Sale ends todayGet 30% off any course (excluding packages)

Ends in --- --- ---

Question About headlight Hi-Low wiring

EFI Wiring Fundamentals

Forum Posts



Tech Articles

Discussion and questions related to the course Motorsport Wiring Fundamentals

= Resolved threads



I Am currently working on rewiring all of my car, a datsun 280z 1977. Everything is going well as I'm basically just mimicking the OEM harness. However I would like to add two 4 pins relays to my headlight switches since from the factory all the current is going trought the switch itself and it has been known to melt the switch.

I've come up with the following wiring diagram (see picture) I know it looks like a 5 years old drawing but I'm not an artist, I work on car. basically there is 4 fuses for the circuit. one 50 amp fuse for the main power wire, 2x 10 and 15 amp fuse after each 87 pin of the relay and one 10 amp fuse for the on/off switch. I have not fused the beam switch as it is only grounding the relays. Each lamp draws about 4,75 amp on high beam and 2,5 on low beam. the Lamp are classic 3 prongs lamp you found in older vehicules. I do not want to upgrade to LED as of now since I like the look of incandescent bulbs.

So I would like to know if the fuses are located at the right place on the circuit and if it is a safe circuit. Thanks!

Attached Files
  • 322300575_883573446173625_7242474459434905425_n.jpg
  • Attachments may only be downloaded by paid Gold members. Read more about becoming a Gold member here.

Normal 7"sealed beam headlights are 75W high, 60W low and if you're using H45 or H43 halogen semi-sealed the standard is 60W and 55W respectively (I usually run 100/80W), so your amperage is a little out. I would suggest checking your figures for the voltage drops in the circuits, as the light output is approximately proportional to the voltage - a 10% (~1.4V) drop across the bulb from 'undersize' wiring that meets the current spec' will mean less than 75% of the possible lighting, and it looks much more when driving.

I was initially a little confused by the wiring, but then remembered it's common for Japanese vehicles to reverse the wiring, the headlight switch powers the common bulb terminals that are normally ground, and each filament is grounded through the dip switch. I would suggest simplifying it by connecting the headlight switch out to the 86 pins and the dip switch to the relevant 85 pins, the easiest way of doing that is to tap the wires at the headlight, I use blade terminals into the OEM sockets and aftermarket plugs with the upsized wiring to plug onto the headlights - this keeps the OEM wiring intact if you want to change back or as a back-up if a relay fails. The headlight side will be isolated then and you can run the conventional common ground.

You may wish to simplify the fuses and relays by using relays that have built in fuses - the same pins are available - and this is what I do, but with a relay for each filament and rather large gauge wiring for both power and earth, because I have a 'thing' about headlight voltages and output. I don't use a fuse on the power feed to the relays, as I have it protected by the routing from possible shorts from a crash, and figure that miniscule risk greatly outweighs the risk of that main fuse blowing and taking out all my lights. I had an alternator fail and blow 5/6 filaments from a voltage spike - VERY scarey at speed and while I can't protect against it happening again, I can avoid the fuse taking them all out.

Thank you for taking the time to answer!

I'm not really sure I'm following you when you're talking about simplfying the circuit when you're talking about the pins 86. Just to be sure, I will be using all new wires, the old ones are corroded and generally not ideal, so nothing of the original harness will remain exepct the switches that is why I want to ground the pins 85 with the dimmer switch and power the pin 86 with the light "on" switch, to keep the originial switches.

The fused relays are a excellent idea, I'll use them to simplify many circuit in my fuse box like the defrost and horn circuit.

Final question about your answer is would'nt you prefer having everything protected with fuses in case of a voltage spike? I know the 50amp fuse would take out both my lights but at least my circuits would be protected. The reason I used a 50amp fuse there in the first place is that because it comes like that from the factory. I have included a picture of the original headlight wirring where you can see a fusible link, it is originaly rated at 50amp. the second wiring diagram is what someone has already figured out but it was made to use the old wires on the car, so that it why I came up with my own version, since I'm going to use all new wires

Attached Files
  • 322429901_1602484756856790_649714513374595118_n.jpg
  • Attachments may only be downloaded by paid Gold members. Read more about becoming a Gold member here.

Sorry, mind-fart there, should hve been 86 and 85 pins, or vice versa if preferred.

The 50A fusable link is normally used to protect multiple circuits and may be supplying the fuse box (/relay box if used) - check the full wiring circuits on that. They are a PITA, because some are physiclly part of the wiring rather than an easily swapped main fuse or link.

First diagram has the fused power to the common light pin from the headlight switch and return through the dip switch, as I suspected - you can copy it or not but I don't know how the dip switch internals are wired so double check it is an insulated ground link before applying power from that end.

The second is as I suggested working around the default earth return of the car. The relays' solinoids are still powered from the headlight switch and grounded through the dip switch, but the headlights now use the common pin as ground and the filaments are powered from the relays, the reverse of the OEM. It's the way I would normally do it.

If you're doing a full rewire, you can do it as you wish, just be aware of the OEM setup as there may be hard-wired circuits that aren't the usual polarities.

We usually reply within 12hrs (often sooner)

Need Help?

Need help choosing a course?

Experiencing website difficulties?

Or need to contact us for any other reason?