Vacuum Reservoir Press Sensor Circ Low
The PCM has detected a fault in the vacuum reservoir pressure sensor circuit.
Code Set Parameters
If the PCM fails to receive a voltage input signal from the vacuum reservoir pressure sensor that is proportionate to the engine RPM and degree of load, a code will be stored and a malfunction indicator lamp will be illuminated. This problem is normally most evident when the brakes are applied and the demand for vacuum is maximized. Some vehicle models will require multiple ignition cycles with a failure of this nature for a check engine soon lamp to be illuminated but a code should be stored on the initial failure. If only the initial failure is exhibited, the code may be stored as a "pending code".
Symptoms may include a hard brake pedal, increased effort required to depress the brake pedal, an illuminated service engine soon lamp, and a stored trouble code. In rare cases, the driver may experience no adverse drivability symptoms.
The most common cause of this code being stored is from vacuum leaks at the vacuum reservoir, a defective vacuum reservoir pressure sensor, or vacuum pump. Other causes may include shorted or open electrical circuitry and connectors, cracked or broken vacuum hoses to the vacuum reservoir or brake booster, or a defective one way valve in the vacuum brake booster supply hose.
Techs report that brake boosters and other vacuum operated components are frequently replaced in error when a defective vacuum pump or vacuum pump motor are at fault.
- In many automobiles, and especially in diesel powered models, a vacuum reservoir is utilized in order to provide vacuum operated accessories with a constant supply of vacuum
- Diesel engines do not produce sufficient vacuum and must be equipped with a belt operated or electric vacuum pump
- It is critical that the reservoir for these pumps maintain a certain amount of vacuum
The vacuum reservoir pressure sensor is designed to provide the PCM with an input signal reflecting the amount of vacuum pressure available
- The PCM uses this input signal for the purpose of monitoring how much vacuum pressure is available to stop the vehicle when it is equipped with a vacuum assisted brake booster
- Vacuum reservoir pressure sensors are generally of a silicon diaphragm design
- A low voltage signal (typically 5 volts) and a ground signal are provided to the sensor
- The higher the pressure becomes in the vacuum reservoir, the less resistance the sensor provides and the greater the voltage signal is to the PCM
- If the vacuum reservoir pressure becomes insufficient, the resistance of the sensor becomes greater and signal voltage decreases to the PCM
- In this scenario, the PCM recognizes the low voltage signal as low vacuum reservoir pressure and a trouble code is stored
Several tools will be instrumental in successfully diagnosing this code
- A suitable OBD-II scanner (or code reader) and a digital volt/ohmmeter will be most helpful in performing a successful diagnosis
Begin with a visual inspection of all wiring and connectors
- Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
- Always retest the system after repairs are completed to ensure success.
If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
- This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
- Continue by clearing the code and operating the vehicle to see if it returns
- This will help to determine whether or not the malfunction is intermittent
After the codes are cleared, test drive the vehicle to see if the code returns
- If the code fails to immediately return, you may have an intermittent condition
- Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
Proceed with your diagnosis with a visual inspection of the vacuum reservoir system
- Make sure that there is no fluid leaking from the rear of the master cylinder into the vacuum brake booster
- Inspect all vacuum supply hoses used to supply the vacuum brake booster
- Also take a look at the pressure sensor, as well as the electrical wiring and connectors for signs of damage from corrosion, burning, or fluid contamination
- Repair or replace faulty components as required.
If the vacuum source checks out and no wiring or connectors are damaged, then connect the scanner (or code reader) and make a note of all stored codes and relevant freeze frame data
- Test drive the vehicle and see if the code persists; if it does, then proceed with the diagnostic strategy.
Disconnect the vacuum reservoir pressure sensor connector and check sensor resistance using the digital volt/ohmmeter
- Compare your findings with manufacturer's specifications and dispose of the sensor if actual readings do not coincide
- If the sensor checks out, test the sensor connector for voltage and ground signals
- If no voltage and ground signals are detected, disconnect the PCM connector and perform a continuity test on the entire circuit in order to isolate a possible malfunction and repair open or shorted wiring as required
If voltage and ground signals are present, connect the manual vacuum gauge
- Consult your vehicle's service manual and compare your findings, replace or repair vacuum supply/engine components as needed.