So, now that I have these way-cool Bosch H4 headlamps on my car, I decided to do something about the stock headlamp wiring. Here's the whole story - please send me any comments you might have...

Howard Chu
October 19, 1998
Modified November 4, 1998

Parts list for heavy-duty headlamp wiring:

Most of these parts can be found at Pep Boys, Chief Auto Parts, Kragen, etc. I used Radio Shack relays. The hard part is finding the universal headlamp connectors - Chief doesn't carry them, and Kragen's uses 18 AWG pigtails. Pep Boys uses 16 AWG. I don't know where to get the connectors with just bare parts that you can crimp your own wire onto. Let me know if you have sources...

Some potential sources I found on the web, which I have not used: Part # 809918, $6.50 each Part # 80300, $12.80/pair Part # 3029, $3.60/pair

Something else to consider would be a dual relay socket, instead of just sliding wires with disconnects onto the relays. I found some cheap ones, but their pigtail leads use 14 and 18 AWG wire - way too thin for what we want: Part # 330-078, $3.25 each

If you have success with any of these suppliers, please let me know so I can update this page. When I ordered the Bosch H4 lamps from Dan Stern, he also sent a pair of headlight connector adapters, which I cut apart and re-used in my own wiring upgrade. It might be easiest just to order these connectors from Dan Stern at $8.00 each, since they already have 14 AWG pigtails attached. -- hyc, 1999-04-20

Before you go any further, I suggest you also read up on the Wiring How-To on Dan Stern's web site: He goes into a lot more of the rationale behind what we're doing, and also talks about some better components you might want to use, such as Hella relays and heavy-duty headlamp connectors. hyc, 1998-11-04

The 10 AWG inline fuse holders were hard to find too, but Pep Boys carries them. They're made by MotorMite, part # 85663. They come with about 6" of 10 AWG wire already attached, and 30 A glass fuses installed. I suggest dropping down to 20 or 25 A fuses initially; you won't really need to handle 30 A. (That would be 360W - who has a pair of 180W lamps??)

Overall approach: we will run one strand each of 10 AWG wire for hi beam, lo beam, and ground. The +12V for hi and lo beams comes from the 1/4" bolt where the battery lead enters the engine compartment fuse box. The GND connects to the 1/4" bolt where the fuse box rests on top of the negative battery bar. The hi and lo beam relays will be mounted inside the housing of the driver's side headlamp. The stock hi/lo connectors will attach directly to the relays' control inputs. +12V will also connect here, and run out to the centerline of the car on 10 AWG wire. From here, each 14 AWG strand will be cut into two equal length strands. One will go back to actually power the driver's side lamp, and the other will go on to the passenger side lamp. Remember that any difference in wire length at this point will result in (miniscule) differences in total circuit resistance, resulting in one side's lamp being brighter than the other. (Of course, for the lengths we're talking about here, the voltage drops add up to a total of about .25V, so it's no big deal. But why go to all this trouble if you're not gonna make it *just right*, eh?)

To begin: The most important thing to begin is to get accurate measurements for the lengths of each wire run. Of course, I didn't do too well at this, but here's what I suggest...

First, get the relays set up. They each need a ground connection for the control circuit in addition to the hi and lo control inputs, but you'll only have one ground connector from the stock headlamp harness. Use the 10 AWG male disconnect and the two 14 AWG female insulated disconnects to assemble a Y connector. I used two 2 inch strands of 14 AWG wire for this connector. Strip each wire appropriately, insert both wires into the 10 AWG male disconnect, crimp firmly. Crimp the other two ends to a female disconnect. Attach the female disconnects onto the ground leads of both relays (onto the 85 leads). The relays should be positioned side by side. I used a wire tie through the mounting holes on the other side to keep things together. This pretty much allows you to handle both relays as a single unit, but gives you some flexibility when you need to maneuver them into tight spaces - which you will definitely be doing!

Remove the driver-side headlamp. Place the relays into the headlamp housing with the connectors facing forward. Attach the stock headlamp wires to the relays (onto the 86 leads). This is going to be tough to manage, and you're likely to get cut on the sharp edges of the headlamp housing. Make sure you keep straight which is hi beam and which is lo. You might as well also remove the other headlamp while you're at it. You'll eventually need to route wire to it and see where it's going to come out.

Next, set up the battery connections with the fuse holders. Attach a ring connector to one end of each fuse holder and seal the heatshrink. Use a 10 AWG butt connector on the other end. It's best to leave the fuses out of the holders until everything else is complete. With the ring connectors attached you can mount the fuse holders onto the bolt where the battery +12V lead enters the fuse box. Just undo the nut, put both ring connectors on, and fasten the nut again. Note that both wires have to follow the same exit path as the battery wire, to allow the fusebox cover to close. Also, all of this work is done right next to the battery. Be extremely careful and don't let your tools touch any other exposed metal while you're working here. I almost turned my ratchet driver into a glow stick by accidentally hitting the negative battery terminal... I managed to break it loose before too much insulation went up in smoke, but my garage was pretty cloudy for a couple minutes after that!

With the battery connections partially in place, go back to the relays. Attach a female disconnect to a strand of 10 AWG wire; this will be used for the lo beam. You will need about 9 inches to go from the relay to the fuse holder, but don't cut it yet. Feed the wire through the rear of the lamp into the housing and attach to the relay's 12V input side (87). Run back to the butt connector on the fuse holder, cut the wire, and attach to the fuse holder. Use another strand of 10 AWG wire and repeat this process for the hi beam.

Next set up the 10 AWG GND cable - put a ring connector on one end, temporarily attach to its mounting post, and run it toward the driver's headlamp. It will curve around the battery, run under the radiator hoses, come back up over the fan, and go straight across to the car's centerline. This should take about 3-4 feet of wire. Don't bother trying to get its final placement at this point, it's easier to run the wiring through all those turns after you've got everything in place and inserted into the spiral flex tube.

OK, now set up the 10 AWG wire for the 12V outputs for hi/lo from each relay. Attach a female disconnect to the wire, feed through the back of the lamp housing and attach to the relay (on the 30/51 leads). Run the rest of the wire over toward the car centerline, as with the GND cable. Do this for the lo and the hi beam wires.

At this point you should have 3 10 AWG wires running from the driver's headlamp housing to the car centerline. Cut these wires at the centerline and attach a butt connector to each. Run some 14 AWG wire from inside each headlamp housing to this center point. Follow the same path as the 10 AWG wire from the driver's side. For the passenger's side, run back from the lamp, around and below the prop-rod mount, up toward the fan housing, under the stock air intake (if it's there!) to the center point. Don't make any cuts yet, just measure each run and then cut two strands equal in length to the longest run. By the way, you probably want the headlamp side to have about 3-4 inches of slack; i.e., the end of the wire should extend about 3 inches out the front of the lamp housing to facilitate lamp replacement... At the center, attach pairs of the 14 AWG wire to the 10 AWG butt connectors. If your wire colors all match it should be no problem to keep hi and lo beams straight. Seal all the heatshrink connections.

Stuff all of the wires into the flex tube and run the tube through its final path. You'll just have to start from the driver's side lamp and snake it all the way over/under things till you end at the passenger's side lamp. Use wire ties to keep the bundle in place; you definitely don't want anything falling into the various fans and belts down there. At each lamp, use the 14 AWG butt connectors to attach each wire to the headlamp connectors. You're pretty much done now - you should be able to plug in the lamps, insert fuses into your fuse holders, and hey - let there be light! By the way, the lo beam goes to the middle position on the connector. The ground connection should be pretty obvious. If you're anxious to check, you can plug in the lamps and leave them resting on the bumper at this point to see if all is well. Then reattach all the lamp mounting hardware.

Congratulations, you now have a headlamp wiring harness capable of driving 720 watts total lighting power! Cool, huh?

Something I didn't consider until after I finished all this - on our cars, the lo beam turns off when you switch on the hi beams. You could easily enhance the relay setup given above such that the lo's stay on with the hi's. You just need to make a T connector from the hi beam's 12V output, and a Y connector for the lo beam's control input, and attach one leg from the hi 12V out to the lo control in. The single 10 AWG GND wire ought to still be able to handle the total current of all hi and low beams on at once.

I floated this by Dan Stern and he cited two very good reasons *not* to make this change:

So - don't bother wiring things so that hi & lo beams stay on together. I myself only thought of it because I saw a "QuadBeam" package in a Kragen store, which was a kit for this very mod. At a guess, the folks selling those kits aren't worried about customer satisfaction...

Also with this in mind, I think you only need a single 10 AWG +12V lead from the battery. You have to feed it to both relays, but since you can't have both hi and lo beams on at once, only one relay will ever be passing the current through at a time. hyc, 1998-11-04