Voltage Drop Test
Measures the voltage lost across a live circuit section to find resistance a continuity test will never see.
What this test tells you
When to use it
- Anything electrical works, but works weakly or slowly
- A circuit passes a continuity test yet the component still misbehaves
- After finding corrosion, a repaired joint, or an aftermarket splice
- Slow cranking with a battery that tests good
Safety
- Keep leads, hands and clothing away from belts, fans and hot exhaust
- Never bridge battery positive to ground with a meter lead or tool
- Set the meter to volts before touching a live circuit
- On hybrid or EV vehicles, do not touch orange cabling — high voltage requires specific training and equipment
Vehicle setup
- Battery in a good state of charge — a flat battery invalidates the result
- The circuit under test must be switched on and drawing its normal current
- Engine off unless the test specifically requires it running
Meter setup
- Tool
- Digital multimeter
- Function
- DC volts
- Range
- Auto range, or 20 V DC — expect readings under 1 V
- Sockets
- Red in VΩ, black in COM
Practise it — interactive simulation
Starter positive circuit
Crank the engine and measure the loss between the battery and the starter.
- GND — Battery → BPOS
- BPOS — Positive cable → J1
- J1 — Cable joint → J2
- J2 — Solenoid contacts → SB
- SB — Starter motor → SG
- SG — Engine earth strap → ENG
- ENG — Battery earth lead → GND
Loading the interactive circuit…
RED Battery positive post (the lead post, not the clamp)
BLACK Supply terminal at the component
Total positive-side drop, including every connection between the two points
RED Component ground terminal
BLACK Battery negative post
Total ground-side drop — check this even when the positive side looks fine
RED One side of a single suspect connection
BLACK The other side of that same connection
Halving down: once the total is high, narrow it one joint at a time
Procedure
- Confirm battery voltage first — record the resting voltage so you know what the circuit started with
- Switch the circuit on so it is drawing its normal current. No current means no drop, and a perfect-looking reading
- Place the red probe at the source and the black probe at the destination for the positive side
- Read the meter while the circuit is loaded, not before or after
- Repeat on the ground side with red at the component ground and black at the battery negative
- If either total is out of spec, halve the run: move one probe along the circuit until the drop appears between two adjacent points
- That last section — between the two probes when the reading jumps — contains the fault
Normal result
Under 0.2 V across a typical low-current wire or connection, under 0.1 V across a good ground, and under 0.5 V total from battery to component on most accessory circuits. On starter cables under cranking load, under 0.5 V per side is the usual workshop limit.
Fault result
Anything meaningfully above those limits. A reading approaching full source voltage means the circuit is open at that point and all the voltage is being dropped across the break.
Interpretation
IfPositive-side drop is high, ground side is fine
ThenResistance is in the feed: fuse box, relay contacts, connector or the cable itself
NextHalve the positive run to isolate the connection responsible
IfGround-side drop is high
ThenPoor body earth, paint under the eyelet, corroded strap or loose fixing
NextClean the earth point to bare metal, refit with a star washer, retest
IfBoth sides are high
ThenUsually corrosion through the whole harness or a battery clamp problem feeding both paths
NextStart at the battery terminals, then work outwards
IfDrop reads near source voltage
ThenThe circuit is open between the probes — nothing is flowing
NextTreat it as an open circuit, not a high-resistance fault
IfDrop is fine but the component still underperforms
ThenThe supply is healthy; the fault is in the component or its control
NextMove to component testing or the control-side signal
Common mistakes
- Testing with the circuit switched off — with no current, even a badly corroded joint reads 0.00 V
- Probing clamps instead of the posts, which hides a bad clamp-to-post connection
- Only checking the positive side and ignoring the ground
- Using a battery that is already flat, so every reading looks bad
- Comparing readings from different current levels and expecting them to match