Relays have been a common part of all kinds of electrical circuits, including automotive applications for decades. Even the Ford Model A had relays for things like the horn and the headlights.
Relays are designed to allow low-current switches to control high-current components like the blower motor, headlights or certain solenoids. Think for a moment about all the small-gauge wiring (some as thin as a strand of spaghetti) that’s used to control high-current loads. Relays are a key part of control circuits that isolate and manage high and low current flows.
A relay is an electromechanical or electronic switch. A common design is the “yoke” relay, with a wire coil wrapped around a soft iron core, an armature that is hinged and held in place with a spring, and one or more sets of contacts. When current is applied, the wire coil generates an electromagnetic field, which causes the armature to activate and close the circuit.
Electronic relays might use a transistor or triac as the switching element, allowing them to control a much larger circuit. Common electronic relay designs include optocouplers (with an LED and phototransistor), thermo-electric (with a bimetallic strip that responds to heat energy) or piezo relays (contacts operate mechanically via piezoelectric effect).
Most automotive relays are single-pole/single-throw or single-pole/double-throw and might draw less than 200 milliamps of current, but are designed to handle circuits with as much as 30 or 40 amps. Some relays are assigned to control more than one system or component. Most automotive relays are designed as a roughly 1-inch cube, with micro-relays being about half that size. Connection is via numbered pins arranged in different patterns. Relays will fail over time, although most are designed for around 100,000 on/off cycles before burnout. We’re going to talk about how to understand relays, how they function in automotive applications, and how to diagnose and repair/replace a failed relay.
Reading wiring diagrams and circuit diagrams
While wiring diagrams can be daunting at first, it’s easiest to think of them as a map that shows a route from the power source to a given component, using simplified, standardized symbols and lines. Circuit diagrams also will indicate color coding of wires, positions of terminals and devices in the system, grounding points and the direction of current flow along the way.
Some typical symbols include:
- Switches: Breaks in a continuous line
- Relays: rectangular box with a coil inside
- Ground: arrows facing downward, ending in a series of tapering horizontal lines
- Battery: a series of long and short parallel lines at a right angle to the wire, with short lines denoting negative and longer lines for positive polarity
Color coding is fairly standardized over model years, with Red indicating hot/switched power, Black indicating ground, Yellow showing constant power and Blue representing wiring to an accessory like a power window or antenna. These are common color codes, but aren’t necessarily universal across the board; consult service manuals and vehicle-specific wiring charts for the year/make/model vehicle you’re working on.
In the case of relays, it’s important to know internal connections and the function of each pin on the relay. Relays will typically have a diagram that shows you those internal connections, including which pins are in the “load” circuit, the “control” circuit, which pin is NO (normally open) and which is NC (normally closed). The diagram also will typically include voltage and amperage ratings, and (with any luck) a part number for easy reference and replacement.
While relays may have different pin layouts and functions, micro-relays usually have five pins with functions, as follows:
- Pin 1 and Pin 2: control/coil circuit
- Pin 3: common
- Pin 4: normally closed
- Pin 5: normally open
Tools to test relays and circuits
To effectively test relays, tools can range from a multimeter or simple light-up circuit tester probe to specialized circuit testers that analyze the entire circuit from the battery to the component in question. Some testers can identify a problem with wiring, ground, the relay or another part at the push of a button, and many feature ports that allow them to be used with other diagnostic tools.
Some also can jump the relay and be used as a remote start switch when testing compression, camshaft timing or compressor function for AC. Induction-style tools eliminate the need to pierce wires with a probe or connect multiple jumper wires. Some relay tester tools can even actuate the fuel pump without starting the engine.
In many cases, you should be able to hear an audible click as the electromagnetic switch inside energizes the coil and completes the circuit. Be mindful, though, that the relay itself may or may not be what’s at fault, and whatever failed and cooked your original relay might quickly do it with the replacement one, too.
It also can be a good idea to invest in specialized relay puller pliers, with longer, thinner jaws and specially designed tips, that hook into the edge of the relay housing, making it easy to remove or replace a relay without damaging it.
• How does a relay know it has been energized and how is it monitored?
Remember about certain pins being connected as Normally Open or Normally Closed? The NO contact connects the circuit when the relay is activated and disconnects when the circuit is deactivated. The NC contact disconnects the circuit when the relay is activated, then connects when the relay is deactivated. Other designs might control two circuits, with one NO and one NC contact and one common terminal, or SPST, SPDT, DPST or DPDT type relays.
On many systems in the vehicle, a failed relay will indicate a reading to the engine’s powertrain control module (PCM) that’s outside of normal parameters, which can store an OBD-II trouble code. The trouble code might not point you straight to the relay itself (especially for something like the blower motor or cooling fan), but it gives a good starting point for diagnostics in the case of a no-start condition, an intermittent or a stalling condition.
• Why swapping relays isn’t agreat idea.
Would you replace a failed household fuse with one that’s rated at a higher amperage? Of course you wouldn’t, and you wouldn’t do the same with an automotive fuse either. Just because another relay’s pins fit doesn’t mean it’ll be a good replacement; relays with different values can cause a voltage spike (sometimes over 100V in a 12V system), or can lead to a short that can burn up a whole system and cause a lot more headaches. Make sure that every time you diagnose and replace a relay, you’re using a replacement that’s an exact match and part number, with the same values to ensure compatibility.
