TM 5-3805-294-23-1
0003
HYEX THEORY OF OPERATION - Continued
Engine Electronic Control System - Continued
in the fuel system along with the principle that water is a better conductor than fuel. If water is present, the voltage
will be lower. The ECU monitors this for engine protection purposes.
The engine is equipped with a glow plug cold weather starting aid. When the operator turns the key switch from OFF
to ON, the ECU calculates the fuel temperature, determines if the temperature is below the set point, turns on the
engine preheat indicator light on the monitor controller, and energizes the glow plug relay. The glow plug relay will
in turn energize the glow plugs located above each cylinder. The ECU will keep the glow plugs energized for an
amount of time that is determined by the measured temperature. When the ECU has determined that the preheat
is adequate, it will turn off the Wait to Start light and de-energize the glow plug relay. If the operator turns the ignition
from ON to START at this time, the engine will crank and start.
If the operator turns the key switch from ON to START without waiting for the engine preheat indicator light to turn
off, the ECU will de-energize the glow plug relay and a key off and key on cycle will be required before preheating
is allowed again.
Anytime the engine cranks but does not start, a key off and key on cycle will be required before preheating is allowed
again.
After the engine has started, the glow plugs will reheat for an additional set amount of time. When the reheat
sequence is complete, the ECU monitors the engine speed for fluctuations, which may indicate the engine is still too
cold to run properly. If the engine speed fluctuations exceed a predetermined limit, the glow plugs will be re-energized
for a set amount of time. The ECU will continue to activate the glow plugs and monitor engine speed fluctuations for
up to 120 seconds. The glow plugs are deactivated when the ECU is measuring so that engine speed fluctuations
will avoid false readings.
Table 1.
Preheat/Reheat Times.
Fuel Temperature
Preheat Time
Reheat Time
-22F (-30C)
15 seconds
10 seconds
-13F (-25C)
15 seconds
10 seconds
-4F (-20C)
11 seconds
10 seconds
5F (-15C)
7 seconds
10 seconds
14F (-10C)
4.5 seconds
10 seconds
23F (-5C)
2.2 seconds
10 seconds
32F (0C)
0 seconds
0 seconds
Table 2. Engine Speed Control System Operation.
Item
Measurement
Specification
Engine Speeds
Slow Idle
Speed
750850 rpm