Redesigned Headlight Wiring Harness

As I mentioned in my last post, I redesigned the wiring in my Corvettes engine bay. Here's how I changed it and why.

Let's start with the why.

While going through the electrical problems in my car, I also learned that the wiring is far from optimal. Not nearly as bad as in the 60's but still there was a lot of room for improvement. I also wanted to add some new electrical stuff to the car and this would be difficult with the factory wiring.

I'm not an expert in car electrics but luckily I found the MAD Electrical web page. They have some great articles about electrical systems in muscle cars. After reading their texts (many times) I had had a pretty good idea what to do. The major problems were:

• Headlights
• New equipment

Headlights

Headlights of the -76 Corvette are the worst part of the electrical system. While looking through the Doc Rebuilds wiring diagram, I saw multiple problems. From the lamp itself the feed wire goes to the main connector at the back of the fuse box. From the fuse box it continues to the dimmer switch which selects high and low beams. Power to it is fed from the headlight switch which in turn has a power wire from the fuse box. Next in line is a fuse and then the main power wire from the engine bay and the battery and/or alternator.

Very simple and direct but crappy. First of all this creates a really long wire from the power source to the lamp thus causing voltage drop. This dims the lights noticeably. Secondly this means that all the current used by the lamps goes through the headlight switch causing corrosion in the connectors and might fry the switch. Third problem is the main power wire. A single 10 gauge wire feeds a lot of devices: radio, interior lights, wipers, rear lights, power windows and, of course, the headlights. That could be a lot of current through a single wire.

The simplest way to correct all these problems is to add relays for the headlights. This way a large power wire can feed the relay and the headlight switch just controls if current goes through it to the lamps. This shortens the way from the power source to the lamp, lowers the current that goes through the switch and naturally does the same for the main power wire.

Now, where to connect the feed wires for the relays? This brings us to the other major problem.

Adding New Equipment

I also wanted to add an electrical fan to my radiator. The clutch in the original fan was not working, the fan shroud was missing and the fan drained a lot of horsepower to turn. After removing it, the engine somehow seemed to run much easier.

Replacing the fan dictated that I would naturally need new wiring for it. In addition to the fan, I would also need a new temperature sensor to control it. I read a bit and decided that I would also add a relay for the fan and then control the relay with the temperature switch. If temperature got too high, the sensor would allow current to go through it which in turn would allow current to go through the relay to the fan.

To add all this new equipment, I decided that I need a central power junction from where it would be easy to distribute electricity. It would also be the place where the alternator charge and sensor wires needed to be connected. This way I would know that the junction had a stable ~14 volts. The factory wiring harness naturally also has a similar junction but it is just a splice of two wires inside the harness. It would be difficult to add any new electrical equipment to it.

That's pretty much it. With these guidelines in mind, I drew a new headlamp wiring harness diagram.

New Wiring Diagram

And there it is. Most of the wiring is the same as in the factory harness but let's go through the changes is detail.

• Junction: There is large power junction marked in the diagram. I also included an example picture how the junction could look like but almost anything will do. I bought a junction meant for car stereo systems and it works great as you can easily screw in more wires. There are two junctions in the diagram but this is just to get more room to draw the wires. If more than one are actually used, they must be connected together. Note that the engine wiring harness is now also connected directly to the junction.
• Battery: In the diagram the battery is connected directly to the junction. This is the best situation but not necessary. There is already a heavy duty cable from the battery to the starter. Factory harness is connected to the starter positive terminal and this is also what I did. I ran a new 8 gauge wire (with 12 gauge fusible link) from the starter to the junction.
• Relays: There are three relays in the new harness. For high beam lights, low beam lights and fan. Light relays are controlled by the light switch and fan relay by the temperature sensor. While building the harness, I wasn't quite sure where to place the relays. In the end I placed them far front near the left hand side marker light. After installing the harness I noticed that a better position would be in the fender near the alternator.
• Fan temperature sensor: The fan temperature sensor needs a constant power source. Connecting it directly to the red wires (ie. battery) is not a good idea because then the fan could be left running even when turning the power off from the ignition switch. Unfortunately there in no wire in the headlight harness that would be "hot" when ignition is on. Luckily the engine harness does have one nearby. The yellow wire going to the windshield washer pump will do nicely. I spliced a connector to this wire and connected it to the temperature switch. This way if the engine is running really hot, I can turn it off and the fan keeps running if necessary.
• Wiring: I tried to draw the connectors in the diagram in a similar position as they would be in the car. Naturally this is not completely possible but I think you can get a pretty good idea which connector is which from the diagram. Especially if you compare my image with the Doc Rebuild harness diagram. To build the harness, I had the original factory harness with me and I cut all my wire lengths according to it. This way I would get them correct right away. The new wires going to the junction are a bit more problematic. I left them quite long and then cut them when the harness was in the car.

And that's it. It took me a couple of nights to solder the new harness and then wrap it up in tape. Installing it to the car took a few hours and everything seemed to work correctly. Lights are now much brighter and the drain on the light switch should be much lower. All in all, I'm really happy with this new system.

Oh, and just one more precaution: if you actually build a new harness according to these instructions and your car goes up in flames or some other damage occurs, do not blame me. You do need to know something about car electrics before trying this stuff :)