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. :-)
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.
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.
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.
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. :-)
Comments? Kudos? Got some parts you'd like to buy/sell/barter/swap? Nasty comments about my web page so far? See Contacting Us.
Page last updated 12/27/2011 10:23:21 AM