TR6 '72 Headlight Relays

[Spock]

I have purchased relays, fuses, and other components to upgrade my headlights. This design uses "dummy" headlights so the original wiring and fuses are left intact. The output from these dummy headlights feeds the low-current side of the relays and allows power to flow to the real headlights on the high-current side. Seems like a good design.

My question is: where should I draw power for the high-current side of the relay?

For obvious reasons this needs to be unstitched power and I see opportunities at the battery, the starter, the alternator, and the fuse box. Wiring diagrams show a direct link from the alternator to the fuse box so that might be my best choice, but I would love to hear from someone who has done this and maybe see pictures of your installation.

Thank you.

 

[poolboy]

I have a Delco alternator on my TR6, so I tapped into it's 12 volt terminal for the battery connection. That's where the Brown (N) wire connected to the circuit breaker is connected.

 

[Spock]

Thank you. I have a plan and wanted to see if anyone thinks it's flawed.

There is a brown wire that connects the battery to the alternator to the fuse box, connected (through a fuse) to the "purple" wires that currently support the headlights (among other things). The fuse box has four slots but only three are in use.

My plan is to jumper from the brown alternator/battery wire to the unused fuse slot, insert a fuse, and then take power from the other side of the fuse box to feed my headlight relay. This should give me a dedicated and fused circuit for my headlights along with a clean/professional looking installation.

I think it's going to work. Let me know if you disagree.

 

[1976 TR6]

Did it? Do you have pictures of the overall wiring? I’d like to do the same.
- 1976 TR6

 

[Spock]

I did it and I'm very happy with the results. I don't have good pictures of the overall wiring but I will share what I have and try to explain. For reference I'm in North America and my car is Left Hand Drive, so if this doesn't match your situation it may be confusing.

1. The fuse box.

The top slot in my fuse box was empty so I used it. The original headlight circuit is second from the bottom. The brown wire on the left is power (from the alternator/battery) and the purple harness on the right runs through the dash switches and feeds the headlights. I jumped the brown wire (3rd row left) to the empty slot (top row left), added a fuse, and then ran two wires from the fuse to the relays (top row right).

2. The relays.

There are two relays, one for the high beam and one for the low beam. Each relay controls both sides. I mounted the relays on the front of the wheel arch, behind the left headlight, because it was easy. Each relay has four wires: power (from the fuse box), switched output (to the lights) and trigger (from the switches inside the car), and ground.

The power is a straight line along the edge of the hood from the fuse box. The ground wire is connected to any convenient spot. The trigger and output wires are connected into the existing wiring harness.

3. The existing harness.

The headlight wiring harness is accessible by removing the driver-side radiator shroud. There are two three-way connections you will need to locate. One for the high beams and one for the low beams. Both connections are located on the left side of the car. Each is a clip that accepts four bullet plugs (two at each end). The three wires for each connection are source power, output to the left light, and output to the right light.

It's important to note that the bullet connectors in these cars are bigger than the modern North American standard. If I use the modern male bullet connectors in the old female clip the result is a loose fit that will fail. If I try to crimp the old female connector it will shatter. The best solution is to source the correct bullet connectors that match the original wiring harness. The exist and can be ordered off the internet. Highly recommended. You will need four male bullet connectors plus two female/female bullet adapters. Basically each connection you pull apart results in one male side and one female side. Your new wires will need the matching ends.

Here is a diagram of the wiring harness (for a 76) with the junctions highlighted in yellow. Note that on some cars the output wires from the connector are on the same end (with the source at the other end) and in other cars this isn't true. You should trace and identify (and label) all six wires before you start.

4. Finishing up.

For the high-beam circuit you disconnect the source wire from the connector and replace it with the switched output from your relay. The original source wire now becomes the input/trigger wire for your relay. Repeat for the low beam circuit, test, and then replace the radiator shroud panel.

For me that's really all there was to the project. It would have been trivial but I mis-identified a couple of wires and didn't know about the bullet connector size mis-match until after I started. So mistakes were made (but corrected later).

It was an easy job, the results are wonderful, and the whole thing can be reversed because no wires were cut.

Best of luck!