The DVOM
Model: All
Production Date: All
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.
- DC Voltage.
- Millivolts.
- Resistance.
- Conductance.
- Capacitance.
- Continuity.
- Diode Test.
- Amps/Milliamps.
- Microamps.
Advanced DVOM's add
- Frequency.
- RPM.
- Duty Cycle.
- Pulse Width.
Scheme 136
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 MoDic, as the measuring system of each contains a highly accurate DVOM.
Choosing a handheld DVOM from a reputable manufacturer, however, leaves the DISplus and MoDic 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 BMW Centers, given the availability of the DISplus and MoDic 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 BMW'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 BMW automobiles, diagnosing with a quality DVOM certainly makes repairing the problem correctly and expediently a more manageable task.
Scheme 137
Power to the meter is turned off.
Scheme 138
Volts AC.
Measures AC voltage.
Ranges 400mV, 4V, 400V, 1000V.
Scheme 139
Volts DC, RPM.
Measures DC Voltage.
Ranges 4V, 40V, 400V, 1000V.
Scheme 140
mV.
Measures DC Millivolts.
Range: 400mV.
Scheme 141
Continuity/Ohms.
Measures Continuity and Ohm.
Ranges: 400ohms,4kohms, 400kohms, 4Mohms, 40Mohms.
Scheme 142
Diode Test.
Test diode operation.
Range: 3.000V.
Scheme 143
Milliamp or Amps DC.
Measures DC Milliamps or amps.
Ranges: 40mA or 400mA for mA input 4000mA or 10 A for A input.
Scheme 144
Milliamps or Amps AC.
Measures AC Milliamps or amps.
Ranges: 40mA or 400mA for mA input 4000mA or 10 A for A input.
Scheme 145
Zero (Relative Reading) Function
Displays difference between the measured value and the stored value.
Scheme 146
Minimum (Min), Maximum (Max), Average (AVG) Recording
Records minimum, maximum and calculates the true average.
Scheme 147
Manual Range or Autorange
In Manual Range user selects fixed range. Meter stays in that range until user changes it, selects Autorange or turns meter off. In Autorange meter selects range automatically.
Scheme 148
Touch Hold
Touch Hold holds last stable reading on display. A new stable reading causes beeper to sound and display to update.
If meter is in MIN MAX Recording, RPM, Duty Cycle, Pulse Width or Hz, Touch Hold interrupts the function. The display is frozen, but recorded readings are not erased.
Scheme 149
RPM / HZ.
RPM 2, RPM 1, or frequency.
RPM 2, 4-cycle engines.
RPM 1, 2-cycle engines.
Hz. counts frequency between 0.5 Hz and 200 kHz.
Scheme 150
Duty Cycle or Pulse Width.
Duty Cycle between 0.0 and 99.9% displayed.
Pulse Width between 0.002 and 1999.9 ms displayed.
Scheme 151
Change Alert, Continuity Beeper or +/- Trigger.
In voltage or current functions selects Change Alert.
In ohms function selects Continuity Tests.
In Duty Cycle or Pulse Width selects trigger slope.
Scheme 152
Smoothing Function and Back-light display (advance model only).
Smooth displays average of last eight readings.
Press Yellow button to turn on or off back-light.
Scheme 153
Scheme 154
Scheme 155
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 0ohms 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 "INFINITY" for Infinity. Be aware of which reading the meter being used will give for infinity or open circuit.
