Every aviation maintenance technician (AMT) relies on electrical test equipment to diagnose circuit faults, verify component operation, and confirm airworthiness. Of all the tools in the shop, the multimeter — and its dedicated cousin, the ohmmeter — are used most frequently. Yet improper use of these instruments is one of the leading causes of inaccurate readings, damaged equipment, and even unsafe aircraft returning to service. Understanding how these instruments work, why they must be used correctly, and what the FAA expects you to know for the AMT General knowledge test is the goal of this article.
Whether you are working with an analog (D'Arsonval movement) multimeter or a modern digital multimeter (DMM), the underlying electrical principles are identical. Both instruments measure electrical quantities by sensing voltage, current, or resistance and displaying the result on a scale or numeric readout. The difference lies in accuracy, internal resistance, and how each type can affect the circuit being tested.
How Multimeters Work
A multimeter combines at minimum three functions into one instrument: a voltmeter, an ammeter, and an ohmmeter. Higher-end units also measure capacitance, frequency, diode forward-voltage drop, and transistor gain — all useful in avionics troubleshooting. The selector switch routes the test leads through different internal circuits, each calibrated for one type of measurement.
Voltmeter Function
When measuring voltage, the meter is connected in parallel with the component or circuit under test. The internal resistance of a voltmeter is designed to be extremely high (often in the megohm range on a DMM) so that it draws negligible current from the circuit and does not disturb the operating conditions. On an analog meter, the needle deflects in proportion to the current flowing through the movement coil; that current is tiny because of the high series resistance inside the instrument. Always connect the red lead to the more positive point and the black lead to the more negative point (or ground). Reversing the leads on an analog meter drives the needle backward against the pin and can permanently damage the movement.
Ammeter Function
Current measurement requires the meter to be placed in series with the circuit — meaning the circuit must be opened and the meter inserted so all current flows through it. The internal resistance of an ammeter is extremely low so it introduces minimal voltage drop into the circuit. Never connect an ammeter in parallel across a voltage source or component. Doing so creates a near-short circuit through the meter, destroying the movement or blowing the internal fuse and potentially damaging the aircraft wiring.
Ohmmeter / Resistance Function
The ohmmeter has its own internal battery (commonly 1.5 V or 9 V on many analog VOM designs, though the exact voltage depends on the specific meter and range selected) and measures resistance by passing a known current through the unknown resistance and reading the resulting voltage drop. Because the ohmmeter supplies its own current, it must only be used on de-energized circuits. Connecting an ohmmeter to a live circuit will push external voltage into the meter's internal battery circuit, giving a completely wrong reading and likely burning out the meter movement or fuse. This point is heavily tested on the FAA AMT General exam.
An equally important rule: when measuring resistance in a circuit, isolate the component by disconnecting at least one lead. If you measure across a component while it remains connected to the rest of the circuit, parallel paths through other components will give a falsely low reading — the meter sees all parallel paths simultaneously.
Analog vs. Digital Multimeters
Analog multimeters use a moving-coil (D'Arsonval) galvanometer movement. The scale is read by aligning the needle with the printed arc. Analog meters are useful for watching trends — a slowly rising or falling needle tells you something about circuit behavior that a flickering digital display may obscure. However, analog meters have relatively low internal impedance on voltage ranges compared to DMMs, which means they can load the circuit slightly and read low on high-impedance circuits.
Digital multimeters display a numeric value and commonly have input impedance around 10 MΩ on most voltage ranges, though this can vary by model and specific range. This high impedance makes them far less likely to affect the circuit. DMMs also have auto-ranging capability on many models, selecting the appropriate scale automatically. However, a DMM can give a confusingly precise-looking number even if the measurement setup is wrong — a common trap for newer technicians who trust the digits without questioning the test setup.
Proper Use Procedures Step by Step
- Inspect leads and instrument: Before any measurement, visually inspect test leads for cracked insulation, broken wires, or corroded tips. A damaged lead can give intermittent readings or create a shock hazard.
- Select the correct function: Turn the selector to V (voltage), A (current), or Ω (resistance) as required. On an analog meter, also select DC or AC as appropriate — connecting a DC meter to an AC circuit gives a mid-scale deflection that means nothing useful.
- Select the correct range: On manual-ranging meters, always start at the highest range and work down to avoid pegging the needle (analog) or overloading the meter (both types). On the resistance function, start at the lowest resistance range for low-value resistors and the highest range for very high resistances.
- Zero the ohmmeter: On an analog ohmmeter, short the leads together and adjust the zero-ohms knob until the needle reads zero ohms, which corresponds to full-scale deflection on the resistance arc (the right end of that scale). This must be repeated every time you change resistance ranges because the internal battery voltage and circuit constants change with each range. Digital meters typically auto-zero and do not require this step.
- De-energize before measuring resistance: Confirm power is off and capacitors are discharged before connecting on the Ω setting.
- Connect leads correctly: Red to positive/high-potential side; black to negative/ground. On the ohmmeter function of some analog meters, internal battery orientation can make the black lead positive at the probe tip — this varies by meter design, so check the specific instrument, and it is a common exam trap on meters where this reversal applies. When checking diodes with such an analog ohmmeter, the diode conducts when the internally positive lead is on the anode.
- Read the correct scale: Analog meters have multiple arcs printed on the face. The resistance scale reads right to left (zero ohms on the right, infinity on the left), opposite to voltage and current scales. Select the arc that matches the selected range and function.
- Disconnect and return selector to a safe position: After measuring, remove leads from the circuit and turn the selector to the highest AC voltage range or to the OFF position. Leaving the selector on ohms means the internal battery drains and the meter could be connected accidentally to a live circuit the next time it is picked up.
Why Proper Use Matters for Aviation Safety
Incorrect electrical measurements can result in misdiagnosed faults. A technician who reads resistance with the component still connected in-circuit may conclude a component is good when it is actually open — or vice versa. An aircraft returned to service with a faulty wire, relay, or sensor because of a bad test technique is an airworthiness problem. The FAA requires that maintenance be performed using methods, techniques, and practices acceptable to the Administrator (14 CFR Part 43). Using test equipment incorrectly violates that standard.
Additionally, analog meters connected incorrectly to live circuits can arc, spark, or create smoke — a serious hazard in a hangar or aboard an aircraft with fuel vapors present. Protecting both personnel and aircraft requires disciplined, correct measurement technique every time.
Key Numbers and Rules
- Voltmeter connected in parallel — always. Never in series.
- Ammeter connected in series — always. Never in parallel across a source.
- Ohmmeter used only on de-energized, isolated circuits — no exceptions.
- Analog ohmmeter: zero leads before every measurement and every range change.
- Resistance scale reads right-to-left on analog meters (0 Ω at right, ∞ at left).
- On many analog ohmmeters, the black lead may be internally positive at the probe tip due to internal battery orientation — this depends on the specific meter design.
- DMM input impedance is commonly around 10 MΩ on most ranges — much higher than analog meters, minimizing circuit loading.
- Best accuracy on analog meters: read in the middle third of the scale where deflection is most linear; change ranges to keep the needle in this zone.