Wiring a Cole Hersee Relay Switch Correctly
The first thing to understand is that a Cole Hersee switch isn't actually a switch. It's a control device that tells a relay to engage or disengage. The switch itself handles only a small amount of current. The heavy lifting is done by the relay mounted nearby. Confusing this distinction is why a lot of installations fail on the first try. Here is how the standard configuration works. You have a power feed coming from the battery, usually through a 30 amp fuse. That goes to terminal 30 on the relay. Terminal 87 runs from the relay out to whatever accessory you're powering — lights, fans, compressors. Terminals 85 and 86 are the control side. One of those goes to a switched 12-volt source from your ignition or accessory switch, and the other goes to ground. The Cole Hersee button itself sits in series with the 85 or 86 wire. When you press it, you complete the ground path through the relay coil, and the relay closes the high-current circuit. The relay coil draws about 80 to 120 milliamps. That is nothing for the switch to handle. The load side, depending on what you're running, can pull 20 to 60 amps. That is why you never wire accessories directly through the button. I've seen people try to run a pair of LED light bars straight through the switch contacts, and the switch melts within minutes. The contacts inside are rated for roughly 10 amps maximum. That's fine for turning things on and off, not fine for carrying the full load.
Wire gauge matters more than most people realize. If you're running a 40-amp load and using 14 gauge wire between the relay and the accessory, you're looking at about a 1.5-volt drop over ten feet. That might seem minor, but it's enough to dim LED bars noticeably or cause a compressor to run hot. Stick with 10 gauge for anything under 30 amps at reasonable distances, and 8 gauge if you're pushing past that or running longer runs. Fuse placement is non-negotiable. Put the fuse within 18 inches of the battery positive terminal. Not two feet away. Not behind a panel somewhere. Within 18 inches. If a fault happens between the fuse and the battery, that unfused cable becomes a incendiary device. I learned this the hard way on a fleet truck back in 2018. A chafed wire somewhere along a five-foot run melted through the insulation, arced to the frame, and took out the entire lighting harness. The fuse had been installed six inches too far from the battery, and by the time it blew, the damage was already done. Terminal identification on Cole Hersee relays follows the standard automotive convention, but it's worth confirming every time because I've encountered aftermarket relays labeled differently, especially the cheaper ones from Amazon. The numbers 30, 87, 85, and 86 should be molded into the relay housing. If they aren't, use a multimeter. Set it to continuity mode, tap the terminals with the probes, and map them out before you connect anything. Taking thirty seconds to verify saves you three hours of troubleshooting later. One thing that catches people off guard is the polarity sensitivity of certain Cole Hersee models. Some of their illuminated switches are designed so that terminal 85 must go to power and 86 to ground, or vice versa, depending on which side the indicator LED is wired to. If you reverse them, the switch still operates the relay, but the light on the button won't illuminate. It works functionally but looks broken. Check the datasheet for your specific part number before you finalize the wiring.
Common Mistakes to Avoid
Running the control wire for the relay alongside high-current load wires is a quiet source of problems. Electromagnetic interference from the load circuit can induce voltage spikes into the control side, causing the relay to chatter or false-trigger. Keep the 85-86 control wiring on its own path, away from the thick power cables going to the accessories. Another frequent issue is using a ground point that isn't clean. A bolt going into painted or anodized metal without a star washer underneath will give you an intermittent ground. The relay clicks lazily, the accessory flickers, and you spend two hours chasing a ghost. Scrape the paint, use a ring terminal, and torque it down. That's it. Some people wire the switched power source directly to the vehicle's ignition switch output rather than using a fused distribution block. That puts additional load on the ignition switch contacts and can cause voltage drop that affects other circuits downstream. Use a relay for the control feed if you're powering more than one accessory switch from the sameIgnition source.
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When This Setup Doesn't Work
The relay-and-switch approach breaks down in two specific scenarios. First, if you're working on a vehicle with a sensitive CAN bus system and you tap into an existing circuit without isolation, you can introduce noise that triggers dashboard warnings or causes modules to behave unpredictably. In those cases, use a fuse tap with a dedicated fuse and route the control wire through a diode to prevent backfeed. Second, if you need to control multiple accessories from a single switch with different timing or sequencing, a simple relay diagram won't cut it. You'd need a programmable relay module or a dedicated control panel. The Cole Hersee system is reliable for straightforward on-off applications, but it wasn't designed for anything more complex than that. The wiring diagram for your specific model can be found on the Cole Hersee website under the product documentation section. Look up the exact part number printed on the relay or switch body. The diagrams vary slightly between models, and using the wrong one will waste your time. I keep a folder of the most common Cole Hersee Switch Wiring Diagram sheets on my phone just in case. It saves looking through manuals when you're already under a dash with wire cutters in your hand.
