This month, we will take a look at those devilish defects known as "sneak circuits." A sneak circuit is any defect in a certain circuit or component system that affects another seemingly unrelated circuit or component system. Most of us have experienced a bulb with an incorrect contact arrangement being installed into a taillight socket. When the brake or another light system is activated, a sneak circuit ends up feeding power to another circuit causing strange behaviors in the second circuit.
Open ground circuits are notorious for feeding voltages into unrelated circuits and wreaking havoc. Identifying which ground circuit is open and causing the problem has become more difficult now that solid-state control modules have added an almost impossible level of complexity to the equation. In addition, not all sneak circuits are open grounds. Some are caused by illegal connections between one circuit and another. Fear not, though. We are going to spell out some techniques to help ferret out these demons.
To start, get a good account of not only the customer's stated complaint but also anything else that might be defective that the customer is not necessarily looking to have repaired today. Also ask the customer if he or she was actuating some other system when they experienced the problem or noticed the warning light. Were they changing the stereo? Turning on the wipers or lights? Making a turn? Hitting the brakes?
Always check all modules for Diagnostic Trouble Codes (DTCs). A DTC set in a module unrelated to the stated customer complaint might provide a clue to the defect and the cause of the stated complaint. Pay careful attention to the Freeze Frame data associated with any powertrain DTCs, as you will at least know what the basic driving conditions were at the time of defect. You then can recreate that drive cycle.
Once you have identified the ECU(s) that are responsible for the defect system and/or any other ECUs that have DTCs, get all relevant data PIDs on screen while actuating the identified defect systems. You might find that as one system is actuated another systems data PID skews.
Figure 1 is a data capture of a test drive of a Ford Taurus. This vehicle was setting a DTC for the Transmission Fluid Temperature (TFT) sensor circuit reading of -40 degrees F or an open circuit. The TFT PID read normally at idle and in first gear. The defect only showed up as the vehicle shifted into second gear on a test drive. Once third gear was engaged, the defect was gone and the TFT read normally again.You can see in the scan data capture that the difference between first, second and third gears are, of course, related to the Shift Solenoid (SS) ON/OFF states. As SS2 turns off, the TFT reading goes open circuit. This circuit is grounded internally by the PCM, and besides, the SS2 is turning off not on, so this is not a lost ground defect. How could this be? When you do not understand why an ECU has readings that are defective, it is time to get out a meter or a scope and actually watch what is happening to the defect circuit.
Graphing It Out More
You can see in graphing meter capture in Figure 2 that indeed there is a stray voltage of over 6V briefly showing up on the TFT signal line. The only way a voltage can show up when a circuit turns off is the collapsing of a magnetic field present around an energized coil. A quick look at the wiring diagram for these circuits revealed that the transmission connector Pin 5 is the TFT sensor signal and next to it is Pin 6, the control side of the No. 2 shift solenoid. A visual inspection found that some pasty corrosion from a recent battery terminal washing had found its way into the transmission connector cavity causing a semi-conductive path for that shift solenoid spike back voltage.In our next case, we found no apparent reason for one circuit affecting another. Seemingly unrelated circuits, according to both function and or wiring layout, might affect each other through an unidentifiable back-feed path through a control unit. This Ford Windstar had a stated complaint of the passenger front, passenger rear and driver's rear windows inoperative from any of the switches. The driver's window worked normally in both directions.
After checking for DTCs, we found that Front Electronics Module (FEM) had a three DTCs: one for the driver's door ajar circuit, one for the exterior temperature sensor for the HVAC system and one for the oil pressure switch circuit. These DTCs set only after clearing when the window buttons were pushed. Watching the Exterior Temperature PID in Figure 3, you can see it react and change temperature each time the window switch is operated. Because we had both window problems and a driver's door jamb switch DTC, we performed a visual inspection and found that a ground wire had broken in the harness running from the driver's door harness to the A-pillar.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.
