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Mastering electrical diagnostics with multimeter tips and tricks

Monday, April 17, 2017 - 07:00
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In our January issue, Motor Age contributor Bernie Thompson presented very detailed information on the theory of and proper testing of voltage drops.  Electrical problems such as voltage drops continue to be some of the most frequent and frustrating problems we face as automotive technicians today. This month we’ll be continuing our dive into electrical diagnostics with practical multimeter tips and tricks!

What’s your most cost-effective diagnostic tool — dollar for dollar?

The answer is the multimeter. The Digital Multi Meter (DMM), also referred to as a Digital Volt-Ohm Meter (DVOM) is to an automotive technician what a rifle is to a soldier. While even the most expensive DMM is only a fraction of your entire tool inventory investment, it often turns out to be the most valuable tool you have in your tool box if you know how to use it to its fullest potential. Most DMMs contain a digital voltmeter, digital ohmmeter and digital ammeter in one package. Many DMMs include a dedicated diode test and other useful features such as a digital thermometer display and tachometer function when coupled with the proper accessories. You most likely are comfortable with those terms like analog, digital and auto ranging but you might want to get on the road to mastering advanced DMM features such as 4 ½ digit mode, Peak-Hold/Relative Delta, Min-Max sampling rates and other practical multimeter tricks. To really know meters, we first take a quick look at why we have the kind of DMMs that we have today.

Figure 1 - If your shop does NOT have a quality analog voltmeter to connect to a charging system, you may want to hang on to an instrument like the one pictured here. Watching for a steady or flickering analog voltmeter does have it’s merits with some types of testing. However, when measuring solid state related circuits (modules, sensors, etc.) and circuits involving airbags, ALWAYS use a DMM rated with at least 10 MΩ impedance. This Simpson 260 meter has been in my dad’s tool box since the 1960’s. The same exact model is still in production today!

Meter evolution & input impedance

Without going back to the original galvanometers with moving coils, it’s not a bad idea to look back upon some equipment that still may be buried in some older technicians’ toolbox drawers – the analog multimeter. Like their modern digital counterparts, the quality and features of these instruments varied all over the place. Many of the lower to medium quality analog multimeters had internal impedances (resistance) that were low enough to load sensitive circuits like ECM 5-volt sensors to the point where a false reading occurred. Some could even damage the 0.45 voltage regulator inside the ECM that fed a bias voltage to the O2 sensor signal wire.  A very cheap digital DMM has the potential to do the same thing. Not all analog meters are low impedance nor are all digital meters high impedance. For most circuits in the electronic systems you encounter on a vehicle, high input impedance meter will be essentially ‘invisible’ to the circuit and associated modules.

Shopping for a new meter?

If you are considering updating your current DMM, besides looking for a minimum of 10 MΩ input impedance, look for an instrument that is rugged and has a large easy to read display. A backlight is a must as is a kickstand to set the meter upright.  High quality banana type universal leads are essential. I’ve owned meters that I loved but gave up on them due to unique non-universal leads that I hated!

Top 10 essential functions to look for in a DMM
  1. Auto Off Function — keeps the meter’s battery from running down constantly
  2. Auto Ranging Capabilities — reduces confusion when testing unfamiliar circuits
  3. Min/Max/Average — for capturing intermittent problems
  4. Hold Button — Allows user to quickly press a button to freeze whatever is on the display
  5. Frequency and Duty Cycle — for pulsing DC square waves and AC sine waves
  6. Fuses for Ammeter’s High and Low Current settings – prevents the meter from being ruined
  7. Analog Bar Graph — for a visual quick look when testing
  8. Diode test capabilities – to forward bias diodes while testing (superior to ohmmeter test)
  9. Category III/1,000 Volt (or higher) rated — for working on hybrid / EV systems
  10. True RMS rated — provides more accurate AC measurements

Many high-end DMMs sport lots of other features, which can be handy as well, but the 10 I’ve just listed are in my opinion are “must haves” for a daily use meter in the bay. Let’s go over a few of these features and cover some advanced tech tips as well.

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