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Upgrade Your Headlamp Wiring


Typically, if you keep your bulbs at 100W or lower, you will have little problem with meltdown of the OE wiring or controls as long as the wiring is in good shape and all connectors in the circuits are clean and free from corrosion. The only problem you might have is less than optimum voltage at the bulb. All automotive bulbs are rated at 13.5V, the output of the typical alternator. Any less than that and you start losing output from the bulb in a hurry. First you want to measure the output of your alternator, at the alternator. With the engine off, hook your VOM leads to the alternator securely. Start the engine and note the voltage. Shut off the engine and move the leads to the back of the bulb you want to check. Start the engine, turn on the lights and note the voltage at the bulb – you can do this for both high and low beam. If the difference between the output of the alternator and the voltage at the bulb is One Volt or more, you might consider upgrading your wiring as described here.

For reference, here are the pin connections for an H4 Bulb or Lamp.  All are looking at the back of the bulb/lamp.


And shown below is the wiring you want to do if you are putting relays in the lighting circuits to handle higher wattage bulbs.

There are two options for putting in the relays. If you are not concerned about keeping the exisitng wiring intact, you can wire in the relays yourself. If you would rather not cut your wiring, or if your wiring skills are questionable, you can buy one of our pre-engineered and tested plug and play harness.

It you decide to do it yourself, we can supply all the components.

I’ve noted on the drawing where various sizes of wire should be used. If you run two separate wires from pins 87 to the two lamps on each relay, you can do that with 14AWG. As I’ve show in the drawing, I have you running 12AWG to a splice point and then 14AWG to each of the lamps. If you prefer to wire it that way, ensure you solder each splice and protect them with shrink tube or good electrical tape. If your application uses a H4666, H6545, H4668, or H4656, change the pin connections shown above as appropriate.

For wire sizing information beyond what i've given you, you may want to check our Sensible Wiring Guide.