This month we will cover testing techniques used to diagnose three wire sensor circuits that vary their resistance in order to output information to an ECU. While there are several types of resistance based three wire sensors that operate differently internally, the external testing techniques we use will be the same for all.
Each type of resistance based three wire sensor shares a common harness arrangement as diagramed in Figure 1. The first harness wires are the sensor reference voltage which originates from the ECU and is sent to the sensor shown in the diagram between points A and B. The second is the sensor Signal voltage that is returned to the ECU along points C to D. Finally, there's the sensor Reference Low, sometimes referred to as sensor ground, which sends the remaining voltage, not used by the signal line, back to ground along points E, F, G and H. Defects along the various lettered points in our diagram ultimately will affect the signal voltage at ECU point D in different ways.We start our testing with a reading of the Signal voltage to the ECU using a digital multimeter (DMM). The signal voltage on a defective circuit will fall into one of three categories. The signal will Fail High when the ECU sees a higher signal voltage than it expected, Fail Low when the ECU sees a lower signal voltage than expected or some signal voltage reading in between but Not Rational compared to other sensor inputs seen by the ECU.
Let's start with a Signal voltage Fails High scenario. If the signal voltage is higher than expected, then we need to check the sensor reference low circuit from points E to H to see if there is an illegal resistance causing an excessive voltage drop in that circuit. Too much resistance in the sensor reference low circuit will not allow enough of our signal voltage to drop to ground as it should. Voltage drop test Sensor Reference Low circuit from harness points E to F and then through the ECM from points F to G and last from harness points G to ground point H to locate the excess resistance. Any points measuring more than 10mV is no good.
A Failed High signal could mean that the 5V reference line is shorted to a higher voltage, a battery circuit say, between points A and B or the ECU's internal regulator circuitry is outputting a reference voltage higher than normal. Test the Reference voltage to the sensor with your DMM and then separate the 5V reference harness from the ECU output at a convenient connector to determine if the excess voltage is originating from the ECU or a harness short.
Be sure to scan test all other sensor signal values for rationality also even in the absence of related Diagnostic Trouble Codes (DTCs). I have seen a poor reference low harness wire (E to G) on one sensor cause excess reference voltage (A to B) on an entirely different sensor circuit. For example, an open reference low wire on a Fuel Tank Pressure Sensor caused the Throttle Position Sensor (TPS) sensor 5V reference from the ECU to be an actual 8V. As you would expect, the TPS signal read higher than normal throughout its range.
In a Signal Voltage Fails Low scenario, we need to check the 5V reference line A to B with our DMM to see if we have sufficient sensor supply voltage to the sensor. We then need to voltage drop check the Signal line from points C to D. We should see less than 10mV drop on that harness. We could also have a bad sensor between point B and C. Once we determined that points A to B and C to D are intact, then a bad sensor (points B to C) is determined.
In a signal Not Rational scenario, we could be failing either higher than the ECU expected or lower than the ECU expected for a given operating condition. The above testing for both signal high and signal low would have to be performed in order to determine the integrity of the sensor and related circuitry in question. If no defects were located during this testing, then we would need to read the ECU's rationality testing operating strategy in the service information and combine this information with the DTC Freeze Frame data to determine where the defect lies.
Let's say we had a TPS signal lower than expected rationality DTC. If the ECU used the Manifold Absolute Pressure (MAP) sensor and RPM reading to determine an expected TPS reading and the TPS circuit tested properly, we would have to test the MAP sensor and RPM sensor next to determine why the ECU thought the TPS signal was not rational.
Some OEMs will report both the sensor reference voltage level and/or the sensor reference ground level as information Parameter Identifiers (PIDs) to the scan tool. Be careful; just because the ECU sees good values at its connector does not mean the actual sensor itself is receiving these stated values at its harness. Be sure to check the tightness of all female connector terminals using appropriately sized terminal test probes.
And remember, OEM specific scan data will, at times, use a false default sensor signal value to continue with its needed operations. Always use your DMM for these tests and do not rely on scan data alone.
Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.
