REFERENCE · LIVE

A6 Voltage Drop Spec Card

Max acceptable voltage drops, by circuit type, under load. Print this and tape it inside the toolbox lid. Knowing the spec turns "high resistance somewhere" into a number you can hit.

RULES THAT APPLY TO ALL

Test under load — no current means no voltage drop.
DMM probes go on the same circuit, one on each side of the suspect part.
Always test ground side too (return path) — most "intermittent" faults live there.
Specs below are SAE/industry-typical for 12V systems. Check OEM service info for outliers.

Spec Table

Battery + cable to starter

Crank load · 150A typical

0.20 V

Drop > 0.3V = internal cable corrosion or undersized cable.

Battery ground / engine to chassis

Crank load

0.20 V

Strip paint at body bond, retorque, retest. Top root cause of "weird" starting.

Each wire run (per side, end to end)

Load current

0.30 V

Chafe, splice corrosion, undersized gauge.

Switch contacts

Activated, current flowing

0.10 V

Pitted contacts → replace. Cleaning is temporary.

Relay contacts (30 → 87)

Energized, load current

0.10 V

Cheap part — replace, don\'t analyze.

Connector / terminal junction

Per pin, under load

0.05 V

Lost pin tension, water intrusion, push-out. Re-pin or replace.

Fuse / fusible link

Per fuse, under load

0.10 V

Higher drop = aging fusible link. Common on older trucks.

Module signal ground

Engine running

0.10 V

Module reference depends on this. Affects sensor accuracy across the bus.

Sensor signal return (5V/0V reference)

KOEO

0.05 V

Drop here biases all sensors on that return. Watch for multiple weird sensor PIDs.

Total system drop (battery+ to load+, ground+ included)

Sum of all parts

0.50 V

If total > 0.5V, drop test each leg separately to find the offender.

SHORTCUT MATH

Ohm\'s Law for Drop Testing

V = I × R. The drop you measure is just the current at the time multiplied by the resistance at the test point. To convert your reading into resistance:

R (Ω) = V_drop ÷ I_load

Example: starter cable dropping 0.6V at 150A crank = 4 milliohms. Spec is 1.3 mΩ (0.2V / 150A). Cable is 3x worse than spec.

BAY-FLOOR DECISIONS

When the drop is over spec

Cable over spec → replace. Internal corrosion can\'t be cleaned.
Ground over spec → strip paint, clean, retorque, dielectric grease, retest.
Connector over spec → un-pin, inspect terminal tension, replace if loose.
Switch over spec → replace. Don\'t clean and reuse.
Relay over spec → replace. They\'re cheap and contacts wear unpredictably.
Fuse over spec → replace or repair fusible link with crimp-and-solder repair.