Wiring Headlight Relays
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This page is all about wiring relays to drive your headlights so they are brighter and/or you can use higher output bulbs (if desired) without the risk of overloading your existing headlight wiring. If you have no idea why you might even want to do that, let alone how to do it, read on. If you don't even know what a relay is - go read my All About Relays page before going any further.

I made this modification years ago on one of my cars after installing higher output headlights - I needed to solve the "flickering headlights" problem that resulted when I used my high beams. The new lights drew so much more power that the existing circuit breaker in the headlight switch was being overloaded. This was both annoying and downright unsafe - so I fixed it with this change. I later found out about the benefits for just getting brighter headlights courtesy of an article written by the Southern California GS chapter of one of the various Buick clubs I know of and get information from. (I can't find the name of the club, but I do have a photocopy of the article itself. If someone cares to remind me who I ought to be giving credit to for this, please remind me so I can update this page.) The voltage drop information and encouragement for me to create an easy to read wiring diagram comes from their article. There was lots of good info in there, but the hand-drawn wiring diagram was less than readable - even for someone like me who actually understands how this all works. :-) So, I decided to put this page up to host a better wiring diagram and explain it in my own way along with details for the high output headlights. Since I'd done this myself years before I'd ever read their article, and I'm giving them credit for some of the extra details, I don't feel like I'm ripping off their idea. :-)


Why you really want to do this

Why would you care about doing this? For one (or both) of two basic reasons. The first is that you have installed high output headlights (off-road units, etc.) and you're having problems with your headlights "flickering" on and off again while you drive. The second reason is to simply improve the brightness of your existing headlights. This is because the factory wiring for the headlights has lots of long "just big enough" wires, and after many years of service, this leads to extra resistance in the wiring and at each connection. That resistance sucks up electrical energy that could be used to produce light at the headlights, so your lights are dimmer than they could be. To put this into perspective, a 10% drop in voltage between the battery and the headlight is not uncommon - and that can cause up to a 30% drop in light output! That's the difference between being able to see to stop in time and having an accident - so this is a very useful safety and drivability modification. The total cost is less than $50 and can be done in an afternoon by anyone who is even vaguely familiar with how to do simple wiring work. $50 to get up to 30% more light from your headlights is very much worth it. So read on and learn how to do this.


Safety tips and application data

The standard set of safety disclaimers apply - this is for your information only and none of this should be attempted unless you are sure you know what you're doing. This is not guaranteed to be 100% correct and you should use common sense when attempting any repairs or modifications to your vehicle. It is not my fault if you fry yourself, anyone else, or your car. I did not tell you that you should do this - only that you could do this. It's up to you to determine if and how this information applies to your car.

On the subject of application information, this entire page is focused on vehicles that use a traditional "positive switched" headlight system like most older American cars. This is where power goes from the + battery terminal to the switch, then to the headlights, then to ground and back to the battery. Some import cars, particularly Toyota's from the early to mid 1980's use a really weird "negative switched" system that runs power direct to the headlights and puts the switch after the headlights in the wiring diagram. You can do the same relay trick in those systems, but several key wires are inverted, and you need to be really careful about what you do because most people have trouble thinking about the system working "backwards".

As a side note, these "reversed" systems are prone to strange behavior when a headlight burns out - things like having all of the headlights burn out at the same time are not uncommon with these systems. That said, you may want to think about doing the extra work to use the relay along with some extra wiring to invert the system so it works "correctly". It's more work, but it can be done. I'd do it if it were my car, but I do things that most folks never notice or care about, so take that recommendation with a (not so) small grain of salt.

Lastly, the physics purists who want to pester me about actual electron flow from negative to positive can save it. I know about this, but it's confusing to most people and not relevant to the discussion here. This entire page is written from the perspective of the traditional positive-to-negative power flow in an electrical circuit. If you know what that means, now you know. If this is gibberish to you, don't worry about it - it was just the elitist purists trying to confuse you. :-)


Technical Summary

This one is pretty easy to conceptualize if you understand how a relay works - and if can't then you should go read my All About Relays page so you can. You splice two 20A relays into the existing wiring harness right out near the headlights so one relay controls the low beams and one controls the high beams. Use the existing high and low beam wires coming from the firewall to trigger the relay, run a new high power feed (with a fuse!) direct from the battery, and hookup the existing high and low beam wires from the headlights to the "normally open" contact on the relays. The hardest part of all this is typically finding the right wires in the existing wiring harness and finding a place to mount the relays - the actual wiring is pretty easy. Any SPST/normally open relay will do, though most automotive relays are of the SPDT variety - just don't hook anything up to the "normally closed" contact on the relay (pretend it's not there) and you'll be fine.

Note that if you pick some really monster sized off-road headlights that draw more power than the ones that you can plausibly use "on the street", you must use higher amperage relays than what is mentioned here. You must also use the appropriate sized power wires and you may very well end up replacing all of the headlight wiring from the relays out to the headlights themselves - don't forget to upgrade to a larger headlight ground if you do this! See my Wire Capacity Chart for more details.


The gory details

Now, for the rest of humanity that has no clue what I just said, here's a step-by-step list of what you need to do. You should read the entire list and understand it before you start this project. If you are knowledgeable in such things, you should be sure to solder all of your connections in addition to crimping them. This helps ensure that you will have a more secure and lower resistance connection that will not degrade over time.

  •  Find the existing wires that go to your headlights in the wiring harness out near the headlights. There will typically be two wires - one for the low beams and one for the high beams. The headlights are usually wired together as part of the harness in a daisy chain fashion - if not, there could be four wires in the harness - one for the left low beam, one for the right low beam and one for the left high beam, one for the right high beam. If in doubt consult a factory wiring diagram for your car or break out the old multi-meter and do some testing and tracing of the wires in your harness to figure out what goes where.
  •  Figure out where you will mount your relays - make sure it's reasonably safe/dry, out of the way, close to the existing wires you just found, has space for the relays, and is reasonably close to the headlights. Make sure it does not interfere with anything like closing the hood or getting to anything else you may need to service on the engine in the future.
  •  Disconnect the negative battery terminal so you don't fry yourself or the car. :-)
  •  Make a careful note of which wires are for what and then cut the existing wires for the headlights where you want to splice in the relays. If you pick a point in the harness after the wires split for the left/right side headlights, you'll have two wires to deal with. You can either tape off one of the low beam wires or connect them both together where you make the relay coil (trigger) connection. I prefer to connect them both together.
  •  Hook up the existing low beam headlight wires coming from the firewall to the relay coil (trigger) as shown in the wiring diagram.
  •  Repeat the same process for the existing high beam wires coming from the firewall.
  •  Hook up the existing low beam headlight wires that go from your "cut point" out towards the headlights to the relay "normally open" output connection as shown in the wiring diagram. If you picked a point for the relays after the wires split for the left/right headlights and had two wires to deal with in the above steps, then you must connect both of the low beam wires together where you make the relay "normally open" output connection. If you do not do this - only one headlight will work.
  •  Repeat the same process for the existing high beam wires that go from your "cut point" out towards the headlights.
  •  Hook up the ground wire from the relay to a new ground or to the existing ground wire for the headlights. This wire only needs to be 16 gauge (it carries very little power) and it should be black.
  •  Run a new red 12 gauge feed wire over to your battery or to any other place (such as the starter solenoid connections) that gets full battery voltage and can be easily connected to. Make sure you put a minimum 30A fuse or circuit breaker into this wire as close to the connection with the battery (or other wiring) as possible. Failure to install a fuse or circuit breaker in this wire will create a safety and fire hazard in your new wiring! This is the wire that will carry all of the power to run your headlights - make your connections securely and cleanly so that your headlights are as bright as possible.
  •  Mount the relays and make any final connections. Make sure all connections are taped and/or covered in "heat shrink" tubing so they are watertight - stuff under the hood gets wet and electricity and water don't play well together.
  •  Go back and double check all of your connections against the diagram. If this is your first time doing wiring work, go back and triple check it to be sure. Seriously.
  •  Make sure the headlights are turned off!
  •  Reconnect the negative battery terminal. Nothing should be out of the ordinary here. If stuff starts smoking or melting, disconnect the battery right away and figure out what you messed up and fix it before reconnecting it again.
  •  Turn on the headlights and make sure they are on low beam and make sure 1) nothing is smoking/melting/burning, and 2) just the low beams are on. If any problems occur, turn off the headlights and unhook the battery immediately - then find and fix the problem.
  •  Switch the headlights to high beam and make the same two checks as on the low beams.
  •  Clean up - you're done. Enjoy your brighter headlights.


Wiring diagram

The wiring diagram below shows what you need to end up wiring to make this work, so if you know how to read a wiring diagram and feel like "skipping ahead", just go click on the thumbnail for the wiring diagram and check it out in full size, full color glory. It's shown for a four headlight system - if you have a two headlight system on your car, pretend the two inner "high beam only" headlights aren't there and you'll be fine. The wire colors shown here represent a typical GM vehicle (the green and tan wires, along with some of the black wires) as well as the proper/correct/desired wire colors to use on any new wiring you do (the red and some of the black wires). Also, this is shown as a typical "Bosch style" automotive relay with the connections numbered as such. If your relay is not numbered like this, then just identify the wires by function and go from there.

HeadlightRelayWiringDiagram.jpg (81505 bytes)

Note that in the original version of this diagram, I had the labels for the right side headlights reversed - it has since been corrected thanks to a sharp-eyed reader who pointed out the mistake to me. The wiring diagram was always correct, but the headlight labels may have been a tad confusing to some. Apologies for the mistake.


Parts list

See the bottom section of my All About Relays page to get some ideas for where to buy parts from. Whatever you do, just be sure to get relays that have a connector and wires with them along with a way to mount them. For reasons I am unable to fathom, some people produce and sell relay kits that have no way to mount the relay. (Go figure...) The circuit breaker can be obtained from your local parts store - they sell units that mount to any flat piece of metal and have simple screw-on terminals for the wires. (I think the one I used was an Echlin CB6339 based on what I looked up on NAPA's website...) The wire, tape/heat shrink, crimp on connectors, and screws to mount things can all be obtained locally at any decent auto parts store. If you know how to solder and opt to solder your connections, then you should already know where to get solder and a soldering iron, aka, your local Radio Shack. :-)

  •  Two 20A or greater mountable automotive relays with connectors
  •  One 30A circuit breaker
  •  10' of red 12 gauge wire
  •  Electrical tape and/or "heat shrink" tubing
  •  Crimp on connectors as required
  •  Screws/bolts to mount the relays and circuit breaker - some sheet metal screws should do the trick
  •  Solder and soldering iron if you decide to solder your connections.

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Page last updated 12/27/2011 10:23:21 AM