Diagnostic Overview - Electric Vacuum Pump (EVP) System
The PCM performs circuit fault detection on the Electric Vacuum Pump (EVP) relay control circuitry and sets the P258A-ELECTRIC VACUUM PUMP CIRCUIT fault if an open or shorted condition is detected. The PCM cannot directly monitor the EVP battery supply or ground circuits. Instead, the PCM performs a system performance diagnostic. When the EVP Relay is energized the PCM compares BARO to the Vacuum Pressure Sensor Signal to make a determination if the Vacuum Pump and system are functional. The PCM expects to see a 1% change in vacuum per 1.0 second of vacuum pump run time until a max of 83% of BARO is reached. If the diagnostic accumulates five failed attempts, the PCM sets the P258B-ELECTRIC VACUUM PUMP PERFORMANCE fault. The performance diagnostic will detect any issue that can affect vacuum in the system. There are several electrical and mechanical failures that could cause the diagnostic to fail:
- Leaking or damaged vacuum tubes or check valves.
- A leaking vacuum brake booster assembly.
- An electrical or mechanical failure with the vacuum pump.
- It's also possible for the system to be functioning properly but fail due to a vacuum pressure sensor that is stuck in range and not changing.NOTE:
It is possible in some cases for pressure sensors to stick mid range if the 5-Volt Supply is missing to the pressure sensor. This is dependent on the type of pressure sensor being used.
A leak in the vacuum hose assembly or brake booster can cause the EVP to fail prematurely. If the diagnostics lead to a failed or weak vacuum pump, the entire system should also be checked for leaks after the EVP is replaced and before releasing the vehicle. Perform the VACUUM SYSTEM LEAK TEST below after the EVP replacement to verify the system is operating properly.
The table below shows the different stages of the vacuum pump performance diagnostic:
| DIAGNOSTIC TABLE | |
|---|---|
| DESCRIPTION | CHART |
| This chart is showing a normally operating system | |
| This chart is showing a system that stops increasing but doesn't reach the maximum threshold. The vacuum pump will turn off to prevent excessive run time which can damage the pump. This system is considered passing because vacuum accumulation is more than75% of BARO (will not set a fault). | |
| This chart is showing a system that stops increasing but doesn't reach the minimum threshold. The vacuum pump will turn off to prevent excessive run time which can damage the pump. This system is considered failing because vacuum accumulation is less than75% of BARO. If the diagnostic accumulates five failed attempts, the PCM sets the P258B-ELECTRIC VACUUM PUMP PERFORMANCE fault. | |
| This chart is showing a system that has a gross failure. The vacuum failed to increase by 1% for 5.0 seconds. When this failure occurs once, the PCM sets the P258B-ELECTRIC VACUUM PUMP PERFORMANCE fault. | |