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How To: Upgrade Your Headlamp Circuit

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How To: Upgrade Your Headlamp Circuit

Postby MMamdouh » Tue Dec 12, 2006 4:57 pm


The success or failure of your lighting upgrade efforts rides on the quality of your parts and the quality of your work. It matters how carefully you route wires to avoid chafing insulation. It matters how well you solder connections (crimps and sloppy or 'cold' solder joints corrode and die). It matters how well you shield added wiring from road spray. It matters that you use fuses in the new wiring to protect against vehicle damage due to a new or old electrical fault. It matters that you use high-quality parts that are designed to stand up to the rigors of automotive usage. Such components must be resistant to a wide range of temperatures, road splash, fumes found under the hood of every car, severe and prolonged vibration, etc. It will pay you to select only the products of companies with well established reputations for quality and durability; your $2.25 bargain no-name relay could easily kill you when it fails on a dark road somewhere, leaving you with no lights. Do not purchase vehicle components based solely on price!

The techniques described in this article will yield excellent results only if the work is carried out carefully and to a high standard, with quality parts and materials and without corner-cutting or sloppy work.


Power for the headlights is controlled by a switch on the dash. This is *not* a great place to tap into the system, for two reasons: The headlamp switch uses tiny, high-resistance contacts to complete circuits, and the wire lengths required to run from the battery to the dashboard and all the way out to the headlamps creates excessive resistive voltage drop, especially with the thin wires used in most factory installations.

In many cases, the thin factory wires are inadequate even for the stock headlamp equipment. Headlamp bulb light output is severely compromised with decreased voltage. For example, normal engine-running voltage in a "12-volt" automotive electrical system is around 13.5 volts. At this voltage, halogen headlamp bulbs achieve 100 percent of their design luminous output. When operating voltage drops to 95 percent (12.825v), headlamp bulbs produce only 83 percent of their rated light output. When voltage drops to 90 percent (12.15v), bulb output is only 67 percent of what it should be. And when voltage drops to 85 percent (11.475v), bulb output is a paltry 53 percent of normal! [Source: Hella KG Hueck AG, Germany]. It is much more common than you might think for factory headlamp wiring/switch setups to produce this kind of voltage drop, especially once they're no longer brand new and the connections have accumulated some corrosion and dirt.

From the headlamp on-off switch, a single wire runs to the beam selector (high/low) switch. Two wires run from the dimmer to the front of the car: one for high beams, one for low.

Here's what we have to start with:


Those are long lengths of thin wire between the battery and the headlamps! Most such circuits produce unacceptable voltage drop. To bring full power from the electricity producer--the battery or alternator Positive (+) terminal--to the electricity consumer--the headlamps--we must minimize the length of the power path between the producer and the consumer, and we must maximize the electrical current carrying capacity, or wire gauge, of that power path. But we still want to be able to control the headlamps remotely (from the driving seat), so how do we do that? Install relays!

A switch is a device that completes or breaks a circuit, sending or interrupting current to whatever device we wish to control. A relay is simply an electrically-operated switch. When we send power to the relay with the headlamp switch, the relay completes a circuit between the the battery or alternator Positive (+) terminal and the headlamps. Unlike headlamps, relays require only a tiny amount of power to operate, so the thin wires that are inadequate to power headlamps are more than sufficient to power relays. We will simply use the existing headlamp wires to switch the relays on and off, and let the relays do the big job of sending or interrupting current to the headlamps. We use relays with plenty of current carrying capacity, which enables us to use heavy-gauge wiring that also has plenty of current carrying capacity. This way, we can bring full current to the headlamps, with virtually no voltage drop, even if we choose to install power-hungry overwattage headlamp bulbs.

A relay only needs a watt or two of power to activate it. On the other hand, even many old-fashioned sealed beam headlamp systems' total power is over 100W on low beam (even more on high beam), which means they need over 10 amps of current. If either the dimmer or headlight switch has a resistance of only 1 ohm due to aging, that means 10 watts (10 amps * 1 ohmm) of heating in the switch. While that doesn't sound like much, remember that these switches can't dissipate heat very well, so they'll get really hot. Don't forget you can *solder* with as little as a 15 watt soldering iron!

So what does the headlamp circuit look like when we install relays?


There are several things to notice in this diagram:

Those seemingly random numbers on the relays are universal terminal designators:
86 is the relay switching (control) circuit input.
85 is the relay switching (control) circuit output.
30 is the power circuit input.
87 is the power circuit output.

Some relays have dual 87 terminals. In this case, you can use one 87 terminal to power the left headlamp, and the other 87 terminal to power the right headlamp. Note that a terminal labelled "87a" is not the same as an 87 terminal. Some relays come equipped with provisions for a fuse right on the relay.

The headlamp power circuit begins at the alternator output terminal, rather than at the battery Positive (+) terminal. This so that when everything is in its 'normal' state (ie, engine running, battery charged) then the power for the headlamps doesn't go thru the car's existing wiring at all. This is especially prudent if your car has an ammeter on the dashboard, because many such gauges must carry *all* current for the entire car. Keeping heavy current loads out of this area reduces stress on the entire wiring system, and eliminates much voltage drop on the charging side of the wiring. The alternator need not push the current for the headlamps through the entire wiring harness to the battery, but rather can supply the headlamps directly via the relays.
You may have heard that it's not good to take headlamp power from the alternator output because of "voltage spikes"; this is a myth. No voltage spikes are present in an electrical system with good voltage regulation, and any spikes that are present in a system with bad voltage regulation are present in equal magnitude across the entire system. If your charging system is "spiky", indicated by vehicle lamps that flash brighter and dimmer with the engine running at a steady speed, then you need to fix the problem that is causing the spikes!

The system incorporates fuses in the power supply side of the headlamp power circuit. This is very important! When you start tapping into places in the wiring harness that weren't tapped originally, you *must* properly protect the wiring system with fuses. In the case of tapping into the "battery" connection on the alternator, for example: suppose your new headlight wiring (or a portion of the old wiring after the relay) shorts to ground. Without a fuse, you *will* start a fire somewhere! The alternator can pump out 60 amps or more, and the battery can contribute another 80 to 100 amps before the vehicle main fuse or fusible link blows. Thats on the order of 130 A flowing through your wires, which will heat them to orange-hot immediately. In the engine bay. Near fuel lines and battery vapors. Not to mention that if you *do* blow the main fuse, you are now *stranded* as well. And if you own an old classic without any sort of main fuse or total-circuit protection, the entire wiring harness can be quick-fried to a crackling, crunchy crisp in a matter of seconds. I have seen/smelled/heard this happen, and it is not soon forgotten. (Incidentally--if you drive such a car, ADD A MAIN FUSE OR FUSIBLE LINK!)

Notice that in the diagram of the upgraded headlamp switch, the wires to the headlamps themselves are heavier. If you are going to the trouble of fixing inadequate factory headlamp wires, do a complete job and run good wires all the way to the headlamps. Various products are available to facilitate such an improvement, such as headlamp sockets with spring-action terminals to accept wire of your own supply, up to 10 gauge. These headlamp sockets are $8.00 apiece.


Wire gauge selection is crucial to the success of a circuit upgrade. Wire that is too small will create the voltage drop we are trying to avoid. On the other hand, it does absolutely no harm to have wire that is larger than necessary. The headlamp power circuit ought to use no less than 12-gauge wire, with 10-gauge being preferable if bulbs of extremely high wattage are to be used. Do not fail to use the large wire size on both sides of the headlamp circuit! Voltage drop occurs due to inadequate grounding, too! you will only sabotage your efforts if you run nice, big wires to the feed side of each headlamp, and leave the weepy little factory ground wires in place. Most factory headlamp circuits run the too-thin ground wires to the car body. This is an acceptable ground--barely--on a new car. As a car ages, corrosion and dirt build up and dramatically increase resistance between the car body and the ground side of the vehicle's electrical system. It takes little extra effort to run the new, large ground wires directly to the battery Negative (-) terminal or to the metal housing of the alternator, and this assures proper ground.


Relays are very compact--about 1 inch by 1.5 inches. Because they take up so little space, it is relatively easy to mount them in an optimal location. Because the main idea with this upgrade is to minimize the length of the headlamp power circuit in order to bring the producer and consumer as close together (electrically) as possible, it is best to mount the relays at the front of the car near the alternator and near the headlamps. Because you will need at least two relays--one for high beam, one for low beam--you may wish to consider the heavy-duty Hella relays that snap-lock together to create tidy relay banks that can be made to look like factory installations if the wiring is done neatly. These relays also use moulded terminal blocks so that all of the wires come together into one relay socket, which is preferable to having individual wires without a supporting plug. Hella snap-together relays with mounting tabs, terminal blocks and all necessary terminals are $13.00 each.

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