IRCM Theory of Operation

ECC Power Relay and Switching Circuit
K1 switches power to the ECC and internal IRCM circuits. The input voltage to the relay is sourced by IRCM pin 8 (constant +12 volts). That voltage connects directly to the relay contact and the relay coil through isolation diode D1. Reverse voltage suppressor diode D6 is connected across K1's coil to eliminate any reverse voltage spikes at the collector of Q1.

Q1 is an NPN transistor used as a switch. The emitter is grounded and the collector drives the coil of K1. The base is held in the normal "off" state by resistor R3. R1 limits the Q1 base current to around 5mA. Diode D2 is in series with the switching voltage, preventing a low-level (ground) from being applied to the base of Q1 when the ignition is off. Finally, C1 appears to be an RFI (radio frequency interference) "short" across D2, preventing RFI from being rectified by D2 and inadvertently turning Q1 on.

When the ignition is off, the base of Q1 is held low by R3 and the collector is in it's high impedance (off) state. When the ignition is on, current flows through IRCM pin 13, D2, and R1 into the base of Q1. This current turns the transistor on, switching the collector to it's low impedance (on) state, thus applying ground to the low side of the K1 coil, closing it's contacts. Contact closure then sources +12 volts to the high sides of the other relay coils, to the internal circuitry of the IRCM, and to the ECC through IRCM pin 24.

Fuel Pump Power Relay
K2 switches power to the fuel pump. K2's contact voltage is sourced from IRCM pin 8, and it's coil voltage is sourced from the K1 contacts. K2 is switched on by applying a ground to IRCM pin 18. Pin 18 is driven by an ECC output.

Secondary Cooling Fan Power Relay
K3 switches power to the secondary cooling fan. K3's contact voltage is sourced from IRCM pins 3 and 4, and it's coil voltage is sourced from the K1 contacts. K3 is switched on by applying a ground to IRCM pin 17, sending power to the fan via IRCM pins 6 and 7.

Primary Cooling Fan Power Relay
K4 switches power to the primary cooling fan. K4's contact voltage is sourced from IRCM pins 3 and 4, and it's coil voltage is sourced from the K1 contacts. When energized, K4 applies power to the primary fan via IRCM pins 1 and 2. K4 is energized via a switching circuit consisting of Q2, Q5, Q6, and their associated components.

IRCM pin 14 is the control input for the primary fan. If pin 14 were open(high), R4 applies +12 volts through the R9 current limiting resistor to the base of Q2. The current flowing into the Q2 base turns it on, making the collector "low". The "low" applied to Q5's base turns Q5 off, making it's collector "high". The "high" thus applied to the base of Q6 turns Q6 on, pulling the Q6 collector low, energizing K4, thus turning on the fan.

R14 and R12 are pull up resistors which normally pull the bases of Q6 and Q5 high. R11 and R9 form a voltage divider that turns Q2 on and off. C4 is used as a time delay that slows the turn on and shut off of the primary fan. Diode D4 is a reverse-voltage protection diode.

IRCM pin 14 is controlled by an output of the ECC. When the engine is below 190*, the ECC applies a "low" to pin 14. The low turns off Q2, allowing Q5 to turn on, causing Q6 to turn off. Conversely, when the ECC determines a need for the cooling fan, it removes the low from pin 14 which allows Q2 to turn on, Q5 shuts off, and Q6 turns on energising K4 and applying voltage to the main fan via IRCM pins 1 and 2. This circuit apparently acts as a kind of "failsafe" for the fan; if the ECC wire to IRCM pin 14 breaks or if the ECC output "opens", the main fan will run continuously.

Air Conditioning Compressor Clutch Control
Power for the air conditioning clutch is switched by the circuit consisting of Q4, Q8, and their associated components. 12 volts is sourced through IRCM pin 21 and is switched by Q8. Q8's switched output is applied to the AC clutch via IRCM pin 23. IRCM pin 22 is the signal used to control the AC clutch state, and originates from various control points depending on the vehicle's configuration (electronic climate control versus manual climate control).

Operation of the circuit is as follows: R7 is a pull-up resistor that applies 12 volts to IRCM pin 22 whenever the IRCM is activated. 12 volts is also applied to the base of Q4 through current limiting resistor R10. The current flowing into Q4's base turns it on, effectively grounding the collector. R5 is a pullup resistor that applies 12 volts to the base of Q8, keeping it biased in the "off" condition; whenever the Q4 collector goes low, Q8's base is also pulled low by resistor R6 turning Q8 on. Current then flows from the source (IRCM pin 21) into the emitter of Q8 and out of Q8's collector to IRCM pin 23 and on to the AC compressor clutch coil turning it on. Conversely, if the AC clutch control lead (IRCM pin 22) is pulled low (ground) Then Q4 is turned off, likewise turning Q8 off thus turning off the AC compressor clutch.


This document is supplied "as-is" and without any warranty for accuracy or correctness, either expressed or implied. While due care and diligence was used in it's writing, without the direct input of the designer the operation of the IRCM as described herein is "assumed" and "to the best of my knowledge". The author assumes no liability for any damages whatsoever that use of this information may cause. That being said, I'm personally using it, but YMMV!

A R T I C L E   L I N K S

IRCM Overview
Disassembly & Reassembly
Schematic & Parts List

Article text, drawings and photos © 2004 by D. Joseph Frazier and
Rustec Applied Technologies; All Rights Reserved.

Please note that while I've been as careful as possible with this article,
it may contain mistakes. I assume no liability whatsoever for any
detrimental effects using this information may cause to you or your vehicle.