Vacuum Brake Booster: Testing and Inspection
VACUUM PUMP DIAGNOSISDiesel Vacuum And Power Steering Pump Assembly:
Diesel Vacuum Switch Location (without ABS):
Vacuum pump diagnosis involves checking pump output with a vacuum gauge. The low vacuum warning switch can also be checked with a vacuum gauge. Refer to the diagnosis procedure in this section.
A standard vacuum gauge can be used to check pump output when necessary. Simply disconnect the pump supply hose and connect a vacuum gauge to the outlet port for testing purposes. Vacuum should hold steady in a range of approximately 8.5 to 25 inches at various engine speeds.
DIAGNOSING LOW VACUUM OUTPUT CONDITION
A low booster vacuum condition or a faulty low vacuum warning switch will cause the brake warning light to illuminate. If the light does go on and indicates the existence of a low vacuum condition, check the vacuum pump, booster and warning switch as follows:
1. Check vacuum pump oil feed line. Verify that line connections are secure and not leaking. If leakage is noted and pump is noisy, replace pump.
2. Disconnect supply hose to booster. Connect vacuum gauge to this hose and run engine at various throttle openings. Output should range from 8.5 to 25 inches vacuum. If vacuum is consistently below 8.5 inches, problem is with vacuum hoses or pump component. If output is within specified limits, continue testing.
3. Check booster operation as described in diagnosis section. Replace check valve, vacuum hoses, or booster if necessary. However, if booster operation is correct but warning light is still on, continue testing.
4. Disconnect vacuum hose at warning switch. Plug hose and connect hand vacuum pump to switch.
5. Start and run engine.
6. Apply 8.5 to 9 inches of vacuum to warning switch and observe warning light. If light goes out, switch vacuum hose is either loose or leaking. If light remains on, leave engine running and continue testing.
7. Apply 20-25 inches vacuum to switch and observe warning light operation. If light now goes out, switch is at fault and should be replaced. If light remains on, continue testing.
8. Reconnect vacuum hoses and replace original warning switch with. known good switch. Run engine and observe warning light operation. If light is now off, old switch is faulty. If light remains on, problem is in wiring between switch and warning light.