Adding Relays For Headlights

I did a final checkup for the engine by removing all plugs and peeking under the valve covers. Everything seemed OK except that plugs for cylinders 3, 4, 5 and 6 were a bit greyer than the rest. Weird because they are exactly at the opposite corners of the engine. I need to get the carb tuned by a professional.

With the engine side handled, I decided to do some electrical work. Some previous owner has replaced the original sealed beam light bulbs with later H4 type headlight. This meant a lot brighter lights but with a much higher current requirement. Unfortunately the original wiring and switches were not made to handle that current and it was only a matter of time before something would burn out.

I went through the same problems with my Corvette and solved the issue by redesigning and building a new harness. I didn't want to do that this time as the original harness was in pretty good condition and I wanted to retain the original look. Or at least be able to return everything to original easily. Then I found this article which covered Reenmachines conversion harness. The idea in it was really simple. The new harness took the relay activation signals from the original headlight connectors and added new, larger wires from the relays to the headlights. Zero modifications for the original harness was needed so I wanted one.

Only problem with the kit was the price: 225 dollars for a few wires! Luckily the harness was so simple that building one would not be a big task. First I drew a diagram of it.

The light red and light green wires control the relays and are pushed in the original passenger side headlight connector. I used 18 gauge wires as not a lot of current go through them. The main wires are 14 gauge and there is a 20 A fuse in the main power feed. I decided to hide the relays under the battery tray and measured the wire lengths accordingly. So after a trip to the car parts store and a few hours of building, this is what I got.

I used some nylon wrap to protect the wires and to make them seem a bit cleaner. The lone black wire that comes out of the harness at the junction is screwed to the original headlight ground bolt on the radiator support. It is missing the connector as I had to cut the wire to correct length in the car.

I decided to use a glass fuse as its box was the simplest and every other fuse in the car are of that type. The yellow wire is still missing the connector in the picture as I needed to measure its correct length.

This picture shows the passenger side harness. Unfortunately I could not find a male H4 connector so I had to use individual terminals. The new headlight ground wire just makes a short loop inside the wrapping and connects directly to the old harness.

I also noticed that the H4 terminals are a bit wider that the normal 6,3mm versions. I could not find those either so I had to solder the already wired terminals from the connector to my own wires. Annoying as this meant that the wire colors change. Why couldn't they just include the separate terminals that I could crimp to my wires?

On the driver side I just had the H4 connector and one terminal to the ground of the original wiring harness.

Installation

I began the installation by removing the headlight housings. Then I turned the lights on and measured the voltage in the original connectors. The car was not running but I saw an 1,5 volt drop compared to the battery. That causes a huge difference in brightness and the difference would probably be even more with the engine running.

Next I had to remove the battery so I could install the harness. The relays were placed out of sight under the battery tray and from there the wires come up to the top of the radiator support and split towards the headlights. After connecting everything up I measured the voltage again. This time the difference was under 0,1 volts. Excellent result!

While at it I also replaced the bulbs with better ones and screwed the housings back. I turned the lights on and the difference was huge! I was really happy as building my own harness had cost only about 30€ in parts.

Original wiring

New harness with relays