DIRECT SUPPORT TROUBLESHOOTING INTRODUCTION - CONTINUED
Voltage measurements. The HET Tractor is equipped with both 12 vdc and 24 vdc circuits. Troubleshoot-
ing procedures will reference 12 vdc and 24 vdc measurements, however these values can vary. When the
batteries are fully charged, 12.6 vdc can be measured on an open 12 volt circuit and 14.5 vdc can be mea-
sured when the engine is running at 1000 rpm. When the batteries are fully charged, 25.2 vdc can be mea-
sured 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 200-volt 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 parentheses (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).
(b) The A/C compressor clutch engages and cycles with A/C evaporator temperature.
(c) The A/C compressor suction line is cool.
(d) The A/C compressor discharge line is hot.
(e) The A/C condenser is hot/warm.
(f) The A/C hose between the A/C condenser and A/C receiver/dryer is warm.
(g) The A/C receiver/dryer is warm.
(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.
The following is a brief description of some of the symptoms or conditions that may exist if a component
fails in the refrigerant circuit of the A/C system:
(a) If the A/C receiver/dryer moisture indicator is pink or white, the A/C receiver/dryer is saturated with
moisture and must be replaced.
(b) A/C compressor failures will show up as abnormal noise, seizure, leakage, or low suction and dis-
(c) A faulty or improperly installed A/C thermostatic switch may cause quick or delayed A/C compressor
(d) A blockage in the A/C condenser coil will cause high system high side pressure. Frost may form at the
location of the blockage.
(e) The A/C receiver/dryer is normally at outside temperature. If the A/C receiver/dryer is cool or cool
spots form, a blockage has formed in the A/C receiver/dryer.
(f) A blockage at the inlet of the A/C receiver/dryer will cause high system high side pressure.