The auxiliary air compressor supplies air for the exhaust brake, air seat, and air horns if equipped. The compressor is energized by a relay that is controlled by the auxiliary air tank pressure switch located in the auxiliary air tank. When the auxiliary air tank pressure switch is closed it supplies voltage to the auxiliary air compressor relay. The relay energizes and supplies battery voltage to the auxiliary air compressor. When air pressure builds to 724-827 kPa (105-120 psi) the switch opens and the auxiliary air compressor shuts off.

Thoroughly inspect the auxiliary air compressor system and related components/accessories for a leak before performing diagnostics.

Schematic Reference

Auxiliary Air Compressor Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Auxiliary Air Compressor Description and Operation

Electrical Information Reference

1. Ignition OFF, disconnect the auxiliary air tank pressure switch.
2. Ignition ON, Connect a test lamp between the pressure switch connector terminal A and ground. Verify the test lamp illuminates.

⇒	If the test lamp does not illuminate, test the air pressure switch ignition voltage circuit for a short to ground or an open/high resistance.

3. Disconnect the air compressor relay. Connect a test lamp between the relay connector terminal 86 and ground. Verify the test lamp does not illuminate.

⇒	If the test lamp illuminates, test the air pressure switch signal circuit for a short to voltage.

4. Connect a 3 amp fused jumper between terminals A and B of the air pressure switch connector. Verify the test lamp illuminates.

⇒	If the test lamp does not illuminate, test the air pressure switch signal circuit for a short to ground or an open/high resistance

5. Disconnect the 3 amp fused jumper.
6. Connect a test lamp between the relay connector terminal 30 and ground. Verify the test lamp illuminates.

⇒	If the test lamp does not illuminate, test the air compressor relay B+ circuit for a short to ground or an open/high resistance

7. Test for less than 1.0 ohm between the relay ground circuit terminal 85 and ground.

⇒	If greater than the specified range, test the ground circuit for an open/high resistance

8. Disconnect the air compressor. Test for less than 1.0 ohm between the ground circuit terminal B and ground.

⇒	If greater than the specified range, test the ground circuit for an open/high resistance

9. Reconnect the air compressor relay.
10. Connect a test lamp between terminals A and B or the air compressor connector. Verify the test lamp does not illuminate.

⇒	If the test lamp illuminates, test the air compressor relay output circuit for a short to voltage. If the circuit tests normal, replace the air compressor relay.

11. Connect a 3 amp fused jumper between terminals A and B of the air pressure switch connector. Verify the test lamp illuminates.

⇒	If the test lamp does not illuminate, test the air compressor relay output circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the air compressor relay.

12. Disconnect the 3 amp fused jumper. Reconnect the air compressor connector.
13. Connect a 3 amp fused jumper between terminals A and B of the air pressure switch connector. Verify the air compressor is activated.

⇒	If the air compressor does not activate, replace the air compressor.

14. If all circuits test normal, replace the air pressure switch.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Auxiliary Air Compressor Replacement