Gen III PCMs are powerful control modules but they are of little use without tuning software to change engine (and transmission) operating parameters.
Before getting too far into a discussion of electric fans and PCM calibrations, it is important to first consider air conditioning control.
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Air Conditioning Considerations
Prior to buying a custom wire harness or doing PCM calibration work, determine if the vehicle will have air conditioning, if electric fan(s) are required when the A/C compressor is on, and if the PCM will be used to control the electric fan(s). (See Chapter 12 for more detail.)
Will the Vehicle Have Air Conditioning?
If the vehicle is fitted with air conditioning, the engine experiences additional load when the A/C compressor clutch is engaged (A/C is on). For engine retrofits, many PCMs (excluding 2003- newer) have an A/C on input that allows the PCM to adjust for the additional engine load, avoiding low-RPM stumble. Wired properly, and if using an LS1 Camaro, Firebird, or Corvette PCM calibration, the electric fan(s) turn on when the A/C is on to pull air through the A/C condenser.
Will the Electric Fan(s) be Required when the A/C Compressor is On?
When the A/C system is turned on, low-pressure refrigerant gases are drawn into the A/C compressor and transferred to the A/C condenser (located in front of the engine radiator) as high-pressure gas. The A/C condenser is essentially a radiator that cools the high-pressure refrigerant gases into a liquid again, in order to cool the interior of the car. To cool the A/C condenser, air must pass through it. One method of cooling the condenser is to install a permanent mechanical fan in front of the engine (as on many GM trucks). A more common method of drawing air through the condenser (and engine radiator) is through electric fan(s).
Will the PCM be Required to Controlthe Electric Fan(s)?
Many owners operate electric fan(s) independently of the PCM by triggering a relay through either the A/C compressor clutch circuit or through a trinary pressure switch. One drawback to these methods when using Gen III PCMs is that unwanted DTCs set as the PCM detects the fan(s) have turned on during conditions that the PCM has determined the fan(s) should be off. One solution is to simply disable these DTCs, which also disables the intended function of the DTC processing. A better solution is to use the PCM’s A/C control functions to gain improved A/C system control and A/C system diagnostics. (See Chapter 11 for more about A/C control.)
PCM Calibration
Electric fan operation is controlled by the PCM based on calibration settings defined by General Motors. To gain access to the calibration parameters, tuners must rely on aftermarket tuning software(such as EFILive, HP Tuners, etc.) to expose the electric fan settings and make desired changes. Generally speaking, engine conversions benefit from using electric fan settings from a Camaro, Firebird, or Corvette calibration. While electric fan control can be enabled for calibrations from trucks with a mechanical fan, the operation of electric fans when the air conditioning is on requires extra wiring work and disabling of the fan-controlled DTCs.
Many GM trucks equipped with a Gen III PCM use a mechanical fan. They use the typical high-speed-fan control output (red PCM connector, pin 33) as A/C automatic recirculation. When using a PCM with truck calibration, change calibration {G0106} from Auto Recirculation to Fan #2 Control.
GM vehicles equipped with an LSseries PCM and electric fans operate both fans in low-speed and high-speed mode only. This requires two PCM outputs and three relays. By using the two PCM outputs and only two relays, two electric fans can be controlled independently.
GM vehicles equipped with an LS-series PCM and electric fans operate both fans in low-speed and high-speed mode only. This requires two PCM outputs and three relays. By using the two PCM outputs and three relays, two electric fans can be controlled in low-speed and high-speed modes.
Camaro, Firebird and Corvette
The LS1 Camaro and Firebird are fitted with two electric fans and manual A/C controls. The PCM is responsible for controlling both A/C operation and electric fan operation. When the A/C controls are turned on, the PCM receives a 12V on signal, and validates A/C operation through a pressure sensor and 12V A/C compressor clutch status circuit. When the PCM is satisfied that the A/C is truly on, the PCM activates low-speed electric fan operation (for series/ parallel operation), or only turns on electric fan 1 relay (on/off operation).
Because the Camaro and Firebird were only available with a cable throttle body, PCMs loaded with the LS1 Camaro or Firebird calibration are the best choice for cable throttle engine retrofits where the PCM controls both A/C and electric fans.
The LS1 Corvette was available with either automatic or manual A/C controls. Those fitted with automatic A/C controls use a Class 2 data signal from the A/C controls to the PCM to indicate A/C control status. A PCM loaded with a Corvette calibration using automatic A/C controls does not provide the necessary A/C on signal to the PCM for electric fan operation. Owners using Corvette electronic throttle equipment on their engine retrofit should use a PCM calibration from a Corvette with manual A/C controls to satisfy the PCMs A/C on input for electric fan control.
Trucks
GM trucks were fitted with a mechanical fan until 2005 so those PCM calibrations do not have provision for electric fan control with A/C on. In 2005, when electric fans were introduced in GM trucks, both manual and automatic A/C controls communicated with the PCM using the Class 2 data stream, a signal not available in early retrofit vehicles. The best solution for retrofits using the PCM for electric fan control is a 1999–2002 Camaro, Firebird, or Corvette calibration.
PCM Operation
All Gen III PCMs are capable of controlling one or two electric fans.
On/Off
Production vehicles operate two fans with low- and high-speed capability using three relays to switch between series and parallel operation. However, since the PCM is only applying a ground to each fan relay signal circuit, two fans can be controlled through two relays for on/ off operation. In this way the electric fans are always wired in parallel so that each fan receives system voltage.
PCMs with a truck calibration, depending on the year, require the fan outputs to be turned on and on/ off temperatures to be set.
Low/High Speed
Production vehicles operate two fans with low- and high-speed capability by using three relays to switch between series and parallel operation. The third relay is responsible for switching from series (low-speed) to parallel (high-speed) operation.
When the PCM triggers only the low-speed fan output, current flows through both electric fan motors. In this way the electric fan motors are wired in a series and voltage is reduced, causing the fan motors to spin at low speed.
When the PCM triggers the two additional relays, the same voltage flows through each electric fan motor. In this way the electric fan motors are wired in parallel, causing the fan motors to spin at high speed.
Written by Mike Noonan and Posted with Permission of CarTechBooks
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