Understanding Relays: Confusing? Yes.
Impossible? Heck No.
By Rob Siegel (Reprinted by permission of Hagerty Media)
The term “relay” is usually only mentioned when a component like a fuel pump or a fan stops working, or some new current-hungry device is being installed. Because they’re literally small black boxes, many people are intimidated by relays. But they’re really very simple. If you understand what relays are, how they work, and what the standard terminal-numbering is, you can easily install them, troubleshoot them, replace them, or if necessary, bypass them.
A relay is nothing more than a remote-controlled switch. Imagine the scene in Young Frankenstein where the giant sparking knife-switch was closed in order to let the lightning flow and shock the creature into life. If you said, “I don’t want to touch that switch with my hands—I’ll connect a pair of ropes to it to open and close it,” you’d be doing pretty much what a relay does. Seriously.
Every little square or round relay you’ve ever seen in a car has a little electromagnet inside that’s used to “throw the switch” by pulling two internal contacts together. That’s really all it is. If, for example, you want to install a bank of driving lights on your bumper that draws tens of amps of current, you probably don’t want to have to run thick wires to and from the switch on the dashboard that turns them on. Instead, you want to keep those thick wires between the lights, battery, and relay, and use thin wires between the relay and the switch. But because the electromagnet requires current to flow through it, and that’s different current than what needs to flow through the thing you’re trying to turn on, it requires a bit of explanation.
Low-Current and High-Current Paths
Every relay has a low-current side and a high-current side. The low-current side goes to the electromagnet (the “switch”). The high-current side feeds the thing it’s trying to turn on (the fan, the lights, the fuel pump, a power amp, the car’s ECU, whatever). For about 50 years, the terminals on the bottom of most relays have been labeled with numbers that come from the Deutsches Institut für Normung (German Institute for Standards, abbreviated as DIN). The ones that apply to relays are:
In Frankenstein-Speak:
The Remote-Controlled Switch
When you feed 12 volts to terminal 86 and connect terminal 85 to ground, it energizes the electromagnet, which pulls terminals 30 and 87 together. Waaaay better than two ropes.
The “Lightning”
If terminal 30 is connected to a high-current source of power (the battery), and terminal 87 is connected to the device you want to turn on (e.g., the fan), current will flow to shock the monster into life.
Add fuses, and this is exactly how you hook up a relay to control, say, a new radiator fan.
Bypassing a Relay
The beauty in knowing all this is that you now can completely understand what’s involved in bypassing a relay. Let’s say that your fuel pump isn’t turning on. When someone says “bypass the relay,” what they mean is “unplug the relay from its socket, read the numbers off the bottom of the relay, carefully transpose them right-to-left onto a piece of paper, set the paper down next to the socket, find terminals 30 and 87, and connect them with a short, thick piece of wire.” If the pump whirs to life, it means that either the relay is bad or there’s a problem in the wiring on the low-current (electromagnet) side. If the pump doesn’t turn on, it means that either terminal 30 isn’t getting voltage from the battery, terminal 87 isn’t reaching the fuel pump, or the pump itself is bad.
Other Stuff
What I’ve described above is a Single-Pole Single-Throw (SPST) relay. These are by far the most common type of relay used in a car. However, there is also a Single-Pole Double-Throw (SPDT) or “changeover” relay that allows switching between two devices. It has a fifth terminal, labeled 87a. When the electromagnet is energized, current flows to terminal 87, but when the electromagnet is de-energized, current instead flows to 87a. Some early low-beam high-beam switches use a changeover relay. I’ve also seen them in early air conditioning systems to ensure that the heater blower fan and A/C fan aren’t on at the same time.
Although the DIN standard prescribes the numbers and what they’re connected to, and although a small cube-shaped SPST relay typically has three vertical terminals and one horizontal one, it doesn’t standardize the physical terminal location. 87 and 85 are often in the same place, but 86 and 30 are sometimes swapped. You need to look for the little injection-molded numbers on the underside of the relay to be certain of their locations.
On a relay for a simple device like a motor or a light, the low- and high-current sides are both bidirectional; it usually doesn’t matter if 30 and 87 are swapped, or if 86 and 85 are swapped. But on a relay controlling an ECU or other sensitive electronic part, there is often a diode in the relay that will only allow current to flow in one direction.
As per the illustration, there is normally a fuse on both the high-current side (between the battery and terminal 30) as well as on the low-current side (between the battery and terminal 86). And what I have labeled as a “switch” can also be something like a temperature sensor triggering a fan to turn on. In addition, although the illustration shows the relay as switching on the positive side (turning on when it’s fed voltage), it can also switch on the negative side. For example, most horns work by allowing the horn contact in the steering column to complete the path between terminal 85 and ground.
That’s most of it. So if a friend’s car won’t start, you can now sound like you know what you’re talking about by authoritatively asking, “Have you tried jumping pin 30 to 87 on the fuel pump relay?”
Editor’s Note: A big thank you to Hagerty Insurance and Hagerty Media for the conscious effort to support the collector car hobby and community with content like this. More is available at Hagerty.com as well as all of their insurance products that assist in keeping our cars on the road. Thank You!!!