Current measurements are very helpful when troubleshooting electrical circuits. The current rating of components can be used to determine their condition. Current measurements allow you to determine the current draw as the component operates.
The engine starter, for example, will draw more than the rated current when overloaded, this higher than rated current flows usually indicates a problem which can cause additional problems.
Inductive Current Clamps:
Most Digital Multi-Meters (DMM) or Digital Volt Ohm Meter (DVOM) only measure DC/AC current up to 10 A. For higher measurements an inductive clamp must be used, which measures current from .01 A to 1000 A, it does so by measuring the strength of the magnetic field around a conductor. Some current clamps can measure AC currents only, which is not useful in vehicle applications. Ensure that the one you use or is capable of both AC and DC.
The clamps use Hall Effect sensor to measure the strength of the magnetic field produced in a ring of material that is temporarily placed around the current-carrying wire to measure DC. The greater the amount of current flowing in the wire, the stronger the magnetic field that is produced in the ring. To measure AC the clamp acts as part of a transformer to produce a measurable value.
Current clamps are a standalone accessories for your DMM or built into multi-meter:
Standalone clamps output a voltage value per measured amp. Most clamps have an output of 1 millivolt per amp. If the multimeter shows a measurement of 10 millivolts, the current flowing in the wire is 10 amps. If the voltage displayed on the multimeter is 100 millivolts, the current flowing in the wire is 100 amps.
In clamps that are built into a multimeter, measuring the current flow is as easy as selecting ‘current’ on the function selector, the value is then displayed in amps.
Setting Up to Measure with Standalone Clamp (clamps that produce voltage):
1. Turn the dial to mVac for ac current, or to mVdc for dc current on DMM.
2. Plug the black test lead into the COM jack.
3. Plug the red test lead into the V jack.
4. Zero the clamp by rotating the “zero” knob (some clamps do not require adjustment). Adjust until reading is on or close to 0mV.
5. Position the clamp over the conductor to be measured. With some clamps current flow orientation must be followed, point the arrow on the clamp towards direction of current flow. To prevent the clamp from picking up stray magnetic fields, separate the test conductor from surrounding conductors by a few inches. If separation is not possible, take several readings at different locations along the same conductor. Do not measure shielded conductors, as the magnetic fields are greatly diminished or even eliminated.
Note: Make sure circuit is energized to perform test.
6. View the reading in the display. On some DMM’s the mV scale only reads up to 600mV, if you expect to read a higher amp value change the setting to the Vdc scale and take your reading there. ensure to keep track of prefixes to the measured value.