testing automotive electronic circuits

Some things to consider when you are making checks on electronic circuits.
First off, be sure to use a high impedance ohm meter. You can damage some
computer controlled circuits with a low impedance meter. A digital Volt/Ohm MultiMeter (DVOM) is the preferred instrument and they can vary immensely in price and quality. Fluke meters are generally the preferred brand for use in automotive shops. If you can afford it, be sure to purchase a model that has an analog (bar graph) display as well as the digital readout. It comes in very handy for monitoring circuits that have constantly varying voltage, for instance an O2S. The higher end models are Digital Storage Graphing meters, but they can run well over a thousand dollars. For under 300 dollars you should be able to get a meter that has functions such as a diode check position, frequency and dwell, even RPM and duty cycle. These are all very handy for engine performance work. There are many accessories that can be bought separately, such as an inductive low amp probe, vacuum and pressure probes. or even a temperature sensor probe. Probably, only techs who do engine performance for a living would get enough use out of these accessories to warrant the price.
Using the meter properly is not as easy as some may think. The biggest mistake novices make is not having the meter set to the correct range for the measurement. The next biggest mistake is failing to read the digital display correctly. The owners manual should have detailed instructions on how to use and read the meter properly, but many fail to practice using the various scales and readings, so when it comes time to use the meter they fall short.
Making proper contact with the test probe is another problem that many beginners have trouble with. Surface corrosion and difficult to reach connector pins often wind up causing faulty readings. Make sure you scratch the surface of the test point with the tip of the probe and apply enough pressure to get a good contact. Be sure to test and retest the reading until you get a repeatable reading that assures you of proper contact with the probe. Always make sure your finger tip is not in contact with the test point. If it is, then you may be reading your body resistance rather than the circuit resistance.
Of course, knowing what to expect and understanding the circuit as well as knowledge of electrical behavior is helpful. Many times a novice will accept a faulty reading as fact where a more experienced tech can tell there must be something wrong with the way he is testing, because of the widely unexpected reading.
A good understanding of the behavior of electricity and electrical circuits is probably the most useful skill when testing. For example, a novice may be testing the resistance of a TPS. The specs call for 4,000 ohms and he gets a reading of 1200 ohms. His first thought is that the sensor has failed the test and is in need of replacement. In fact, there may just be some oil, moisture, or even dirt allowing the low current of the Ohm meter to short across the two terminals being tested, especially if the meter probes are not making good contact with the test points. In reality that electrical path measured by the ohm meter is parallel to the circuit being tested and will not carry the current that is flowing though the sensor. For example, if you hold the meter probes, one in each hand, you will show a high resistance reading of your skin conductivity. The meter is showing some continuity, but if you take those same two fingers and touch them to the battery posts no current will flow thorough your body. You simply have too much resistance for twelve volts to push a current through. The same is true about those high resistance shorts caused by conductive surface contamination. The five volt reference voltage is not strong enough to push enough current through this parallel circuit to have any measurable affect on the TPS signal output. Current will always take the path of least resistance. I'm not saying there will be no loss, but in the overall scheme of things, it won't be enough to cause a performance problem. This kind of experience is what helps knowledgeable techs keep from replacing good parts.
Sorry for the long rant but I was bored and had nothing better to do right now.
Kevin Mouton
Automotive Technology Instructor
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