RF600 Posted March 24 Share Posted March 24 I’m not have any luck with my headlight relay wiring kit. The kit I bought has really long wiring and I didn’t like the non standard relays that came with the kit. I shortened the wiring and added the standard style relays. The ground wire was in a different spot compared to my harness so I swapped the wires. When I put power to the relay, headlight switch off, one of the headlights turned on. Which shouldn’t happen. I have looked at the diagrams on how the wiring is supposed to be, and everything looks proper. I'm a bit confused and hoping someone can assist me Link to comment Share on other sites More sharing options...
FirstGenFreak Posted March 24 Share Posted March 24 Sounds like a fault with your headlight switch, or a crossed wire on one of the relays. Link to comment Share on other sites More sharing options...
Slartibartfast Posted March 24 Share Posted March 24 Assuming it's like the harness I made for mine (one relay for low beams, one for high beams), then, yeah, that shouldn't be possible. Even if one relay was stuck closed, or wired to normally closed on the relay, that would be two lights on, unless you've also got one bad headlight bulb (or a faulty plug, or a broken wire). First thing I'd check is the splice where you shortened the harness. Make sure you didn't cross something up. Second, you swapped out the relays. Are the new relays the same pinout as the old ones? Did it work normally before the relay mod? Did you test the harness before modifying that? If in doubt, try to draw out a wiring diagram of the relay harness. Make sure it makes sense on paper, then check that it's wired the same way in the truck. Link to comment Share on other sites More sharing options...
RF600 Posted March 24 Author Share Posted March 24 I didn’t double check the kit harness before shortening things. The headlights work fine plugged into the factory harness. So no bad bulbs. I actually just replaced both headlight bulbs. It has to be something I’m missing, but I just can’t seem to figure it out. Link to comment Share on other sites More sharing options...
RF600 Posted March 25 Author Share Posted March 25 I think I figured it out. I went back and started over. I went with a 5 pin relay so I wouldn’t have to put 2 wires together. My plan was to use pins 87 and 87a. I assumed that 87a would be powered up the same as 87. Apparently that’s not how it works. 87a has no power when the relay is activated. That is where I went wrong. Lesson learned. 2 Link to comment Share on other sites More sharing options...
FirstGenFreak Posted March 25 Share Posted March 25 At least you got it sorted! 1 Link to comment Share on other sites More sharing options...
Slartibartfast Posted March 25 Share Posted March 25 That'll do it! 87A is normally closed (opens when the coil is powered), 87 is normally open (closes when the coil is powered). Link to comment Share on other sites More sharing options...
RF600 Posted March 26 Author Share Posted March 26 I don’t understand the point of having 87a if there is no power to it. Link to comment Share on other sites More sharing options...
Slartibartfast Posted March 26 Share Posted March 26 There is power to 87a, but it works opposite to 87. If there's power to the coil, 30 and 87 connect. If there's no power to the coil, 30 and 87a connect. This is wasted in most circuits (which is why many relays only have four pins), but it can be useful in some applications. If you had halos or markers or something that you wanted on only when the headlights weren't, you'd wire those to 87a. 1 1 Link to comment Share on other sites More sharing options...
RF600 Posted March 27 Author Share Posted March 27 That makes sense. It’s weird, but makes sense. I will try and stick that in my head for future reference. Thanks. 1 Link to comment Share on other sites More sharing options...
RF600 Posted March 28 Author Share Posted March 28 Wouldn’t 87a have constant power? It wouldn’t be switched power, right. Link to comment Share on other sites More sharing options...
Slartibartfast Posted March 28 Share Posted March 28 87 and 87a are both switched, but they're switched opposite of each other. 87 is turned on by the coil. 87a is turned off by the coil. If you do better with pictures (I know I do), try this. As drawn, the coil is not energized (headlight switch off, in your case). Spring tension is holding common (30) against normally closed (87a), and away from normally open (87). When the coil is energized, its magnetic field pulls on the armature, which pushes that middle contact up, holding common (30) against normally open (87) and away from normally closed (87a). If it's still clear as mud, maybe this guy can explain better than I can. TL;DR, your relays have a feature that is not useful in this application. Ignore 87a and treat them like four-pin relays. While we're on the topic of relay harnesses, make sure yours is connected to the battery through a decent fuse. Mine's connected to an aux fusebox I added on for that and a few other circuits, but when I first set it up, I didn't know better, and I used something like this crap. Water got in, the weak little contacts corroded, the corroded contacts got hot under normal load, the plastic melted, and then my headlights quit working mid-corner in the dark on a rural highway. I replaced that garbage with a rubber fuse holder (this style, though I think I bought mine locally). Much better connections, takes the same fuses as the rest of the truck, never let me down, highly recommended. I only swapped that for the aux fusebox because I had three or four of those things bouncing around, and that bothered me. 1 Link to comment Share on other sites More sharing options...
adamzan Posted March 29 Share Posted March 29 Great explanation. 1 Link to comment Share on other sites More sharing options...
RF600 Posted April 2 Author Share Posted April 2 Make sense. I was trying to understand how to wire up 87a so it’s not powered up constantly, for halos or something similar. Link to comment Share on other sites More sharing options...
Slartibartfast Posted April 2 Share Posted April 2 Power to 30, headlight to 87, halo to 87a, 85 to ground, 86 to signal. If there's no signal, 30 and 87a connect, halo is on, headlight is not. If there is signal, 30 and 87 connect, headlight is on, halo is not. It only powers one pin at a time. If 87a does not disconnect from 30 when the coil is powered, then your relay is defective. 1 Link to comment Share on other sites More sharing options...
RF600 Posted April 3 Author Share Posted April 3 But wouldn’t 87a have constant power? Link to comment Share on other sites More sharing options...
Slartibartfast Posted April 3 Share Posted April 3 No, it would not. Link to comment Share on other sites More sharing options...
RF600 Posted April 4 Author Share Posted April 4 The relay is hooked up directly to the battery. Yes the switch activates the relay to turn on the lights. The lights are normally given full power all the time. On/off with the switch. Where does 87a get its power then? Is there some kind of switched power that would turn on 87a? Link to comment Share on other sites More sharing options...
Slartibartfast Posted April 5 Share Posted April 5 On 4/3/2025 at 8:03 PM, RF600 said: Where does 87a get its power then? 87 and 87a both get power from 30 (which is wired to battery +), but through separate contacts inside the relay, which allows them to be switched opposite of each other. On 4/3/2025 at 8:03 PM, RF600 said: Is there some kind of switched power that would turn on 87a? Switched power to the coil (from the headlight switch) turns off 87a, as it turns on 87. Removing switched power from the coil turns off 87, but turns on 87a. Here's another way to look at it. Imagine a standard household light switch. Switch up, light on. Switch down, light off. Now imagine two light switches, side by side. But one of them is upside down. If you flip both switches up, you've turned one on, and the other off. Flip them both down, and you've turned one off, and the other on. The right-side-up switch controls 87. The upside-down switch controls 87a. Both switches get power from 30. And both are controlled by the coil, which can only move them both together. 1 Link to comment Share on other sites More sharing options...
RF600 Posted April 7 Author Share Posted April 7 Maybe I’m not asking the question properly. If the relay is hooked up to power, wouldn’t that give 87a constant power? Link to comment Share on other sites More sharing options...
peejay Posted April 7 Share Posted April 7 On 3/28/2025 at 9:26 PM, adamzan said: Great explanation. ALWAYS. Link to comment Share on other sites More sharing options...
Slartibartfast Posted April 7 Share Posted April 7 5 hours ago, RF600 said: Maybe I’m not asking the question properly. If the relay is hooked up to power, wouldn’t that give 87a constant power? With power to 30, and no power to the coil (headlights off), yes, 87a has power. I would not however call it constant power, because when the coil is powered, 87a will lose power. Link to comment Share on other sites More sharing options...
RF600 Posted April 11 Author Share Posted April 11 So how would one wire halos up to 87a and have them shut off when turning ho headlights? Link to comment Share on other sites More sharing options...
Slartibartfast Posted April 13 Share Posted April 13 Just like that. Wire the halos to the 87a. Wire the headlights to 87. Buy a set of alligator test leads and a couple of indicator lights and wire it up yourself. Seeing it work might help make sense of it. 1 Link to comment Share on other sites More sharing options...
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