Test 2A - Modulator Circuits
Modulator Circuits:
NOTE: You must perform Test 1A before proceeding with this test. Test 1A - Checking For Diagnostic Trouble Codes
MODULATOR CIRCUIT
When Its Monitored
During the drive-off cycle, or five seconds after key on and anytime thereafter until the next ignition reset. The valves are tested about every half second while in antilock mode and every 4 seconds in normal driving mode.
Set Condition
The modulator is a group of four solenoid valves controlled by the Controller Antilock Brake (CAB) to modulate brake pressure during Antilock Brake System (ABS) control. There are four build/decay valves, one for each wheel. During normal braking (non-ABS control), the solenoids are functionally invisible to the brake system. Modulator power comes from the solenoid drivers in the CAB and is activated by command of the CAB for ABS control. The solenoid drivers are powered through the system relay when energized. In this way, ABS can be disabled by deenergizing the system relay.
ABS Disabled
Upon detection, the ABS warning lamp is illuminated and the Modulator Circuit trouble code is set. ABS function is disabled and the lamp is illuminated until the next ignition reset.
Possible Causes:
^ LF decay solenoid control circuit open, shorted to ground, or shorted to B+
^ RF decay solenoid control circuit open, shorted to ground, or shorted to B+
^ LR decay solenoid control circuit open, shorted to ground, or shorted to B+
^ RR decay solenoid control circuit open, shorted to ground, or shorted to B+
^ Open or short in any of the four decay solenoids
^ CAB failed
^ Wiring harness connector terminals spread or pushed out
SOLENOID CIRCUIT OPEN (ALL WHEELS)
When Its Monitored
Any time the system relay is energized, system relay output voltage must be above 9 volts and below 20 volts. The static solenoid test is conducted only when not in ABS cycling. It runs once each 630 milliseconds. The dynamic solenoid test is run every minute when not in ABS cycling, and every 620 milliseconds when cycling. Each solenoid is tested separately at 5 milliseconds intervals, with a complete test taking 20 milliseconds to complete.
Set Condition
The modulator is a group of four solenoid valves controlled by the CAB to modulate brake pressure during ABS control. One solenoid valve controls each wheel. Solenoid power is supplied through the CAB's internal solenoid drivers. The CAB receives power for the solenoids from the system relay located in the relay box. The CAB monitors each driver circuit when it is off and will store a trouble codes if the solenoid voltage is above 2 volts during a static test or above a calculated threshold referenced to the system relay output voltage during a dynamic test.
Possible Causes:
^ Open decay solenoid control circuit
^ Open decay solenoid coil
^ Failed solenoid diver
^ Loose or corroded connector terminals
^ Decay solenoid control circuit shorted to B+
^ High resistance in the decay solenoid circuit
SOLENOID CIRCUIT SHORTED (ALL WHEELS)
When Its Monitored
Any time the system relay is energized, system relay output voltage must be above 9 volts and below 20 volts. The solenoid channels are tested every minute when not in ABS cycling, and every 620 milliseconds when in cycling. Each solenoid os tested separately at 5 milliseconds intervals, with a complete test taking 20 milliseconds to complete.
Set Condition
The modulator is a group of four solenoid valves controlled by the CAB to modulate brake pressure during ABS control. One solenoid valve controls each wheel. Solenoid power is supplied through the CAB's internal solenoid drivers. The CAB receives power for the solenoids from the system relay located in the relay box. The CAB monitors each driver circuit when it is off and will store a trouble codes if the solenoid voltage is below a calculated threshold referenced to the system relay output voltage.
Possible Causes:
^ Decay solenoid control circuit shorted to ground
^ Shorted solenoid coil
^ Failed solenoid driver
TEST PROCEDURE
1. With the DRB, select "Erase DTC's."
2. Turn the ignition off, then turn the ignition on.
3. With the DRB, select "Miscellaneous" and Bleed Brakes". Actuate the Bleed Brake selection.
4. With the DRB, select "Read DTC's." Refer to the chart below and perform the appropriate test.
Problem Area And Test
Modulator Circuit: Test 2B - Modulator Circuits
LF Solenoid Circuit Shorted: Test 21A - Left Front Solenoid Circuit
LF Solenoid Circuit Open: Test 21A - Left Front Solenoid Circuit
RR Solenoid Circuit Shorted: Test 22A - Right Rear Solenoid Circuit
RR Solenoid Circuit Open: Test 22A - Right Rear Solenoid Circuit
LR Solenoid Circuit Shorted: Test 23A - Left Rear Solenoid Circuit
LR Solenoid Circuit Open: Test 23A - Left Rear Solenoid Circuit
RF Solenoid Circuit Shorted: Test 24A - Right Front Solenoid Circuit
RF Solenoid Circuit Open: Test 24A - Right Front Solenoid Circuit
5. If no trouble codes return, refer to the schematic. Inspect the wiring harness and connectors of the modulator circuits.
6. If a problem is found, repair as necessary and perform Test VER1A. If no trouble codes return, perform Test VER2A.
^ Verification Test VER-1A - ABS
^ Verification Test VER-2A - (Road Test) - ABS