With a 4-pin automotive relay, you can only control one accessory or appliance. Meanwhile, a 5-pin automotive relay can control two loads from one power source or one load from two power sources.
An automotive relay is one of those small components your car, van, or truck needs to function properly. It has also proven very useful for controlling power to accessories and appliances in adventure vehicles. But should you get a 4 pin relay or a 5 pin relay? Any type of relay is essentially a low-voltage electronic switch designed to safely and effectively control high-current loads. In vehicles, these loads can include the starter motor, stereo, lights, dashcam, wipers, horn, windows, seat warmers, cigarette lighter, et cetera.
For caravanners, RVers, and boondockers, though, 12V power relays manage more than just the vehicle's electronics. You can use a relay to power one or two loads without risking draining the starter battery. And in this guide, we'll show you how to achieve that effectively.
What's the Difference Between a 4 Pin and 5 Pin Automotive Relay?
When shopping for an automotive relay for your vehicle or camper project, you can either choose a 4-pin or 5-pin version. The 5-pin relay is the most popular in the industry, but most users underutilise its full functionality.
| 4-Pin Automotive Relay | 5-Pin Automotive Relay | |
|---|---|---|
| Type | SPST | SPDT |
| Switching Mode | NO only | NC & NO |
| Integrated Fuse | YES | NO |
| Voltage Rating | 12V – 14V | 12V – 14V |
| Current Rating | 30 Amps | 40 Amps |
| Wiring Schematic | YES | YES |
| Mounting Bracket | YES | YES |
| External Terminal Points | Pre-wired harness | Pre-wired harness |
What Is a 5 Pin Relay?
Simply, a 5-pin automotive relay is a type of relay that has five terminal pins for connection—86, 85, 30, 87, and 87a. If you remember our previous guide on how to choose the right power relay, the five terminal points basically make it an SPDT (Single Pole Double Throw) configuration.
- 86 pin — the terminal point for the positive wire connected to the coil (input side) that triggers the relay
- 85 pin — the terminal point for the ground wire connected to the coil (input side) to return current back to the source
- 30 pin — the terminal point for the common (COM) connection of the relay's switch (output side) that powers your accessory or appliance
- 87 pin — the terminal point for the NO (Normally Open) connection (output side) that powers your accessory or appliance
- 87a pin — the terminal point for the NC (Normally Closed) connection (output side) that powers your accessory or appliance
What Is a 4 Pin Relay?
Meanwhile, a 4-pin automotive relay is a type of relay that has four terminal pins for establishing a connection—86, 85, 30, and 87. This makes it an SPST (Single Pole Single Throw) configuration, which is the most basic relay available.
- 86 pin — the positive terminal point for the coil (input side)
- 85 pin — the ground terminal point for the coil (input side)
- 30 pin — the common (COM) connection for powering your accessory or appliance
- 87 pin — the NO (Normally Open) connection for powering your accessory or appliance
Therefore, we can say that a 4-pin relay is basically a 5-pin relay without the 87a (NC) terminal connection. The wiring is exactly the same for the four identical terminal pins, as we'll break down in the next section.
How to Wire a Relay in Your Vehicle, Caravan, or Motorhome
Again, wiring a 4-pin or 5-pin automotive relay is the same for all identical terminal pins. The key difference comes when you use the extra 87a terminal pin on the 5-pin version to control an additional load or power source.
Whichever of the two relays you end up using, most brands include a wiring schematic on the top or side of the unit. The relay wiring diagram gives you a visual illustration of how the device is internally connected between the primary (coil) side and the switch side.
For instance, you'll notice our available automotive relays have the coil side with the 86-pin terminal on top and the 85-pin terminal at the bottom. The 30-pin terminal has continuity with the 87a terminal, so the relay's resting position (when off) will always connect to it. Then, the 87-pin terminal sits between the 86-pin and 87a-pin terminals, but closer to the 87a terminal for maximum contact with the switching armature when the coil's electromagnet activates.
As for the connection side, the external pins of an automotive relay can either be male quick-disconnect spade connectors or lead terminals. A relay with spade terminals requires female quick-disconnect connectors, whereas a relay with terminal wires can use either type of connector.
In our little project here, we'll be working with pre-wired automotive relays. However, you can always reach out to us if you need help with a spaded relay setup.
What You'll Need
- A 5A blade fuse
- 2 × 6.0mm² butt connectors
- A 6.0mm² ring terminal connector
- A blade piggyback tap fuse holder
- A butt connector (splice connector)
- A 30A inline fuse holder with fuse
- A 1.5mm² ring terminal connector
- 2 × 1.5mm² butt connectors (splice connectors)
- Properly sized electrical wires — 1.5mm² wire for the coil input and 6.0mm² auto cable
- A 12V 4-pin automotive relay — we'll be working with our 30A model that includes a fuse holder
4 Pin Relay Wiring: Step-by-Step Guide
- Prepare Wire1. Take a suitable length of 1.5mm² wire (Wire1) and crimp a ring connector onto one end. Then, crimp one of the extra butt connectors onto the other end.
- Prepare Wire2. Grab a second 1.5mm² wire (Wire2) and crimp a butt connector onto one end. Then, crimp the other end to the butt connector on the tail lead of the piggyback tap fuse holder.
- Connect the positive input of the relay. Take your 4-pin relay and crimp its white 86-pin lead onto the butt connector on the now-extended wire from the piggyback fuse holder.
- Connect the ground input of the relay. Onto the butt connector you created on Wire1 (in Step 1), crimp the relay's black 85-pin lead. Then, connect the ring connector on the other end of Wire1 to your van's common ground point.
- Make the NO connection. Next, take a suitable length of 6.0mm² cable (Cable1) and crimp a butt connector onto one end. Then, crimp the relay's red 87-pin lead to this butt connector.
- Hook up the COM terminal. Get a second 6.0mm² cable (Cable2) and install the 30A inline fuse on one end. Then, crimp a butt connector onto the other end and connect the relay's blue 30-pin lead to it.
- Power up the COM terminal. Crimp a ring connector onto the free wire end of the inline fuse holder on Cable2. Then, connect this ring terminal to the primary busbar linked to your van's starter battery. Also, make sure the cable running from the battery to the busbar is protected with the correct fuse size.
- Connect the load. With Cable1 (from Step 5), connect its other end to the accessory or appliance you want to control.
- Identify your ignition-on fuse. Next, head to your van's fuse panel and locate a low-current IGN (ignition-switched) fuse—5A, 7.5A, or 10A—and remove it. Then, insert this fuse into the bottom slot of the piggyback fuse holder connected to the relay.
- Fuse up the relay. Insert the new 5A blade fuse you bought for the relay into the remaining top slot of the piggyback fuse holder. Then, plug the piggyback fuse holder into the now-empty non-critical ignition-on fuse slot identified in Step 9.
PS: In our test, we used a 20A DC-DC battery charger, as we did not have another available high-current load. We connected this cable to the charger's positive input terminal, while the negative terminal (of the charger) was connected to the van's ground stud.
How the 4 Pin Relay Setup Works
At this point, your relay is ready for use, while the load is waiting for the relay to send power to it. When the ignition is OFF, the relay's control circuit remains open, so the power circuit also stays disconnected. Once you switch the ignition ON, though, the relay energises, closing the circuit and allowing power to flow to the connected load.
How to Wire a 5 Pin Relay
As we said, the process of wiring a 5-pin automotive relay is mostly the same as wiring the 4-pin version. The main difference is that you must also connect the additional 87a pin to your desired load.
It's also worth noting that the extra terminal wire can slightly disrupt the colour coding of pre-wired relay terminal wires. The 86-pin wire is still usually white and the 85-pin wire black. But the remaining wires may be interchanged between blue, red, and yellow. That means you may now have:
- White 86-pin wire
- Black 85-pin wire
- Red 87-pin wire
- Blue 87a-pin wire
- Yellow 30-pin wire
In other cases, you may encounter a White 86-pin wire, Black 85-pin wire, Blue 30-pin wire, Red 87a-pin wire, and Yellow 87-pin wire. Sometimes, it may even be a White 86-pin wire, Black 85-pin wire, Blue 30-pin wire, Red 87-pin wire, and Yellow 87a-pin wire.
Whichever colour coding the wires use, the wiring process is the same across all 5-pin automotive relays. Below is a brief breakdown of how you can wire one based on the 4-pin relay wiring guideline above:
- Take your relay and crimp its 86-pin lead onto the butt connector on the wire of the piggyback fuse holder.
- Attach the 85-pin lead onto the butt connector you created on Wire1. Then, connect the Wire1 ring terminal to your van's common ground point.
- Next, connect the 87-pin lead to the butt connector of Cable1. Then, connect the other end to the load you want to control on the NO (Normally Open) terminal.
- Add the 30-pin lead to the butt connector connected to Cable2. Then, connect the other end to the ring connector to your starter battery busbar.
- Connect the 87a-pin lead to the positive wire of the second load you want to control on the NC (Normally Closed) terminal. Then, attach the load's negative wire to a ground point.
- Now, remove the IGN (ignition-switched) fuse you want to tap into and insert it into the bottom slot of the piggyback fuse holder connected to the relay. Then, insert the new 5A blade fuse for the relay into the top slot.
- Finally, plug the piggyback fuse holder into the empty “ignition-on” fuse slot from Step 6.
How the 5 Pin Relay Setup Works
Since we've connected our 5-pin relay to two separate load circuits, the setup will provide two different operating scenarios:
Scenario 1: When the ignition is OFF, the 87a-pin (NC) terminal will still receive power because it remains mechanically connected to the 30-pin wire linked to the starter battery. Therefore, the load connected to it will continue receiving power and remain functional.
Scenario 2: When the ignition is ON, the relay's electromagnet pulls the switching armature away from the 87a-pin terminal contact and onto the 87-pin (NO) terminal contact. This connects the 87-pin terminal to the 30-pin terminal, allowing power from the starter battery to flow to the load connected to the 87-pin terminal, thereby turning it on.
Why You Should Only Tap Into a Non-Critical IGN ON Fuse
Our illustration here was simply meant to help you understand how an automotive power relay works. Modern DC-DC battery chargers include either an IGN (ignition) signal wire or a smart voltage-sensing feature. In either of these cases, you may not need an external relay—unless you notice your charger is still drawing power from the starter battery.
Secondly, you should know that the IGN ON fuse we tapped into can remain powered even when the vehicle's engine is not running. This is because the IGN ON (ignition on) mode activates most of the vehicle's electrical systems—including those required to crank the engine.
On the bright side, it's rare to leave the car, van, or truck in IGN ON mode—unlike ACC mode, which many users commonly leave active. In IGN ON mode, the dashboard will usually display several illuminated indicators even when the engine is not running. However, ACC mode can remain active without any obvious visual or audible signs.
Worse still, ACC mode can draw 1–3 amps or more even when everything appears to be off. If not managed properly, this can flatten your starter battery within 12–24 hours, leaving you unable to start your vehicle the next morning.
Therefore, tapping into an IGN ON fuse may not be 100% foolproof, but it is still safer than using ACC mode. Just avoid tapping into critical IGN ON circuits for systems such as airbags, the steering system, the engine control unit (ECU), or fuel injectors.
Instead, tap into non-critical ignition circuits for accessories such as the cigarette lighter, USB charger, heated seats, washer pump, interior lighting, or rear camera. Vehicle manufacturers also often leave a few spare fuse slots for add-on devices.
When Should You Choose a 5 Pin Relay Over a 4 Pin Relay?
The choice between a 4-pin and a 5-pin automotive relay ultimately depends on what you want to achieve. Most people simply want to control a single accessory or appliance with each relay, so a 4-pin relay is usually sufficient.
However, a 5-pin automotive relay is more versatile for caravans, campervans, RVs, and motorhomes. You can use it to control either a single load or two separate loads when needed. A perfect example of a two-load setup is creating a dual-lighting system, where the NC terminal activates the interior lights while the NO terminal operates the exterior lights—or vice versa.
Other applications for a 5-pin automotive relay include:
- Make a dual-lighting switch for interior cabin and steps lights
- Build a DIY solar/alternator charger for your leisure battery bank
- Modify a single-speed fan to operate in both low-speed and high-speed modes
- Create an automatic dual-power system for high-current loads such as a fridge
Frequently Asked Questions
What is the difference between a 4 pin and 5 pin relay?
A 4-pin relay is an SPST (Single Pole Single Throw) switch with a Normally Open (NO) connection only, while a 5-pin relay is an SPDT (Single Pole Double Throw) switch with both Normally Closed (NC) and Normally Open (NO) connections. In practice, a 4-pin relay is basically a 5-pin relay without the 87a (NC) terminal.
What does the 87a pin on a 5 pin relay do?
The 87a pin is the Normally Closed (NC) terminal. It has continuity with the 30-pin (COM) terminal when the relay is at rest, so the load connected to it receives power until the coil energises and switches the armature over to the 87 (NO) terminal.
Can a 5 pin relay control two loads at once?
Yes. A 5-pin relay can control two separate loads from one power source—for example, a dual-lighting system where the NC terminal activates the interior lights while the NO terminal operates the exterior lights, or vice versa.
Do I need a relay with a modern DC-DC battery charger?
Modern DC-DC battery chargers include either an IGN (ignition) signal wire or a smart voltage-sensing feature, so you may not need an external relay—unless you notice your charger is still drawing power from the starter battery.
Ready to sort the switching in your build? Browse our range of 12V automotive relays for DIY campervans and caravans and get the right relay for your next project.
