Troubleshooting guide

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Section 9: Air Brake System Troubleshooting Tests

Please follow all standard safety precautions, including, but not limited to, the general

precautions listed on page 4 of this handbook.

Air Brake System Troubleshooting Tests

TEST 1

Governor cut-out / Low pressure warning / Pressure build-up

VEHICLE PARKED, WHEELS CHOCKED

;

Not

1. Drain all the reservoirs to 0 psi.

2. Start the engine and run at fast idle. The low pressure warning should be on. Note: on vehicles equipped with ABS, the

warning lamp will also come on momentarily when the ignition is turned on. On some systems, such as the Bendix

AD-IS

dryer system (illustrated), reservoirs may not ﬁ ll simultaneously and one reservoir may ﬁ ll to 110 psi before the other starts

to ﬁ ll.

3. Low pressure warning; dash warning lamp should go off above 60 psi.

4. Build-up time; pressure should build from 85-100 psi within 40 seconds.

5. Governor cut-out; cuts out at the correct pressure, usually 125-135 psi.

6. Governor cut-in; reduce the service air pressure to governor cut-in. The difference between cut-in and cut-out pressure must

not exceed 30 psi.

MAKE ALL THE NECESSARY REPAIRS BEFORE PROCEEDING TO TEST 2;

SEE CHECKLIST 1 FOR COMMON CORRECTIONS.

CHECKLIST 1

If the low pressure warning lamp or buzzer doesn’t come on:

1. Check the warning lamp wiring.

2. Check the warning lamp bulb.

3. Repair or replace the buzzer, bulb or low pressure warning switch(es).

If the governor cut-out is higher or lower than speciﬁ ed by the vehicle manual:

1. Check the dash gauge with test gauge known to be accurate.

1. Repair, replace, or adjust the governor as necessary after ensuring the compressor unloader mechanism is operating correctly.

If the low pressure warning occurs below 60 psi:

1. Check the dash gauge with test gauge known to be accurate.

2. Repair or replace the faulty low pressure indicator switch.

If the compressor build-up time exceeds 40 seconds (at maximum governed RPM) or is considerably greater than the permanent record

ﬁ gure:

1. Examine the compressor air inlet ﬁ lter and inlet line checking for restrictions, damage or wear. Clean or replace the ﬁ lter or inlet line as

necessary.

2. Check the compressor discharge port and line for excessive carbon. Clean or replace the discharge line as necessary. If there is carbon, ﬁ nd

the cause of the excessive heat.

3. With the system fully charged and governor in the unloaded mode, listen at the compressor inlet for leakage. If leakage can be heard, remove

the unloaders and repair or replace as necessary.

RETEST TO VERIFY PROPER OPERATION OF ALL ITEMS REPAIRED OR REPLACED.