412-04 Control Components: Description And Operation (page 12 of 21)

During the engine warm-up period, the ATCM manages the electrical load to make sure that the battery voltage is 
maintained above a pre-determined level. The battery voltage level that is maintained and the duration of the start period 
varies with ambient air temperature and engine coolant temperature. After the engine warm-up period, the ATCM 
manages the electrical load to make sure that the requested electrical load does not exceed the generator output. 
 
The duration of the engine warm-up period depends on the ambient air temperature and the engine coolant temperature 
when the ignition is switched on, as detailed in the following table: 
 

Engine Warm-up Times 

 
The ATCM calculates the electrical load from the battery voltage and generator output voltage, and compares the result 
against the maximum load available from the generator. The calculation is averaged across the first 20 seconds after the 
engine starts, and subsequently averaged every 60 seconds. When the ignition is turned off, the ATCM stores the status 
of the electrical load management for 20 seconds. If the engine is re-started within the 20 seconds, the ATCM resumes 
electrical load management using the stored status. If the engine is re-started after the 20 seconds, the timers are reset 
and the ATCM re-calculates the status. 
 
If the electrical load is more than the maximum load available, the ATCM requests an increase of engine idle speed using 
the medium speed CAN bus message to the ECM. If an electrical load imbalance remains after an increase in engine idle 
speed, or if the electrical load is more than the capacity of the charging system, the ATCM reduces the electrical load by 
reducing the power of some vehicle systems or inhibiting their operation. The number of systems controlled depends on 
the electrical load reduction required. The systems controlled, and the order in which their power is reduced or they are 
inhibited, are contained in three priority tables. The table used depends on the ambient air temperature, battery 
temperature and engine coolant temperature: 
 

The cold start table is used when the ambient air temperature is less than 5 °C (41 °F) and the engine coolant 
temperature is less than 30 °C (86 °F).  
The hot start table is used when the ambient air temperature is 5 °C (41 °F) or more and the engine coolant 
temperature is less than 30 °C (86 °F).  
The continuous table is used when battery temperature is more than 5 °C (41 °F) and the engine coolant 
temperature is more than 50 °C (122 °F).  
If none of above conditions are met, the ATCM adopts the last used table.  

Cold Start Electrical Load Management 

Ambient Air Temperature, °C (°F)  Engine Coolant Temperature, °C (°F)

<10 (<50) 

>10 to <30 (>50 to <86) 

>30 to <60 (>86 to <140)  >60 (>140)

Warm-up Period, Minutes 

>10 (>50) 

15 

15 

15 

15 

>5 to <10 (>41 to <50) 

15 

15 

15 

15 

>0 to <5 (>32 to <41) 

10 

15 

15 

15 

>-10 to <0 (>14 to <32) 

10 

10 

15 

15 

<-10 (<14) 

5 

5 

10 

15 

Priority 

System 

Power Reduction  Inhibited 

1 

- 

Air suspension 

2 

- 

Front seat heaters 

3 

- 

Entertainment system 

- 

4 

Front seat heaters 

5 

- 

Auxiliary climate control blower 

6 

- 

Rear window heater 

7 

- 

Windshield washer jet and exterior mirror heaters 

- 

8 

Windshield washer jet and exterior mirror heaters 

9 

- 

Windshield heater