Wiring, Grounds & Relays
Most A6 electrical faults aren't a dead component — they're resistance hiding in a connector, a ground, or a corroded splice. This reference covers the wiring, grounds, relays, and circuit protection that carry every circuit, and the one habit that finds the problem: measuring voltage drop under load. Learn what a relay does, why a bad ground mimics a dozen unrelated faults, and how protection devices fail, and the A6 circuit questions stop being guesswork.
Why circuits fail where they do
Current has to flow through a control device, a load, connectors, splices, and back to ground. Resistance anywhere in that path steals voltage the moment current flows — which is why a circuit can read full voltage at rest and fail under load. A6 builds question after question on this, so the system here is less about parts and more about the path and how to test it.
Component reference
| Element | Job | Common failure | Symptom |
|---|---|---|---|
| Ground | Return path to battery negative | Corrosion, loose connection | Dim/erratic operation, odd unrelated faults |
| Connector | Junction in the harness | Corrosion, backed-out terminal | Voltage drop under load, intermittent |
| Relay | Switch a large load with a small signal | Burned contacts, open coil | Load won’t energize; clicks but no output |
| Fuse | Protect against overcurrent | Opens on a fault | Dead circuit (find the cause, not just the fuse) |
| Fusible link / breaker | Protect feeds / resettable circuits | Open link; breaker cycling | Dead high-current circuit; cycling load |
| Splice | Join wires | Corrosion, breakage | High-resistance point, voltage drop |
Voltage drop is the test
The A6 method for nearly all of this: with the circuit on and under load, measure the voltage dropped across each connection, ground, and the load. Voltage shows up where resistance is. A meaningful drop across a ground or connector — not across the load — is the fault. Checking “is there voltage present?” with the circuit dead finds nothing; measuring drop under load finds it every time.
Voltage-drop test (circuit live under load)
- Probes placed across the connection, not to ground
- Measures voltage the connection steals while current flows
- Finds a corroded ground or connector a continuity check passes
- The A6 go-to for 'voltage present but device fails'
Continuity check (circuit off)
- Tests whether a path exists, not how much resistance it has
- A corroded ground can pass continuity and still drop voltage
- Misses the most common A6 wiring fault
- Use for checking for opens; not for resistance under load
The call: When a circuit passes continuity but the device still underperforms, the fault is resistance under load. Voltage-drop the ground and each connector with the circuit energized.
The parts techs mix up
- A blown fuse is a symptom. Replacing it without finding the overload (a shorted load or chafed wire) just blows another. The test wants the root cause.
- Relay control side vs. load side. A relay that clicks but won’t power the load has a good control circuit and a bad load-side contact; one that won’t click has a control-side (coil/trigger) problem. Diagnose which side.
- Ground as an afterthought. A bad ground can make several unrelated circuits act up at once — chasing each load wastes time when the shared ground is the fault.
| Symptom | Real cause | Exam trap / correct call |
|---|---|---|
| Device weak or dim, voltage 'present' at the connector | High resistance in a ground or connector under load | Voltage present does not mean the circuit is good — voltage-drop test the ground and feed side with the circuit on |
| Multiple unrelated circuits acting up at once | Shared high-resistance ground | Don't chase each load separately — find and test the common ground first |
| Fuse keeps blowing on replacement | Short to ground or overload in the circuit downstream | Replacing the fuse is not the repair — find what's drawing excess current |
| Relay clicks but the load won't run | Burned or open load-side contacts | A click means the control circuit works; the fault is on the load side of the relay |
| Relay won't click at all | Open coil or no trigger signal (control side) | No click = control-side fault — check the coil voltage and trigger, not the load contacts |
ASE traps
- A blown fuse is the symptom — find what overloaded the circuit.
- Voltage present ≠ circuit good; resistance only reveals itself as voltage drop under load.
- A single bad ground can cause several unrelated symptoms at once.
Study the test in the A6 Electrical guide, pair with starting & charging components, and see electrical testing tools for how to take these measurements correctly. And before you can trace any of this on a schematic, make sure you can read every automotive electrical symbol on it.