Scope & Scan: Two Sisters, One Always Lies

So begins the ancient riddle in which a questioner must ask a series of questions to a pair of sisters in order to determine which one of them is the liar and which one is the truth teller. Sometimes diagnostics can take on this same level of frustration if you do not know that your scanner can actually be lied to by design!

The deceptions I am referring to are designed-in functions of the Powertrain Control Module (PCM). One is PCM software called Failure Mode Effects Management (FMEM), and the other is PCM software scaling design inadequacies.

FMEM is the PCM's way of handling defective sensor inputs. If the PCM determines that a sensor is providing inaccurate data, the PCM will use a substitute default value in place of that sensor's input in order to make its engine management calculations. That default sensor value also will be reported to the scan tool's Enhanced (OEM) PID data list. Of course, during this type of defect occurrence, a DTC will be set indicating the particular sensor circuit has a fault.

Figure 1 shows the OEM scan data of a fuel level sensor reading of five volts while I am jouncing the vehicle to create fuel slosh. Figure 2 is the fuel level signal voltage from the same event as measured live with a graphing meter.

Obviously, Figure 1 shows that the vehicle is displaying a default FMEM value. The thing we must remember is that during our diagnostic process, we cannot use this default FMEM sensor value to make any kind of determination as to the actual condition of that sensor, its circuit or even the PCM. Also, we cannot measure the sensor circuit directly using a DMM or scope and make any kind of prediction as to the vehicle's behavior in response to that sensor's measured value, because the PCM is now ignoring it and using a default value in its stead.

One possible way around being fooled by sensor values is to use the OBDII generic side of our scanner. It was written into the OBDII regulations that substitute default PID values must not be reported to the scan tool because of the potential to mislead technicians. However, there certainly are times when the OBDII generic side of the scanner might not offer the particular enhanced OEM PID you are interested in. In this case, you must use a DMM or scope to determine the actual sensor values and take into account that the PCM may be ignoring that value while making its calculations.

The second deception, PCM software scaling design inadequacy, can manifest itself in several different ways. First is what I call inadequate scaling range values. When the software conversion for a particular sensor's voltage input reading is written, it is scaled for a "normally expected" sensor voltage range. Any voltage input that falls well outside that range has no PID software written to correlate to it.

Say for instance that an O2 sensor normally ranges between 0 volts and 1 volt, and the software for that PID might be able to show only a reading no greater than 1.2 volts. If that same O2 sensor is shorted to battery voltage, the software still will read only a top-of-the-scale 1.2 volts and not the actual 12.6 volts the circuit is really carrying.

This dynamic applies to not only powertrain management PIDs, but also to PIDs of all kinds. An airbag resistance PID might read 6.2 ohms, when in actuality the circuit is open. Of course this condition would also generate a DTC.

The other software scaling inadequacy is that of polarity. If a circuit is only ever expected to read a positive voltage, what happens when the sensor outputs a negative voltage? The software may have no provision for adding a negative (-) sign in front of the PID reading.

A common example of this is an O2 sensor that either loses its ground reference circuit or has its outside air reference vent contaminated with some type of fluid. The O2 sensor still will vary in a range of 1volt, but it will actually range from a slightly positive reading of say +100mV to a very negative reading of -800mV.

The O2 sensor waveform shown in Figure 3 has lost its reference ground circuit. The PCM reads this input as +170mV to +790mV, which inverts the mixture reading from what it actually is and causes drivability issues. The PCM also reports this incorrect voltage to the scanner.

When making your own measurements with a DMM or scope, always be sure to use the same ground reference point the PCM uses for that sensor. This point is typically called "Sensor Low" or "Reference Low."

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.