A voltage drop test shows how much voltage you're losing in a wire, a connection, or a ground. It's the best way to find a bad cable or a corroded connection, and it's all over the A6.
The big rule: the circuit has to be working when you test it. A bad connection only shows up when current is actually flowing through it.
Why the circuit has to be loaded
Resistance doesn't show up as voltage loss unless current is flowing. A corroded cable can read fine with nothing running and then lose a bunch of voltage the second the starter kicks in. That's why starter cable tests are done while cranking.
Where the leads go
Put one lead on each end of the part of the circuit you're checking. The meter reads the voltage lost between those two points.
- Starter positive cable: battery positive post (not the clamp) to the starter's battery terminal, while cranking with the engine disabled
- Solenoid contacts: across the solenoid's battery terminal and motor terminal, while cranking
- Charging output: alternator output terminal to battery positive, engine running with loads on
What the readings mean
Service information gives you the limit for each part of the circuit. As an example, a positive cable might be allowed 0.5 V. If it drops 1.2 V, more than twice that, the cable or its connections need to be fixed.
A clean, tight connection should drop almost nothing. Losing 0.8 V between a battery post and its own clamp, when the limit is 0.1 V, means corrosion or a loose clamp is robbing the starter.
Don't forget the ground side
Grounds go bad just like positive cables. A ground with too much drop can cause all kinds of weird stuff. A good example: an aftermarket amp grounded to a painted seat bolt with 0.6 V of drop lets alternator whine come through the speakers. Move it to clean, bare metal and recheck.
Slow crank with a good battery
If the battery passes a load test but the engine cranks slow, voltage drop testing is how you find out if the cables are the problem. Check the positive side and the ground side separately, so you know which one is bad.
Cheat sheet
- Voltage drop = voltage lost in a wire, connection, or ground
- The circuit has to be working when you test
- Put the leads on each end of the section you're checking
- Compare to the limit in service information
- Test the positive side and the ground side separately
Common questions
Why do a voltage drop test instead of checking resistance?
A voltage drop test checks the circuit while it's actually carrying current, which is when a bad connection shows up.
Where do the leads go for a starter cable test?
Battery positive post to the starter's battery terminal, while cranking with the engine disabled.
What does a high voltage drop mean?
Too much resistance in that section, usually a bad cable, a loose or corroded connection, or a poor ground.
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