Inspection of the Air Dryer
- Park the vehicle on a level surface, shut down the engine, and set the parking brake. Chock the tires.
- Check for moisture in the air brake system by opening the reservoir drain cocks. Examine
the discharge. Some trace of water in the discharge is normal. A discharge of a milky
gray liquid indicates excessive moisture is present in the air system and the desiccant
cartridge needs to be replaced. See Group 42 of the vehicle workshop manual for troubleshooting
procedures. The following conditions could cause small amounts of moisture to be found
in the air system:
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An outside air source has been used to charge the air brake system. This air did not pass through the drying bed.
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Air usage is unusually high and not normal. This may be due to accessory air demands or some unusual air requirement that does not allow the compressor to load and unload in a normal fashion. Check for air system leaks.
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The air dryer has been installed on a system that had previously been operated without an air dryer. This type of system will be saturated with moisture, and several weeks of operation may be needed to fully dry the system.
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The temperature range in your area fluctuates more than 30 degrees in one day. Small amounts of water can accumulate in the air brake system due to condensation. Under these conditions, the presence of moisture is normal and should not be considered an indication of poor air dryer performance.
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- Inspect the air dryer for external damage, and check that the unit is tightly mounted on the frame. If the air dryer mounting is loose, see Group 42 of the vehicle workshop manual for mounting instructions.
- Turn on the ignition switch, but do not start the engine.
- Disconnect the harness connector from the air dryer.
- Connect a voltmeter to circuit 94 on the air dryer's harness connector and to a body ground point. Check for battery voltage. If voltage is not found, check and repair the open or short to ground in wire 94.
- Connect a voltmeter between the two terminals on the air dryer's harness connector. If voltage is not found, repair the open in the ground circuit wiring.
- Use an ohmmeter to check resistance between the terminals on the end cover. See Fig.. The end cover contains the heater/thermostat. If the heater/thermostat is between
40 and 90°F (5 and 32°C), the heater/thermostat measured resistance will be infinite
ohms. If the measured resistance is less than infinity, replace the heater/thermostat.
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End Cover Assembly
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Heater/Thermostat Terminals
Fig. 1, End Cover Terminals Location
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- Remove the heater/thermostat assembly and place it in a freezer for about five minutes to cool it below 40°F (5°C). Remove the cold heater/thermostat assembly and check resistance between the heater/thermostat terminals. Resistance should be 1.5 to 3.0 ohms. As the temperature of the heater/thermostat assembly increases, the ohm reading will increase. When the temperature goes above 45°F (7°C) measured resistance should be infinite ohms. If the heater/thermostat does not operate as outlined, replace it. If it does operate as outlined, install it.