From this information, the computer can then determine the  fuel required for the combustion cycle for either best power  or best economy mode of operation. The computer precisely  times the injection event, and the duration of the injector  should be on time for the correct fuel-to-air ratio. Then, the  computer sets the spark ignition event and ignition timing,  again based on percentage of power calculation. Exhaust  gas temperature is measured after the burn to verify that the  fuel-to-air ratio calculations were correct for that combustion  event. This process is repeated by each computer for its own  assigned cylinder on every combustion/power cycle. The computers can also vary the amount of fuel to control  the fuel-to-air ratio for each individual cylinder to control  both cylinder head temperature (CHT) and exhaust gas  temperature (EGT). An ECU is assigned to a pair of engine cylinders.  [Figure 4-22] The ECUs control the fuel mixture and spark timing for  their respective engine cylinders; ECU 1 controls opposing  cylinders 1 and 2, ECU 2 controls cylinders 3 and 4, and ECU  3 controls cylinders 5 and 6. Each ECU is divided into upper  and lower portions. The lower portion contains an electronic  circuit board, while the upper portion houses the ignition  coils. Each electronic control board contains two independent  microprocessor controllers that serve as control channels.  During engine operation, one control channel is assigned to  operate a single engine cylinder. Therefore, one ECU can  control two engine cylinders, one control channel per cylinder.  The control channels are independent, and there are no shared  electronic components within one ECU. They also operate  on independent and separate power supplies. However, if  one control channel fails, the other control channel in the  pair within the same ECU is capable of operating both its  assigned cylinder and the other opposing engine cylinder as  backup control for fuel injection and ignition timing. Each  control channel on the ECU monitors the current operating  conditions and operates its cylinder to attain engine operation  within specified parameters. The following transmit inputs to  the control channels across the low-voltage harness: All critical sensors are dually redundant with one sensor  from each type of pair connected to control channels in  different ECUs. Synthetic software default values are also  used in the unlikely event that both sensors of a redundant  pair fail. The control channel continuously monitors changes  in engine speed, manifold pressure, manifold temperature,  and fuel pressure based on sensor input relative to operating  conditions to determine how much fuel to inject into the  intake port of the cylinder.  The ignition system consists of the high-voltage coils atop the  ECU, the high-voltage harness, and spark plugs. Since there  are two spark plugs per cylinder on all engines, a six-cylinder  engine has 12 leads and 12 spark plugs. One end of each lead  on the high-voltage harness attaches to a spark plug, and the  other end of the lead wire attaches to the spark plug towers on  each ECU. The spark tower pair is connected to opposite ends  of one of the ECU’s coil packs. Two coil packs are located  in the upper portion of the ECU. Each coil pack generates  a high-voltage pulse for two spark plug towers. One tower  fires a positive polarity pulse and the other of the same coil  fires a negative polarity pulse. Each ECU controls the ignition  spark for two engine cylinders. The control channel within  each ECU commands one of the two coil packs to control  the ignition spark for the engine cylinders.  [Figure 4-23] The high-voltage harness carries energy from the ECU spark  towers to the spark plugs on the engine.  For both spark plugs in a given cylinder to fire on the  compression stroke, both control channels must fire their  coil packs. Each coil pack has a spark plug from each of the  two cylinders controlled by that ECU unit.  The ignition spark is timed to the engine’s crankshaft  position. The timing is variable throughout the engine’s  operating range and is dependent upon the engine load  conditions. The spark energy is also varied with respect to  the engine load. Figure 4-22.  Electronic control unit.  PowerLink Ignition System Electronic Control Unit (ECU) 1.  Speed sensor that monitors engine speed and crank  position 2.  Fuel pressure sensors 3.  Manifold pressure sensors 4.  Manifold air temperature (MAT) sensors 5.  CHT sensors 6.  EGT sensors 4-14