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Powertrain; Electrical

Chassis, Running Gear

Technical Notes

Digital Multimeter (DMM) V/Ω/mA/A

NOTES, REQUIREMENTS, INFORMATION, ETC.

In addition to the standard types of electrical measurement for current, voltage and resistance many multimeters also offer the possibility of measuring the engine speed, ON/OFF ratio and injection time. These types of measurements and tests are generally performed with engine diagnostic analyzers and testers. Moreover, with the LH injection system, for example, the injection time can also be measured with the HHT.

Relative measurement (measured values corresponding to difference in relation to stored reference value) eliminates the necessity of zeroing the instrument for a connected external crimp-on probe and allows display of the resistance reading without taking the resistance of the connection cable into consideration (facilitates work and increases measurement accuracy).

To measure the ON/OFF ratio (digital displays only), e.g. for control unit fault code readout in % and for checking the idle speed and lambda control (CIS-E systems) it is necessary to find and make contact with the proper ends in the vehicle test socket, e.g. with Fluke (red to ground (2), black to signal (3). If incorrectly connected an ON/OFF ratio of 80% instead of 20% may be measured. This measurement possibility is NOT a substitute for a lambda tester.

Engine-speed measurement, usually via a clip-on probe which must be fitted over an ignition cable (not via the TDC sensor).

Temperature measurement, usually via a simple all-purpose sensor. No substitute for the specialized air-temperature sensors of the temperature meters listed in Group 83.

Dwell-angle measurement, usually only on clean square-wave signals (e.g. terminal TD) via the ON/OFF ratio measurement in %. Compared with engine testers etc., relatively low technical complexity for reliability of measurement and malfunction (e.g. for vehicles with coil ignition systems, breaker contacts, etc.).

Quality Assurance and Utility Value Retention:

Observe the low supply voltage indicator and replace (all) batteries at the same time.
When switched on, all the display segments are lit for a few seconds. Check that all the segments are properly lit.
Replace blown fuses with genuine replacements.
Use recommended DMM's to avoid erroneous measurements of sensor signals with uneven voltage curves.
Avoid extreme changes of temperature of the DMM during measurement (e.g. avoid attachment near exhaust system or outdoor use at sub-zero temperatures).
Concerning measuring range resolution and accuracy, the recommended DMM's far surpass the requirements arising in the vehicle repair sector. The conditions of use in this sector, however, are not without their challenges. In case of any doubt concerning accuracy of readings a comparison test with other instruments (DMM's) can be performed. The resistance measurement can be checked with the aid of the resistance decade box (MB special tool). Furthermore, the DMM manufacturers recommend annual calibration according to the manufacturer's instructions. Because of the low demands on measurement accuracy and the positive experience with the brand multimeters recommended, a calibration interval of two to three years seems reasonable, depending on the conditions of service.