Headlight Relay Modification 240Z

[SteveJ]

Here is a DIY headlight relay modification that I implemented on a friend's 73 this weekend. This will not work on early Z cars where the headlight switch completed the path to ground.

Parts needed:

1. 2 SPDT or SPST automotive relays with bases
2. 14 AWG wire - It is good to have many different colors to keep things straight
3. 12 AWG wire
4. Electrical tape to wrap your wire bundles
5. 1 3-pin 6.3mm non-latching connector (male and female)                                                                                                                                                                             
6. 1 6-pin 6.3mm non-latching connector (male and female)                                                                                                                                                                   
7. 2 4mm bullet terminals with insulators and 1 4mm dual socket terminal with insulator                                                                                                               
8. 2 inline fuse holders with 10A fuses
9. 2 5A glass fuses
10. 1  yellow ring terminal (solderless lug) (BTW I'm guessing on the size. I can't remember for sure)
11. 2  blue ring terminals (solderless lug)

Steps

1. Remove the negative battery cable
2. Remove the clamshell cover on the steering column
3. Remove the center console and loosen the fuse box for access underneath
4. Figure out where you can mount the relays. If you can get relay bases with mounts built in, that is preferred.
5. Estimate the distance from the turn signal connector to the relays and from the fuse box to the relays
6. Cut four lengths of 14 AWG wire for the run from the turn signal connector to the relays and wrap the wires together
7. Cut a short jumper wire to run between the 3-pin male and female connectors per the drawing                                 
8. Note which wire you plan on landing on the connectors 
9. Crimp the pins onto the wires and insert them into the connectors                                                                         
10. Break apart the 3-pin turn signal connector and plug your new connectors into the turn signal connector in the steering column
11. Route the new wire down the steering column and over to where the relays will be mounted.
12. Break apart the 6-pin connector at the fuse box that has 2 red and 1 red/yellow wire.
13. Cut 2 14 AWG wires to run from the fuse box to the relays. Cut two shorter runs of 14 AWG wire (I prefer black) for the grounds.
14. Cut a length of 12 AWG to run from the fuse box to the relays.
15. Cut 4 short 14 AWG jumpers to go between the male and female 6 pin connector.
16. Wrap the 12 AWG with the 2 long 14 AWG wires. Leave enough length of 12 AWG wire to route into position
17. Crimp pins to the wires and insert them into the appropriate connectors.                                                                                                                                                                      
18. Route the 12 AWG through the underside of the fuse box and attach the yellow ring terminal.
19. Lift the rubber boot covering the terminal on the white/red wire on the underside of the fuse box.
20. Remove the screw, and add the new ring terminal on the screw with the white/red wire.
21. Secure the screw back onto the fuse box and cover with the rubber boot.
22. Secure the fusebox to the car.
23. Connect the jumper harness into the fuse box connectors.
24. Route the ground wires over to a screw. Trim the length of the wires if necessary. Attach the blue ring terminals and secure the ground wires to the car.
25. Route the 3 wire bundle over to the relays.
26. Trim the length of the wires if necessary. Attach the 4mm dual socket terminal to the 12 AWG wire. 
27. Attach 1 4 mm bullet terminal to each inline fuseholder. 
28. Attach the fuseholders to the 12 AWG wire.
29. You can do one of two things at this point. You can remove the existing wires from the relay bases and use 6.3 mm female terminals on the new wires, or you can use an 8-pin 6.33 non-latching connector to connect the relay bases to the new wires. Make the connections per the attached schematic.                                                                                                                                                                                                                                                    
30. Change the headlight fuses in the fuse box to 5A.
31. Attach the negative to battery. Test your headlight relays by turning on the headlight switch and alternating between high beam and low beam.
32. Once you verify relay operation, install 10A fuses in the inline fuseholders.
33. Replace the center console and clamshell.

Here is the schematic:

This reverses the original design to have 2 positive wires going to the headlights and 1 negative. This allows the use of LED headlights.

Edited August 16, 2020 by SteveJ