GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 1998 🢒 Monte Carlo 🢒 Brakes 🢒 Antilock Brake System 🢒 DTC C1251 Rear ABS Motor Free Spins

Object Number: 215275 Size: SF

Circuit Description

DTC C1251 detects a stripped nut or gear assembly during initialization. During the homing sequence, the piston should reach the top of the bore. Then the ABS motor stalls. If the ABS motor does not stall, the ABS motor must be spinning with little or no resistance. This indicates a nut/screw or gear malfunction.

Conditions for Setting the DTC

DTC C1251 can only set during initialization.

A malfunction exists if the feedback current is less than the command current for a specified period of time.

Action Taken When the DTC Sets
    • A malfunction DTC stores.
    • The ABS disables.
    • The amber ABS warning indicator turn on.
    • The red BRAKE warning indicator turns on.

Conditions for Clearing the DTC
    • The condition responsible for setting the DTC no longer exists and the Scan Tool Clear DTCs function is used.
    • 100 drive cycles pass with no DTC(s) detected.

Diagnostic Aids

The following conditions may cause an intermittent malfunction if the conditions exist in a mechanical part of the system:

    • Sticking
    • Binding
    • Slipping

Use the enhanced diagnostic function of the Scan Tool in order to measure the frequency of the malfunction. Refer to the Scan Tool manual for the procedure. DTC C1251 may set after modulator disassembly if the modulator pistons are positioned at the bottom of the modulator piston bore. If DTC C1251 only occurs once and DTC C1264 also occurs, refer to DTC C1264. If intermittent and enhanced diagnostics shows this DTC occurs during ABS, refer to DTC C1264.

Depending on the frequency of the malfunction, a physical inspection of the mechanical parts suspected of causing the malfunction may be necessary.

Clear the DTCs after completing the diagnosis. Test drive the vehicle for three drive cycles in order to verify that the DTC does not reset. Use the following procedure in order to complete one drive cycle:

  1. Start the vehicle.
  2. Drive the vehicle over 16 km/h (10 mph).
  3. Stop the vehicle.
  4. Turn the ignition to the OFF position.

Important: Zero the J 39200 test leads before making any resistance measurements. Refer to the J 39200 user's manual.

Step

Action

Value(s)

Yes

No

1

Was the Diagnostic System Check performed?

--

Go to Step 2

Go to Diagnostic System Check

2
  1. Turn the ignition switch to the RUN position.

    2. Do not start the engine.

  2. Use the Scan Tool in order to read the DTCs.

Does DTC C1261 also set as a current DTC?

--

Go to DTC C1264 Rear ABS Motor Circuit Open

Go to Step 3

3
  1. Pump the brake pedal until it is firm in order to deplete the brake booster vacuum reservoir.
  2. Select Special Functions on the Scan Tool .
  3. Select the Manual Control function on the Scan Tool .
  4. Select the rear ABS Motor Apply function on the Scan Tool .
  5. Use the Scan Tool in order to apply the rear ABS motor.

    6. Observe the following currents:

    • The rear ABS motor command current
    • The rear ABS motor feedback current

Were the rear ABS motor command current and the rear ABS motor feedback current approximately the specified current?

10 A

Go to Step 4

Go to Step 6

4
  1. Apply firm pressure to the brake pedal.
  2. Select the rear ABS Motor Release function on the Scan Tool .
  3. Use the Scan Tool in order to release the rear ABS motor.

    4. Observe the following currents:

    • The rear ABS motor command current
    • The rear ABS motor feedback current
  4. Release the rear ABS motor and observe the brake pedal height.

Have the following conditions been met?

    • The rear ABS motor command current and the rear ABS motor feedback current approximately the specified current
    • The brake pedal falls

6 A

Go to Step 5

Go to Step 6

5
  1. Continue to apply firm pressure to the brake pedal.
  2. Select the rear ABS Valve Relay function on the Scan Tool .
  3. Use the Scan Tool in order to apply the rear ABS motor.

Does the brake pedal rise?

--

Go to Step 22

Go to Step 6

6
  1. Select Special Functions on the Scan Tool .
  2. Use the Scan Tool in order to perform the Gear Tension Relief function.
  3. Turn the ignition switch to the OFF position.
  4. Disconnect the 6-way ABS brake motor pack connector.
  5. Disconnect the 8-way EBCM connector C2
  6. Use the J 39200 in order to measure the resistance between the 6-way ABS brake motor pack connector terminal D and the 8-way EBCM connector C2 terminal F.

Is the resistance less than or equal to the specified value?

1.5 ohms

Go to Step 7

Go to Step 15

7

Use the J 39200 in order to measure the resistance between the 6-way ABS brake motor pack connector terminal C and the 8-way EBCM connector C2 terminal E.

Is the resistance less than or equal to the specified value?

1.5 ohms

Go to Step 8

Go to Step 16

8

Use the J 39200 in order to measure the resistance between the 6-way ABS brake motor pack connector terminal C and the 6-way ABS brake motor pack connector terminal D.

Is the resistance less than or equal to the specified value?

1.5 ohms

Go to Step 9

Go to Step 17

9
  1. Remove the ABS brake modulator/master cylinder assembly from the vehicle.
  2. Remove the ABS brake modulator gear cover.
  3. Inspect for a stripped ABS brake motor pack rear gear.

    4. (The ABS brake motor pack is the unit with the three small gears. The ABS brake motor pack rear gear is the middle gear when the ABS brake modulator is mounted in the vehicle.)

Is the ABS brake motor rear gear stripped?

--

Go to Step 17

Go to Step 10

10

Inspect for a stripped ABS brake modulator rear gear.

(The ABS brake modulator unit is the unit with the three large gears. The ABS brake modulator rear gear is the middle gear when the ABS brake modulator is mounted in the vehicle.)

Is the ABS brake modulator rear gear stripped?

--

Go to Step 18

Go to Step 11

11
  1. Reconnect the 6-way ABS brake motor pack connector.
  2. Reconnect the 8-way EBCM connector C2.
  3. Carefully secure the ABS brake modulator/master cylinder in the vehicle.

    4. Secure the assembly in a position that permits observation of the gears.

  4. Turn the ignition switch to the RUN position.

    5. Do not start the engine.

  5. Select the Manual Control function on the Scan Tool .
  6. Select the rear ABS Motor Apply function on the Scan Tool .
  7. Use the Scan Tool in order to apply the rear ABS motor while observing the ABS brake modulator rear gear set.

    8. (The ABS brake modulator rear gear is the middle gear when the ABS brake modulator is mounted in the vehicle.)

  8. Select the rear ABS Motor Release function on the Scan Tool .
  9. Use the Scan Tool in order to release the rear ABS motor while observing the ABS brake modulator rear gear set.
  10. Select the rear ABS Motor Apply function on the Scan Tool .
  11. Use the Scan Tool in order to apply the rear ABS motor while observing the ABS brake modulator rear gear set.

Does the ABS brake modulator rear gear set move at least one full revolution in both directions?

--

Go to Step 12

Go to Step 17

12
  1. Use the Scan Tool in order to perform the Gear Tension Relief function.
  2. Turn the ignition switch to the OFF position.
  3. Separate the ABS brake motor pack from the ABS brake modulator.
  4. Grasp the ABS brake modulator rear gear.
  5. Rotate the ABS brake modulator rear gear.

Does the ABS brake modulator rear gear rotate more than 8 full turns from lock to lock?

--

Go to Step 19

Go to Step 13

13
  1. Disconnect the 6-way ABS brake motor pack connector.
  2. Disconnect the 8-way EBCM connector C2.
  3. Inspect the connectors for the following conditions:
    4. • Poor terminal contact
    • Corrosion

Do any of the terminals exhibit poor terminal contact or corrosion?

--

Go to Step 20

Go to Step 14

14
  1. Reinstall the ABS brake modulator gear cover.
  2. Reinstall the ABS brake modulator/master cylinder assembly in the vehicle.
  3. Reconnect the 6-way ABS brake motor pack connector.
  4. Reconnect the 8-way EBCM connector C2.
  5. Start the engine. Do not apply pressure to the brake pedal.
  6. Let the engine run for at least 10 seconds.
  7. Turn the ignition switch to the OFF position.
  8. Repeat the above cycle (steps 5 through 7) two more times.
  9. Use the Scan Tool in order to read the DTCs.

Does DTC C1251 set as a current DTC?

--

Go to Step 21

Go to Step 22

15

Repair the high resistance in CKT 1284.

Is the repair complete?

--

Go to Diagnostic System Check

--

16

Repair the high resistance in CKT 1285.

Is the repair complete?

--

Go to Diagnostic System Check

--

17

Replace the ABS brake motor pack.

Is the repair complete?

--

Go to Diagnostic System Check

--

18

Replace the ABS brake modulator gear.

Is the repair complete?

--

Go to Diagnostic System Check

--

19

Replace the ABS brake modulator.

Is the repair complete?

--

Go to Diagnostic System Check

--

20

Replace the terminals that exhibit signs of poor terminal contact or corrosion.

Is the repair complete?

--

Go to Diagnostic System Check

--

21

Replace the EBCM. Refer to Electronic Brake Control Module Replacement .

Is the repair complete?

--

Go to Diagnostic System Check

--

22

The malfunction is intermittent or is not present at this time.

Is the action complete?

--

Go to Diagnostic Aids

--