Voltage drop is a prevalent electrical issue in modern automotive repair shops. As vehicles incorporate more connections and wiring, their electrical systems become increasingly susceptible to voltage drops. Practicing safe electrical service involves measuring voltage drop before drawing conclusions. Performing a voltage drop test on a circuit can reveal if it's too restricted to operate a component — such as a motor, relay, or light bulb — correctly. If you find a restriction, repair the circuit and retest. If there's no restriction and the component still malfunctions, consider replacing the component. 
 

What is Voltage Drop?

Voltage drop refers to the reduction in electrical potential as current flows through a circuit. It occurs due to resistance in wires, connectors, and other components, which impedes the flow of electricity. In an ideal circuit, all voltage should be delivered to the intended component, but in reality, some energy is lost as heat due to resistance.

A small voltage drop is normal, but excessive voltage drop can cause electrical components to underperform or fail. Technicians need to measure and diagnose voltage drop in order to maintain the efficiency and reliability of automotive and industrial electrical systems.
 

Symptoms of Voltage Drop

Voltage drop symptoms vary based on the circuit's function and the severity of the drop. Common indicators include dimming headlights, slow cranking during engine start, malfunctioning sensors, and intermittent electrical component failures. These symptoms can be confusing and contradictory, making accurate diagnosis all the more important.

• Inoperative electrical parts
• Sluggish, lazy electrical devices
• Erratic, intermittent devices
• Devices that work sluggishly or erratically during periods of high electrical loads
• Excessive radio interference or noises in the radio
• Damaged throttle or transmission cables or linkage
• Repeated throttle or transmission cable failures
• Damaged drivetrain parts
• Engine or transmission performance complaints
• No starts or hard starts
• High sensor or computer voltages
• Erratic engine or transmission computer performance
• False trouble codes in the memory of any on-board computer
• Premature or repeated A/C compressor clutch failure

This symptom list brings up several points:

1. Visual inspections miss most cases of electrical voltage drop. You usually can’t see the corrosion inside a connection or the damaged wire that is causing the problem.
2. Ground-side voltage drop, a commonly overlooked cause of electrical trouble, can cause most of these symptoms. Any circuit or component is only as good as its ground.
3. The more sophisticated electrical systems become, the more important their grounds are. The number of electrical components has increased rapidly and most do not have separate ground wires. Instead, these devices are grounded to the engine or body. Rust, grease, vibration, and/or careless repairs often restrict the circuit from the engine/body back to the battery.
4. Many components, such as engine sensors, share a common ground. Therefore, a bad ground complicates diagnosis because it affects several components at once.
5. Some shop manuals and diagnostic charts or fault trees recommend checking grounds last. It is much quicker to check ground circuits before you climb that fault tree.
6. It is quicker and smarter to routinely check a circuit’s voltage drop than it is to memorize long lists of symptoms. If experience has taught us nothing else, it is that chasing symptoms is no substitute for routine and thorough voltage drop testing.

There are other reasons to check voltage drop first, too. Voltage drop, usually on the ground side, causes inaccurate or bizarre digital multimeter (DMM) readings and oscilloscope patterns. Moreover, when you connect a digital multimeter or scope to a system with bad grounds, the test equipment itself can create a good substitute ground, depending on the instrument’s impedance. If impedance is low enough, this can be frustrating since your equipment is connected, the circuit works, and you can’t find anything wrong.

Causes of Voltage Drop

Several factors contribute to voltage drop in automotive electrical systems:

• Resistance in conductors: Corrosion, loose connections, or damaged wires can increase resistance, leading to voltage drops.
• Length of wiring: Longer wires have higher resistance, which can cause more significant voltage drops.
• Current load: High-current components, such as starter motors, can cause substantial voltage drops if the wiring isn't adequately sized.

Negative Consequences of Voltage Drop: Why Is It Bad?

Voltage drop can cause a range of performance issues in automotive electrical systems, often leading to frustrating and costly repairs. Here’s why excessive voltage drop is problematic:

1. Reduced component performance

Many electrical components in a vehicle, such as fuel pumps, headlights, and ignition systems, require a consistent voltage to function properly. A significant voltage drop can result in:

• Dim headlights: Reduced brightness affects visibility.
• Weak fuel pump operation: An underperforming pump may lead to poor engine performance or even failure to start.
• Slow or weak cranking: If the starter motor doesn't receive sufficient voltage, the engine may struggle to turn over.

2. Increased heat and potential damage

Excessive resistance in a circuit due to corroded connections, frayed wires, or poor grounding can generate heat. This can cause:

• Wire insulation damage: Overheated wires may melt, shorting circuits.
• Premature component failure: Excess heat can degrade sensitive electronic components, shortening their lifespan.

3. Electrical system instability

Modern vehicles rely heavily on sensors and computer modules. A voltage drop in these systems can result in:

• Erratic sensor readings: Misleading data can trigger false warning lights or cause the engine control unit (ECU) to make incorrect adjustments.
• Malfunctioning electronic systems: Automatic transmissions, anti-lock brakes, and infotainment systems behave unpredictably.

4. Inefficient power distribution

When voltage drops occur in a vehicle’s wiring, other components may try to compensate, leading to:

• Increased current draw: Components pull more current, putting additional strain on the electrical system.
• Battery drain: A struggling alternator and increased electrical demand can drain the battery more quickly, reducing its lifespan.

5. Potential safety hazards

In severe cases, excessive voltage drop can compromise vehicle safety by:

• Disrupting critical safety systems: Airbags, brake lights, and traction control systems may not function as intended.
• Causing electrical fires: If unchecked, high resistance and excessive heat can lead to fire hazards