Auto Service World
Feature   October 1, 2009   by Jim Anderton, Technical Editor

Sensors Simplified

You can learn a lot about automotive sensors with a little common sense

When it comes to drivability, emissions and even no-start troubles, these days it’s more and more about electronics.

And with increasing complexity, it’s a natural instinct to go straight to the book … but is that the best first move? In many cases, sensors are the cause of the codes, and often they’re easy to diagnose quickly, narrowing down the troubleshooting options. The logic works like this:

1 Start with grounds. While this is a given for experienced techs, most manuals neglect this step. Poor grounding is the number one cause of erratic operation of every electrical circuit from the power antenna to the ABS, and grounding points are often buried or coated with grime. Before diving into the schematic, check the harness diagram for grounds and inspect visually and with a simple pull with your fingers. A good test lead can be made with a large crocodile clip on one end and a small alligator or spring hook on the other. Clip it to the negative terminal or sure ground and probe sensor ground wires. If you see a spark, or the sensor springs to life, you have your issue. Sometimes it’s difficult to access body grounds … simply add a new one to the ground wire circuit. Remember, grounds cannot do damage if they short to the chassis, by definition, so tie into any clean metal as close to the sensor as possible. The engine pigtail is especially vulnerable. If it’s broken don’t forget to check the engine mounts and “dog bone” for excessive engine movement. Always use a new pigtail that’s as long or longer than the original.

2 Is there enough current? Modern vehicles can start with minimal cranking, so it’s possible for a bad alternator diode or regulator to wear out a battery, but to keep it limping along and causing havoc with sensors. If it’s a regulator, the test is easy, but a bad diode can leave marginal DC voltage, superimposed with serious ripple, again causing erratic sensor/ECU operation. Only a scope will show this dirty DC current. If the alternator is bad, stop the drivability troubleshooting until the issue is fixed because like computers, for electrical devices it’s “garbage in, garbage out.” If the ripple is serious, it could induce a faint pulse in adjacent wires in a bundle, generating a spurious input to the ECU that’s almost impossible to find. Start with good clean DC.

3 What kind of signal does it send to the ECU? There are two possibilities: analog or digital information, and circuit completion. An example of the former is the oxygen sensor, which by cross-counts lets the ECU infer the O2 content of the exhaust for mixture control. The simplest example of the latter is the oil pressure warning sensor, which grounds the lamp circuit when a pre-set pressure level is reached. A single wire sensor is a grounding device, so there are two easy tests, continuity and short circuit. Short this sensor and you can test the rest of the circuit, or use continuity to tell whether it behaves like it should. For a coolant temperature switch, for example, it will be open or closed in cold coolant, so something as simple as an electric kettle can simulate its operation. If there are multiple contacts to the sensor, it’s likely sending information back to the ECU either as pulses (digital) or as varying voltage (analog).

If it’s analog, can you use a potentiometer to simulate its action? Or can you tap into the sense wire and watch it work (or not work) in real time? If it’s digital, can you tap into the signal and use a scope at the signal source? A good example is Hall Effect crankshaft position sensors and ABS sensors, which feed a crude square wave signal back to the ECU. For many, you can check operation key on engine off by manually rotating the toothed reluctor wheel, either by cranking the engine or turning a brake rotor.

If you can’t use a scope, the next best tool is an analog VOM. Modern digital voltmeters are handy and rugged, but it’s hard to tell the pulse from a Hall Effect device from the natural voltage fluctuation in the digits as the probes’ contact varies. There are times when the digital meter is the best choice, however. That’s the case when a sensor uses low voltage values, where an older analog meter may load down the circuit and cause erratic operation. It’s ideal to have one of each in your box.

3 Does the sensor have a mechanical action? Thermo sensors are usually built with bimetallic thermostat-like technology, or as solid-state thermistors, but throttle position sensors have moving mechanical parts like a potentiometer. On your stereo, a bad one can make a crackling noise in your speakers … something similar in the ECU can cause chaos. Testing these requires solid connections between contacts and your meter, leaving hands free to cycle the shaft and watch for intermittents. The same goes for steering angle sensors … use alligator clips or spring hooks to minimize intermittents induced by your hands. Remember that some vehicle speed sensors operate off the speedometer, so a broken speedo cable can look like a dead sensor … and you won’t notice a bad speedometer in the parking lot.