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10-8

Chapter 10

– Component Repair or Replacement

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Table of Contents

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Index

Problem Activating AC Clutch Circuit.

Inspect the low pressure switch located on the air block. The switch

is normally closed. It must have air pressure to disengage the clutch.

If there is no power to the clutch proceed as follows.

1.

Make sure there is power to the pressure switch—use a

voltmeter or trouble light to check.

2.

Make sure there is air to the controls.

3.

If there is air and power available, place a jumper across the

two terminals on the low pressure switch. If the clutch en-

gages, there is a problemwith the pressure switch or air block.

4.

Remove the switch from the air block, move the lever to heat

mode. If air exhausts from the port, replace the pressure switch.

If no air is vented from the port, the air block is defective.

5.

If the air block and pressure switch check out OK, inspect

AC system clutch, thermostat, Trinary™ switch, or other

pressure switches, and wiring.

CTC™ (Heater) and CTC™ II (Heater–A/C System)

The constant temperature control systems maintain a pre-selected cab

temperature by pulsating the flow of hot engine coolant through the

heater core. In the CTC II configuration the compressor is cylced to

control temperature in the AC mode. The four major components of the

system are illustrated in Figure 10-3 and described as follows:

TRUCK

AIR IN

AC MODE

PRESSURE

SWITCH

TO ACTIVATOR

AIR TO

CONTROL PANEL

E/C TO

CONTROL PANEL

CONTROL PANEL

VARIABLE RESISTOR

HOT WATER IN

FREON

COMPRESSOR

AIR

EVAPORATOR

EXPANSION

VALVE

THERMOSTAT

AIR OPERATED

WATER VALVE

AIR

TEMPERATURE

SENSOR

DEFROST

HEATER CORE

TREATED

AIR

COOL

HEAT

CONDENSER

RECEIVER-DRIER

SOLENOID

ELECTRONIC

CONTROL

MODULE

E/C

Circuit Board Assembly—

The assembly is located on the heater/AC

unit. It supplies power to resistor and solenoid, and monitors air output

temperature.

Variable Resistor—

Located on the control panel and used with tem-

perature control lever to select desired temperature. It translates the

desired temperature into a control voltage for the circuit board.

Figure 10-3

The CTC II system is shown

in this drawing.