Introducing the DVOM
The ability to measure voltage, current flow, and resistance is important in the diagnosing of electrical problems. Without the results of these measurements troubleshooting in an electrical system is a futile process.
The instrument most commonly used to make electrical measurements is called the Digital Voltage-Ohm Meter (DVOM).
Basic DVOM's are capable of measuring
- AC Voltage
- Millivolts
- Conductance
- Continuity
- Amps/Milliamps
- DC Voltage
- Resistance
- Capacitance
- Diode Test
- Microamps
Advanced DVOM's add
- Frequency
- Duty Cycle
- RPM
- Pulse Width
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The DVOM provides for a method of accurate measurements.
Even though accurate measurements are the key to electrical diagnosis, the following four factors determine the effectiveness of the measurements
- Accuracy of the measuring instrument.
- Correct installation in the circuit of the measuring instrument.
- Ability of the Technician to read the instrument.
- Skill of the Technician in interpreting the results.
As it is clearly seen, only one of the factors depends on the DVOM (e.g. accuracy), the rest will always depend the ability of the technician to read and interpret the results.
Choosing a DVOM
A good choice of a DVOM is the DISplus or the GT1, as the measuring system of each contains a highly accurate DVOM.
Choosing a handheld DVOM from a reputable manufacturer, however, leaves the DISplus and GT1 free to perform other tasks that a DVOM can not do (e.g. Retrieval of fault codes).
In choosing a DVOM several factors need to be considered, one of which is Impedance.
Impedance is the combined resistance to current created by the resistance, capacitance and inductance of the meter. Impedance is measured in 'Ohms per Volt'.
Meters with the highest 'Ohms per Volt' impedance are the most accurate. More importantly using a meter with high impedance will not cause damage to sensitive electronic circuitry.
When a Meter is connected across a circuit to measure voltage, it must be connected in parallel. This adds parallel resistance. The total resistance in a parallel circuit is less than the lowest resistance in that circuit (Ohms Law). Using a Meter with low impedance will reduce the total resistance of the circuit and allow more current to flow.
A meter with low impedance can draw enough current to cause inaccurate measurement, voltage drops or damage sensitive electronic circuit boards. A high impedance meter will draw little current and insure accurate readings.
Using older type meters with low impedance values (20,000 to 30,000 ohms-per-volt) can damage modern electronic circuits and components or give inaccurate readings.
Test lights should be avoided for the same reason. They lower the total resistance of the circuit and cause increased current flow.
Other factors in choosing the proper DVOM are
- Cost
- Features
Basic DVOM's are available reasonably priced. These basic models may be more than sufficient for use in MINI Centers, given the availability of the DISplus and GT1 for advanced measurement and scope functions.
Advanced features and price go hand in hand. The more features added the higher the cost. Some of those features may be worth the increase in cost (e.g. frequency, duty cycle and pulse width). Other features may not (e.g. oscilloscope, graphing).
Choose a DVOM wisely based on personal preference and cost. Like many other tools it is valuable in the diagnosis and repair of MINI's. Experience has shown if the technician is not comfortable with the DVOM or confident in the results of the measurements, the DVOM will not be used.
Considering the technology in MINI automobiles, diagnosing with a quality DVOM certainly makes repairing the problem correctly and expediently a more manageable task.
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Infinity Display
While most displays of DVOMs are standard (i.e. mV means millivolt, mA means milliamp) the display or symbol for infinity or open circuit can be confusing. A display of 0W indicates no or little resistance. It means the circuit or portion of the circuit being measured has continuity or is complete. A reading of OL means the circuit is open or not complete, the resistance is said to be "INFINITY". Some meters may use the symbol B for Infinity. Be aware of which reading the meter being used will give for infinity or open circuit.
