antilock brake system

The Delphi DBC-7 ABS antilock brake system was first used in 1999 on 1999 Buick Regal and Century, and Chevrolet Tracker. The system replaced the earlier Delphi ABS-VI (which has been around since 1991 on many General Motors passenger cars). For model year 2000, the new DBC-7 system was added to the Chevrolet Impala, Monte Carlo, Malibu, Cavalier and Venture, Pontiac Sunfire and Montana, and Oldsmobile Cutlass and Silhouette.

Like other late model ABS systems, DBC-7 is a compact, lightweight, low-cost ABS system that combines the controller and valve body in one assembly. It is a nonintegral, four-wheel system for either three or four channel operation, and offers traction control capability.

DBC-7 is totally different than the ABS-IV system GM has used on so many of its passenger cars. Gone are the gear pack and motor driven pistons. DBC-7 uses solenoid valves like other ABS systems manufactured by Bosch, Kelsey-Hayes and Teves, and the controller, valves, solenoids and pump are combined into one compact unit.

The valves and pump are mounted in a metal block called the "Brake Pressure Modulator Valve" or BPMV. This is bolted to the "Electronic Brake Control Module" or EBCM that houses the controller, relay and solenoids that actuate the ABS valves. The unit can be taken apart so the BPMV or EBCM can be replaced separately if needed.

The Brake Pressure Modulator Valve has one inlet (apply) valve and one outlet (release) valve for each brake channel, plus two accumulators (one for each brake circuit). So the BPMV contains a total of six ABS valves for three channel system applications, eight ABS valves for four channel systems, or 10 ABS valves if the vehicle has TCS traction control. The inlet valves are normally open, and the outlet valves are normally closed. The solenoid for each valve is energized when the EBCM provides a ground to complete the circuit.

The GM passenger car applications are all four channel ABS systems, so each of the four BPMV outlets is connected to the brake line for a separate wheel. The lines are color coded for identification: left rear is purple, right rear is yellow, left front is red, and right front is green.

TRACKER SYSTEMThe Tracker is a three channel system, so the BPMV has only three outlets. Two are connected to each front wheel while the third serves both rear wheels.

Additional system components include a brake pedal switch that closes when the pedal is depressed to signal the controller the brakes are being applied. Input from the brake pedal switch is not required to trigger an ABS stop on the passenger car applications. But if the switch is closed (pedal depressed) it will disengage traction control.

The Tracker also has a "longitudinal accelerometer sensor" located inside the parking brake lever bezel. The sensor monitors the fore and aft motions of the vehicle so the controller can vary the operation of the ABS to suite different braking conditions. The sensor receives a 5 volt reference signal from the EBCM, and returns an acceleration signal to the EBCM. When the sensor's voltage goes high, the EBCM decreases front brake pressure to limit front end diving.

The Tracker also has a "four-wheel drive switch" for changing ABS operation when the vehicle is being driven in the four-wheel drive mode.

DBC-7 BRAKE PROPORTIONINGSomething you won't find on a DBC-7 equipped vehicle is a brake proportioning valve. DBC-7 uses the ABS solenoids to reduce pressure to the rear brakes during normal braking to maintain proper brake balance. This is done by monitoring front and rear wheel speeds, and reducing pressure to the rear brakes as needed when there's a difference in wheel deceleration rates.

The controller energizes the inlet valve solenoids for both rear brakes to hold pressure in the lines, and then energizes both rear outlet valve solenoids to release pressure as needed. The pump may also run to clear the accumulators if a sufficient number of release cycles are required.

The "dynamic" rear proportioning function is enabled at all times unless there is a failure of the EBCM or two wheel speed sensors on the same axle both fail simultaneously (a code C1248 will be set if the system is disabled). But as long as there is at least one functional speed sensor on the front and rear axles, the EBCM can compare the relative speeds of the front and rear wheels.

Another new feature of the DBC-7 is the ability to detect low tire pressure. A tire that is underinflated will have a slightly smaller rolling radius than one that is properly inflated. This will create a difference in the wheel speed sensor reading if the difference in inflation pressure is 12 or more pounds. The EBCM will then turn on the "Low Tire Pressure" warning lamp to warn the driver his tires need attention.

DBC-7 DIAGNOSTICSWhen the ignition is switched on, the warning lamps are all illuminated for a few seconds for a bulb check, then turned off. The solenoids are also commanded on and off, as is the pump motor. The EBCM looks for proper component operation and does a self-check of its own internal circuits.

As soon as vehicle speed reaches 10 mph, a dynamic test occurs if the brake pedal is depressed. The driver may notice pedal movement while applying the brake and a slight loss of speed as the system checks itself. The operation of the wheel speed sensors is also monitored continuously while driving.

If a fault is detected, a code will be stored in the EBCM's memory, the ABS warning lamp will be illuminated and the ABS system will be deactivated for the remainder of the drive cycle. The red brake warning light may also come on if there is a possibility that the ABS malfunction may affect dynamic rear proportioning.

If the malfunction is no longer present the next time the vehicle is driven, the code will become a history code and the warning lamp will not come back on. Codes will be automatically cleared from memory if no codes are set off during 100 consecutive drive cycles.

There are NO flash codes available with this system so a scan tool such as a GM Tech 1, Vetronix MasterTech or similar scan tool with ABS software is required to read and clear stored codes. The DBC-7 system also provides "enhanced" data which can be read through the scan tool, including:
ï؟½Brake switch status;
ï؟½TCS switch status;
ï؟½ABS activity status;
ï؟½ETC/TCS activity status;
ï؟½Vehicle speed; and
ï؟½Number of drive cycles since a code was set.

DBC-7 FAULT CODESIf a fault code is present, refer to the appropriate diagnostic chart to isolate the faulty component in the circuit or to verify the failure.

When checking ground connections, EBTCM ground resistance should not be more than 5 ohms. Resistance across the pump motor terminals should be less than 2 ohms.

Wheel speed sensors should generate a signal over 100 mV when a wheel is rotated by hand at a speed of one evolution per second.

RELAY & PUMP RELATED CODESC1214 System relay contact or coil circuit open
C1217 Pump motor short to ground (GM cars)
C1217 BPMV pump motor relay circuit open (Tracker)
C1218 Pump motor circuit shorted to voltage
C1242 Pump motor circuit open
C1243 BPMV pump motor stalled

WHEEL SPEED SENSORSC1221 Left front wheel speed is zero
C1222 Right front wheel speed is zero
C1223 Left rear wheel speed is zero
C1224 Right rear wheel speed is zero
C1225 Left front excessive speed variation
C1226 Right front excessive speed variation
C1227 Left rear excessive speed variation
C1228 Right rear excessive speed variation
C1232 Left front WSS circuit open or shorted
C1233 Right front WSS circuit open or shorted
C1234 Left rear WSS circuit open or shorted
C1235 Right rear WSS circuit open or shorted

MODULE CODESC1236 Low system supply voltage
C1237 High system supply voltage
C1238 Brake thermal model exceeded (brakes hot)
C1241 Variable effort steering circuit malfunction
C1245 Low tire pressure detected
C1247 Low brake fluid detected
C1254 Abnormal shutdown detected
C1255 EBCM internal malfunction
C1261 Left front inlet valve solenoid
C1262 Left front outlet valve solenoid
C1263 Right front inlet valve solenoid
C1264 Right front outlet valve solenoid
C1265 Left rear inlet valve solenoid
C1266 Left rear outlet valve solenoid
C1267 Right rear inlet valve solenoid
C1268 Right rear outlet valve solenoid
C1272 Left front TCS valve solenoid
C1274 Right front TCS valve solenoid

DYNAMIC REAR PROPORTIONING (DRP) CODESC1248 DRP function disabled (set by combinations of other codes that affect DTC operation)

TRACTION CONTROL CODESC1275 PCM requested ETS/TCS to be disabled (engine problem)
C1276 Delivered torque signal circuit (serial data problem)
C1277 Requested torque signal circuit (serial data problem)
C1278 TCS temporarily inhibited by PCM (engine problem)

BRAKE SWITCH CODESC1291 Open brake lamp switch during deceleration
C1293 Code C1291 set in previous ignition cycle
C1294 Brake lamp switch circuit always active
C1295 Brake lamp switch circuit open

TRACKER CODESC1215 Idle up circuit malfunction
C1257 Acceleration sensor malfunction
C1258 4WD switch malfunction

CLASS 2 COMMUNICATION CODESC1298 PCM class 2 serial data link malfunction
U1016 Loss of communication with PCM
U1255 Serial data link malfunction
U1300 Class 2 short to ground
U1301 Class 2 short to voltage

DBC-7 BLEEDING PROCEDURESNormal bleeding procedures can be used to bleed the individual brake lines provided no air has entered the ABS modulator. But if air has entered the modulator, the system will have to be bled using a pressure bleeder and scan tool.

To bleed the system:
1. Connect the pressure bleeder to the master cylinder reservoir.
2. Turn the ignition on.
3. With bleeder screws closed, pressurize the system to 35 psi.
4. Select the "Automatic Bleed Procedure" on the scan tool. The scan tool will then energize and cycle the ABS solenoids in the BPMV for one minute.

After this, the scan tool will instruct you to bleed each wheel. During this step the pump runs and the respective release valve cycles for one minute. This is repeated for each wheel. Finally, the scan tool performs a final 20 second cycling of the ABS solenoids to purge any remaining air.

5. Relieve pressure at the bleed tool and remove it from the master cylinder. Then check pedal height and feel to make sure all the air has been removed and the pedal is firm