Fuses, Relays, MOSFETs and Drivers
Between the battery and any component there is protection and a switch. Knowing which kind of switch is in the way changes how the circuit is tested.
In plain language
Nothing on the car is connected directly from the battery to the part. Power passes through a fuse, which protects the wiring, and then through some kind of switch that decides when the part is allowed to work. That switch may be a relay — an electrically operated mechanical switch — or it may be a solid-state device inside a module with no moving parts at all.
The important consequence is that a component with power sitting at it is not a component that has been switched on. Most modern circuits are controlled on the other side, by the module, and that is the side that has to be tested.
What the customer sees
A function that works intermittently, works when cold, or stops working when several things are switched on at once. A repeatedly blown fuse. A component that a parts swap did not fix.
What the module is doing
Switching the load through a driver stage and then watching it. Modern drivers sense the current they are passing and protect themselves against short circuits, open circuits and overheating by shutting the output down and recording a fault. That is a deliberate, protective behaviour, not a failure.
What a technician tests
Which side of the load is switched, and then that side. Supply and ground under load, whether the control signal is actually being issued, and whether the load responded. A fuse is checked for continuity, but a fuse that blows repeatedly is a symptom to be traced, never a part to be up-rated.
The technical detail
| Part | What it does |
|---|---|
| Fuse | Protects the wiring by opening when current becomes excessive. It protects the harness, not the component. |
| Relay | Uses a low-current electromagnetic coil to operate higher-current mechanical contacts, isolating the control side from the load side. |
| MOSFET | A fast semiconductor switch with no moving contacts, able to switch precisely and repeatedly — which is what makes pulsed control possible. |
| Power driver | The full output stage. Typically a MOSFET plus current sensing, thermal protection and fault detection, so the module knows what the circuit did. |
A typical supply path runs battery → fuse → relay or driver → the component's supply, with the module switching the control side of the load. For an injector, for example, supply arrives at one side while the module's driver switches the other side precisely; the component only works when both are correct.
Contacts wear; semiconductors do not, but they do get hot. A relay can develop resistance across its contacts that only shows itself under load, which behaves exactly like the voltage-drop faults described in the electricity page. A driver stage, by contrast, is more likely to protect itself and report than to degrade quietly.
Pulsed control is normal. Rapid switching is how modules vary the effective power delivered to motors, lamps and solenoids. A simple measurement on a pulsed circuit can read as something between on and off, and be entirely correct.
High-side and low-side switching both exist. Whether the module switches the supply side or the ground side of a load determines what a measurement at the component actually means. Establishing which one applies before interpreting a reading avoids a very common wrong turn.
Why this matters when something goes wrong
Output-side fault codes are written from the driver's point of view. "Open circuit", "short to ground" and "output-stage fault" describe what the driver detected in the circuit it was switching — which includes the connector, the wiring, the load and its ground, not only the module.
Repeat failures deserve the same reasoning. A relay or driver that fails twice is usually being asked to switch a load that is drawing more than it should, and replacing the switch without measuring the load simply schedules the next failure.