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This month, I want to talk about pressure sensors – used in many areas in a vehicle, from manifold pressure to air conditioning and brake pressure to fuel rail pressure. These sensors all take a physical input and turn it into a voltage output. The actual method of measurement inside the sensor can vary, but for us, the common factor is that they all produce a voltage output.
The method of testing, as always, starts with the analysis of live data and you are looking for readings that stand out or seem implausible. Testing may have been initiated by a fault code giving you a guide as to where to look, but issues can often be caused by a sensor that has gone out of spec but not enough for the ECU to recognise it as faulty. Are the readings you are getting reasonable? If you have a car that you are not familiar with, you may not know. Expected readings can sometimes be gained from a technical data source, either an aftermarket supplier or direct from the manufacturer, but another good way is by comparison to another car that you know is fault free.
With pressure sensors there are three areas that need to be tested. The first is whatever is producing the pressure – is it a mechanical or a sensor issue? Is the ECU logging a sensor issue as a result of the system not producing the required pressure? The best way to check is to ‘T in’ a mechanical pressure gauge and compare the reading to both the technical data and the live data reading. If we have a difference in the readings then we must move on and check the next step.
Next, check the power supply voltages at the sensor. Most pressure sensors will have three wires; a live, a ground and a signal output. It is vital that you check the supply voltage and ground are good. As per my voltage drop article, this should be done with the circuit live and with the sensor connected. Voltage checks made with the sensor unplugged are unreliable.
I have seen several cases where a faulty sensor pulls the supply voltage to ground, this can cause issues with the faulty sensor and any other sensor that shares the same supply. If power and ground are good then we move onto the voltage output. Best practice when measuring the output voltage is to measure at the ECU, but often access issues mean it is quicker and easier to test at the sensor. Back probing the connector is my method of choice. Remember to use the sensor ground as a reference and not a chassis or battery ground as these can lead to inaccurate results. You can use a multimeter to test sensors that measure slow moving changes, but often the best method is an oscilloscope. If you are lucky, your data source will give you a range of pressures and the voltage readings that should be produced – if not, you may have to compare with a good known car again. Comparing these with your readings should tell you whether your sensor is within specification.
If these tests are all OK then we need to check the signal voltage again at the control unit. The voltage should be the same within a few millivolts. If necessary, carry out voltage drop testing on the signal cable to find any errors.
You’ll notice that at no point have I used any resistance testing. Many of the sensors will have internal signal conditioning circuits that are designed to work on 5 Volts, so the 9 Volt battery in your multimeter could potentially damage them. More importantly, the results are not reliable.
Knowing how the ECU tests the sensor can help us understand how and why fault codes are logged. The typical working range of the sensor is 0.5 to 4.5 Volts. Let’s look at the circuit. Figure 1 shows a simplified version of a typical circuit. In many ways it looks similar to the voltage divider circuits that I discussed in the last issue.
Inside the ECU there is an input to an analogue to digital converter and a resistor between the input and 5 Volts, however the function of the resistor in this circuit is different. It has a typical value of around 60KOhms. The input to the analogue to digital converter is high impedance, meaning it takes very little current to change the voltage. If the wire from the sensor to the ECU was to break then the voltage would float and interference from circuits like the ignition or injection could induce a signal on the input. To stop this happening, the resistor is used to pull the input up to 5 Volts and the sensor pulls the voltage down against it. Think of it as like a throttle return spring, where the pedal and cable pull the throttle open and the spring ensures it closes. It is a safety device, so that should the cable fail under full acceleration, the throttle will be closed. In the same way, if the wire to the ECU fails, the input is pulled to 5 Volts and a fault logged.
Typically, a fault code will be set if the voltage goes outside the 0.5 to 4.5 Volt range. Below 0.5 Volts, you’ll get a short to ground fault code and above 4.5 Volts you’ll get an open circuit or short to positive code. The difficult faults to pin down are when a sensor goes out of calibration and produces a plausible but incorrect reading, often no code is logged. This is where comparison to your pressure gauge is useful, but for sensors where the information is more dynamic, more detail can be gained by using an oscilloscope to watch the changes in voltage.
To sum up, as always, look at live data and verify mechanical condition. Compare the mechanical reading to live data and check readings are within specification, according to technical data.
