Electrical Boot Camp · Module 00

Voltage-Drop Testing

A6 · General Electrical ≈ 8 min Most-tested A6 procedure
01 The Concept

Why a "good" wire can still starve a component.

Every connection and length of wire has a little resistance. Push current through it and that resistance uses up voltage — the voltage that was supposed to reach the load. That lost voltage is the voltage drop. A healthy circuit wastes almost none; a corroded connector or loose ground can eat 1–2 volts, and the component runs weak, slow, or not at all.

V = I × R
Volts dropped = Amps × Resistance

More current or more resistance = more voltage lost. This one line explains every reading in this lesson.

≤ 0.5 VPower side — total allowable drop from battery + to the load, key on and loaded.
≤ 0.2–0.3 VGround side — tighter, because grounds must be near-perfect. Tighter still on starter circuits.
⚑ The trap ASE loves

You must test with the circuit ON and loaded. An ohmmeter checks resistance with the circuit off and almost no current — so a bad connection can read near 0 Ω yet drop 1–2 V under load. When a question describes a weak or intermittent part, the answer is a voltage-drop test, not a resistance check.

You measure in two places: the power (feed) side — battery positive through the fuse, switch, and wiring to the load — and the ground (return) side — from the load back through the ground connection to battery negative. High drop on either side steals from the load. The demo below lets you break the ground and watch it happen.

02 See It Live

Two headlights, same feed. Corrode the right one's ground and watch it starve.

+ – 12.6 V FUSE 15A HEADLIGHT · LEFT HEADLIGHT · RIGHT BAD GROUND 0.60 Ω VOLTMETER · GND 2.35 V A B C
Right headlight — ground resistance0.60 Ω
LeftGood ground
12.5Volts
4.78Amps
59.7Watts
Brightness100%
RightBad ground
10.2Volts
3.92Amps
40.1Watts
Brightness67%
Voltage drop on ground 2.35 VFails > 0.2 V

Point B is the ground side of the right bulb — it should sit near 0 V. On "Corroded" it climbs past 2 V, so the bulb only sees what's left (A − B) and dims. That wasted power (12.1 W) becomes heat at the bad joint.

03 What If…

Run the reasoning before you reveal the answer.

?The corroded ground reads 0 Ω on your ohmmeter, but the headlight is still dim. How is that possible?
?You measure 0.6 V on the power side and 0.1 V on the ground side. Which side is the problem?
?A blower motor runs slow. Battery is 12.6 V, but you read only 9 V at the motor's feed with it running. Next step?
04 Practice

ASE-style. Pick an answer to see why.

05 Recap
  • Voltage drop = voltage lost to resistance. Test with the circuit on and loaded.
  • Limits: ≤ 0.5 V total power side, ≤ 0.2–0.3 V ground side (tighter on starters).
  • An ohmmeter can read 0 Ω and still hide a connection that drops volts under load.
  • High drop on either side starves the load — dim, slow, or dead.
  • V = I × R interprets every reading you take.
Redline · Electrical Boot Camp · Module 00