Relays: reading the numbers on top, and testing one in the car

A relay is a switch operated by a magnet instead of a finger, and its whole purpose is to keep heavy current away from places heavy current should not go.

The original puts that better than most: without relays the wiring loom would have to carry full load current all the way to the dashboard and back, so it would be longer, heavier, and hot. Instead a thin wire carries a few hundred milliamps to the coil, the coil pulls a contact closed, and the fat wire from the battery to the headlamp or the fuel pump never leaves the engine bay.

That is the entire idea. What follows is how to read one and how to test one — and the original, which is 534,000 views and the most-read electrical piece in the archive, covers neither.

The numbers on top

They are not arbitrary and they are not the manufacturer's. They come from DIN 72552, and once you know four of them you can wire or test any standard relay without a diagram.

So a relay with 30, 85, 86 and 87 is a simple on-off switch. Add 87a and it is a changeover — it can switch one feed between two destinations, which is how a headlamp stalk sends power to dipped or main beam.

Which way round 85 and 86 go usually does not matter — a coil is a coil. There is one exception and it is worth knowing, because it is a short circuit rather than a non-event. A relay with a suppression diode across the coil is polarity-sensitive. Fit it backwards and the diode conducts, which either blows the fuse feeding the control circuit or, on a module-driven relay, damages the module output that was switching it. Look for a diode symbol on the case, and if there is one, respect 85 as earth and 86 as the positive.

Why the diode is there at all: switching off a coil makes it generate a large reverse voltage spike as its magnetic field collapses, and the diode gives that spike somewhere harmless to go instead of into whatever switched it. A resistor across the coil does the same job less well and does not care about polarity.

Testing one, and why the usual method sometimes lies

Swap it with an identical one. This is the right first move on any car with several matching relays in the box, it takes ten seconds, and it is conclusive when it works.

And when it does not work, the corpus has the reason: a corroded relay socket fails exactly like a bad relay, and swapping in a known-good relay proves nothing because the fault is the thing you plugged it into. A high-current contact that has arced generates heat, so the evidence is discolouring and softening around the socket terminals rather than anything on the relay itself. Look into the socket before ordering a part.

Then, in order of what each proves:

• Listen for the click. It tells you the coil is being commanded and is pulling in. It tells you nothing about the contacts.
• Check the coil. A few tens to a couple of hundred ohms across 85 and 86. Open circuit means a dead coil.
• Bridge 30 to 87 with a fused jumper. If the headlamp or pump now runs, everything downstream is fine and the fault is in the control side or the relay. If it does not, the fault is downstream and the relay was innocent.
• And the one that finds the fault the others miss: with the relay energised and the load actually running, measure the voltage drop between terminal 30 and terminal 87. It should be a few hundredths of a volt. Burnt contacts can pass enough current to click, to light a test lamp and even to run the load weakly, while dropping a volt or more — which starves the load and heats the relay. Using a multimeter explains why a resistance test cannot see this and a loaded voltage-drop test can.

A relay that clicks is not a relay that works. That sentence is the whole of relay diagnosis.

The worked example the original is really about

Almost all of the source is a pin-by-pin account of one specific part — the double relay fitted to the Peugeot 405, Samand, Pride, Xantia and their relatives. It is two relays in one housing on a twelve- or fifteen-way connector, and between them they feed the engine module, the fuel pump, the injectors, the coil, the canister purge valve, the oxygen sensor heater and the throttle heater.

That is worth keeping, because it is a good illustration of a pattern found on every fuel-injected car under a different name. The corpus calls it the engine control module power relay, and the behaviour is the same everywhere:

• Ignition off — a permanent feed keeps the module's memory alive.
• Ignition on — the relay energises for a couple of seconds, priming the fuel pump and waking everything up. That two-second whirr from under the back seat is the pump, and hearing it is a free confirmation that this relay did its job.
• Running — everything stays powered.
• And after switch-off the module holds the relay in for a while itself, to finish writing what it has learned and to run any after-run functions, which is why some cars click and hum for a minute in a quiet garage.

The source also gives the single best diagnostic in the whole piece, and gives it in one line without noticing: if the warning lights do not come on when you turn the key, suspect this relay. No relay means no module, and no module means no bulb check. It costs nothing to observe and it separates "the car is dead" from "the starter is dead" instantly.

One caution on the pin lists it prints. They are specific to particular looms and models, and the source says so — different manufacturers of the same part number use different arrangements. Use them as an illustration of the shape, and the wiring diagram for the actual car for anything you intend to probe.

What a failing relay looks like from the driver's seat

• Intermittent everything. A relay whose contacts are marginal works when cold and drops out when hot, so the car cuts out, restarts twenty minutes later and tests perfectly in a workshop. This is the same shape as the crank sensor fault in the crankshaft position sensor, and it is misdiagnosed just as often.
• A single click and nothing else when you turn the key — the coil is fine and the contacts are not.
• No click at all — no command, so look at the control side: the module output, the switch, the earth, or the fuse feeding the coil.
• A relay that gets hot, or a socket that has discoloured. That is resistance where there should be none, and it is on its way to melting the connector.
• A relay that never releases. Welded contacts, and the load stays on. The corpus notes this specifically for cooling fans, where it is irritating rather than dangerous; on a fuel pump it is a different matter.

What relays are worth knowing about generally

They are cheap and they are consumable. A relay switching an inductive load — a motor, a pump, a fan — erodes its contacts a little every time it opens, so a hard-working one is a wearing part rather than a permanent fixture.

They are also the cheapest thing in most circuits, which is why the corpus's advice on the cooling fan applies to nearly everything: check the relay before the motor. It fails more often and costs a fraction as much.

And the fuse protects the wiring, not the relay. A relay whose contacts have welded is not a fault a fuse can see, because the current is normal — it is simply flowing when it should not be.

Video guides

Video, for the parts of this that are easier watched than read. The English-language ones come first.