Circuit Description
The primary brake system consists of the following:
| • | 4 wheel disc hydraulic brake system with 4 channel ABS. |
| • | Hydro-boost brake power assist system which utilizes hydraulic pressure form the power steering pump. |
The primary brake indicator illuminates when ground is applied to the lamp. A brake fluid level switch, located in the master cylinder fluid reservoir, supplies ground to the primary brake indicator if the brake fluid drops below a minimum level. The multi-function alarm module performs a diagnostic after the engine has been running for 4 seconds in order to verify the power brake booster fluid flow switch has opened, indicating hydraulic fluid flow. On Allison automatic transmissions, the EBCM may also illuminate the PRIMARY BRAKE indicator if DTC C0287 is set.
Test Description
The number below refers to the step number on the diagnostic table.
Step | Action | Yes | No |
|---|---|---|---|
Schematic Reference: Brake Warning System Schematics | |||
1 | Did you review the Hydraulic Brake System Description and Operation and perform the necessary inspections? | Go to Step 2 | |
2 | Start the engine while observing the primary brake indicator. Does the primary brake indicator illuminate for 4 seconds and then turn OFF? | Go to Step 3 | Go to Step 4 |
3 | Is the only concern that the primary brake indicator does not illuminate when the brake fluid level is low? | Go to Step 5 | Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems |
4 | Are the auxiliary brake and low coolant indicators also inoperative at bulb check? | Go to Step 10 | Go to Step 7 |
5 |
Does the primary brake indicator illuminate? | Go to Step 6 | Go to Step 16 |
6 | Test the ground circuit of the brake fluid level switch for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 19 | Go to Step 17 |
7 |
Does the test lamp illuminate for 4 seconds and then turn OFF? | Go to Step 8 | Go to Step 14 |
8 |
Did you find and correct the condition? | Go to Step 19 | Go to Step 9 |
Test the brake warning indicator control circuit for an open or a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 19 | Go to Step 12 | |
10 |
Did you find and correct the condition? | Go to Step 19 | Go to Step 11 |
11 | Test the neutral safety switch signal circuit for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 19 | Go to Step 14 |
12 | Inspect for poor connections at the instrument panel cluster harness connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 19 | Go to Step 13 |
13 | Replace the instrument panel cluster. Refer to Instrument Cluster Replacement in Instrument Panel, Gages, and Console. Did you complete the replacement? | Go to Step 19 | -- |
14 | Inspect for poor connections at the MFAM harness connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 19 | Go to Step 15 |
15 | Replace the MFAM. Refer to Multifunction Alarm Module Replacement in Instrument Panel, Gages, and Console. Did you complete the replacement? | Go to Step 19 | -- |
16 | Repair the open in the brake warning indicator control circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | Go to Step 19 | -- |
17 | Inspect for poor connections at the brake fluid level switch harness connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 19 | Go to Step 18 |
18 | Replace the brake fluid level switch. Refer to Brake Fluid Level Indicator Switch Replacement . Did you complete the replacement? | Go to Step 19 | -- |
19 | Start the engine while observing the primary brake indicator. Does the primary brake indicator illuminate for 4 seconds and then turn OFF? | System OK | Go to Step 3 |
