UNIT TROUBLESHOOTING INTRODUCTION - CONTINUED
Voltage measurements. The HET truck is equipped with both 12 vdc and 24 vdc circuits. Troubleshooting
procedures will reference 12 vdc and 24 vdc measurements, however, these values can vary. When the bat-
teries are fully charged, 12.6 vdc can be measured on an open 12 volt circuit and 14.5 vdc can be measured
when the engine is running at 1000 rpm. When the batteries are fully charged, 25.2 vdc can be measured
on an open 24 volt circuit and 29 vdc can be measured when the engine is running at 1000 rpm.
(a) Connect the red test lead to the volt-ohm input connector and the black lead to the COM input on the
meter. If a DC-AC switch is present, make sure it is set to the DC position.
(b) Set the function/range switch to the desired volts position. If the magnitude of the voltage is not
known, set the switch to a range which will be able to read most voltages seen on the truck (typically,
a 200V range will do). Then reduce the range until a satisfactory reading is obtained.
(c) Connect the test leads to the circuit being measured. Following the voltage measurement point, the
color test lead tube used is given in parenthesis (red is volt-ohm connection and black is the COM
A/C system performance checks. Troubleshooting procedure for the HET tractor includes system perfor-
mance checks. A properly functioning A/C system will have the following characteristics.
(a) The A/C control switch is set to the desired A/C blower speed position (low, medium, or high).
(c) The A/C compressor suction line is cool.
(f) The A/C hose between the A/C condenser and A/C receiver dryer is warm.
(g) The A/C receiver/dryer is at outside temperature.
(h) The A/C binary switch is closed.
(i) The inlet to the A/C expansion valve is warm.
(j) The A/C evaporator is cold.
(l) The A/C discharge air temperature is approximately 20°F cooler than the ambient air temperature.
(m) The A/C high pressure switch closes and activates the A/C engine fan control solenoid when system
high side pressure reaches 300 psi.
Wire repair. Refer to TM 9-2320-360-20 for the repair of wire harness connectors. Refer to TM 43-0158 for
detailed instructions concerning electrical wiring repairs. Wire harness repair is limited to splicing and taping of
wires at Unit Maintenance. If a wire harness cannot be repaired, notify DS Maintenance.
Abbreviations and Commonly Used Terms.
A/C - Air Conditioner: either component, subsystem, or system.
ACC - Accessory.
BAT - Battery.
CKT - Circuit.
COM - Common.
ECB - Electronic Control Box Assembly.
Erratic - Intermittent.
mag - Magnetic