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320 Headlight Relays


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I’m planning on eventually upgrading my headlights to H4 halogen bulbs, so I needed to modify my headlight wiring circuit. Halogen lights are power hungry and need good connections and heavy wire to ensure proper operation. The headlights are by far the biggest drain on the circuit, and the factory wiring routes all of the headlight power from the fuse block, through the light switch, through the dimmer switch and finally to the headlight bulbs. Relays can reroute the headlight power so it no longer passes through the light switch or the dimmer switch. That should increase the reliability and improve the function of the stock headlight circuit, and increase the longevity of the light switch contacts.



The fuse in the following diagram feeds the light switch and the brake switch. I’ve already modified my tail lights to use LEDs instead of incandescent bulbs, and won’t make any changes to that part of the circuit, or to the front parking lights. The relays will only control the headlights. The dotted line shows where I cut into the existing wiring to splice in the relays.



The modification requires at least 2 relays. One relay controls the low beams and one controls the high beams. I could optionally use 4 relays, wiring each headlight with its own pair of relays. That would reduce the amount of current passing through the contacts of each relay by 50%, and possibly increase the longevity of the components. I’m using 2 relays for now, powering both low beams through one (the LO relay), and both high beams through the other (the HI relay)



The dimmer switch mounted on the floor has two possible states, Low Beams and High Beams. Each time you step on it, it toggles between those states.


Here's a truth table of the possible combinations:




Here’s the modified schematic showing the relay and headlight connections. The dotted lines indicate where the stock wiring harness was cut, and connected to the relay coils and headlights.





The floor mounted dimmer switch receives its power from the light switch over the RY (Red with Yellow stripe) wire according to my schematic. It only receives power when the light switch is in the fully ON position (headlights on).



There are two wires coming out of the dimmer switch, RB (Red with Black) for high beams and RW (Red with White) for low beams. When the light switch is in the ON position, one of these wires receives power and one does not. Which wire receives power depends on the position of the dimmer switch (LO or HI).


The two wires from the dimmer switch previously carried the power directly to the headlights, but now will control the relays, and allow the relays to switch the power to the headlights. A good place to tap into them between the dimmer switch and the headlights is right next to the fuse block as shown in the following diagram.





There is a connector right next to the fuse block that includes both of the needed wires, shown by the arrow in the next picture. I cut them just in front of the connector, leaving enough for a soldered connection to the leads from the coil terminal 86 of the HI and LO relays.





I could tie terminal 87 of each relay to the other ends of the cut wires, but I positioned the relays forward of the horn relay and there is a shorter route to the headlight wires.There is a bulkhead connector there that passes the wires from the main harness through the inner fender to the passenger side headlight. The wires continue inside the main harness to the driver side headlight, so I only need to tap into those two wires with my HI Headlight and LO Headlight connections to terminal(s) 87 of the relays. I can put a piece of heat shrink over the cut ends of the wires that I am not making any connection to.


The arrow in the following picture shows the spot where I tapped into the headlight wires. You can also see where I mounted the relays.






I ran a heavy gauge power wire from a spare fused terminal on the fuse block to the common terminal 30 of both relays. This will supply the power for both headlights through a 30 Amp fuse for now. It’s important to fuse this connection to protect your wiring against a possible meltdown and electrical fire!



I connected the two 85 terminals together and soldered a ring terminal on the end, then fastened it to a good bare metal ground with a machine screw, washer and nut.



Here's a summary of the relay connections:






For now, I tied into the stock headlight wiring, but when I do the upgrade to H4 halogen lights, I will install new wiring between the relays and the headlights. An alternator swap for the generator is in the cards before the headlight upgrade.


If you wanted to use 2 relays for each headlight, run separate wires to each headlight, and wire them individually the same way as shown.


The whole process only took an hour or so, once I identified the right place to splice into the wires. It works great and the stock headlights are definitely brighter than they were. Now a few tie wraps and some electrical tape and I'm done.


This worked for me but may not be right for your application or experience level. If you decide to try this modification, do so at your own risk.


Let me know if anyone spots any issues or has any suggestions for improvement.




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This is a relay from Sunmmitracing.com.


It has two 87 terminals, and no 87a terminal.

Here is a direct link to the part, from Summit.




Thanks for the link. I thought about using the dual output relay, but there are three issues I have with it; cost, availability and function.


I wanted to be able to go to any auto parts or farm supply store and get a replacement. The stores around here don't carry the dual output relay, at least not that I've found, and I already had the standard type on hand. I bought 5 of the single 87 terminal relays on Ebay for $15 and they came with sockets and pigtails, so at around $3 per set it's super cheap. Plus I used 3 of them for my LED taillight conversion, and had 2 left over to use for this project. What can I say, I'm cheap. The KC relay is $6.75 without a socket or pigtails, so it costs more and may be harder to find in a pinch.


Function wise, I'm not sure how the KC works internally. If it has two separate sets of contacts, that could be an advantage. If it has one set of contacts and an internal connection between two output terminals, that's no better than what I've already installed. If that is the case the only benefit of using the KC relay is to possibly have one less splice or crimp connection, depending on what you are hooking it up to. In my case it wouldn't save me any splices or crimps at all. WIthout popping the cover off and seeing what's in there, it's impossible for me to say. If it does have one set of contacts and an internal jumper, it might be possible to take the cover off a standard relay and reconfigure it to give the same functionality as the KC relay. Maybe I'll get a KC relay and take it apart to see exactly what's in there.


FWIW I did a search on the KC3300 relay, and found a lot of chatter on the Jeep forums about how, when the relay failed, they tried to get a replacement locally and were unable to, or just plugged in a standard relay not realizing the KC is wired differently. The symptom of plugging in a standard relay when your vehicle is wired for the KC3300? One headlight lights with the switch off, but goes off when the switch is turned on. Although any parts store that doesn't have it in stock can get it in a day or so, I'm into instant gratification when it comes to fixing stuff!


If I were to go with the KC when I upgrade the lights, I'd buy a spare and put it in the glove box just in case. The KC relay wil plug right into the sockets that I have already installed, and I'm going to rewire between the relays and the new headlights anyway, so I won't rule it out until I can look into it further.

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I understand your concerns about the KC3300 "non standard" relay. I use the two outputs on the relay to go to each side of the vehicle, and have one relay as the high beam relay, and the other relay as low beam.

I would be curious about the internal construction of the relay also. KC is calling this a 40 amp relay. I would hope it is robust on the inside.


When I redid the headlight wiring on a 1974 Ford F-250 truck, last spring, I used two of these relays. I mounted the relays inside a relay box I got from a more modern Ford, at Pick-n-Pull.


The total cost of two relays, shipped to my doorstep was $18.45. More than the relays you got, but that was to my door, and I spent no time or gas to go get them.


This is a schematic on the wiring.



This is the relay box I mounted them in, with two fuses, one fuse for each relay.


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I like that relay box, especially the self-contained fuses.


Given the rating that you mentioned, I suspect that the KC relay is identical to the standard 40 Amp relay that I used except for an internal connection to the second 87 terminal. That would mean that the relay is really capable of passing a maximum of 20A per 87 terminal at the same time. It is functionally identical to my setup except that my headlight circuits tie together outside of the relay rather than internally. That is, I have two headlights on one 40A relay contact. I modified your diagram to show how it would be with the standard relay, and there is very little difference. Really just the convenience factor of having 2 termination points rather than one per relay for the outputs. The tradeoff seems a little one sided as far as the downsides vs the upside, but if I had it I wouldn't hesitate to use it if 20A max per light was enough.. H4 high beam is 130 watts, or 10.8 Amps. Running two would be 21.6 Amps total. If I needed more capacity than 20A per light, I would use two conventional relays per side.



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