-To-Fan, IGN Start/Run and GND.

- LFC Relay

- HFC Relay

- Cooling Fan

- PCM