Good advice.

Datsun 1200 doesn't use any headlight relays, and so far so good on that, works as well as the Datsun 510s that have them, and more reliably than the 510s.

Definitely needed with dedicate power circuit if you go with high-power headlights.

I forget if those had "grounded" relays or "hot-side" relays

Little matter, check your grounds, especially the ones below the batt4ery..

Look for wire chafing around the headlight buckets and anywhere else.

If you installed aftermarket lighting, the relays may be drawing too much current, forgive me for rambling, it's almost 4 am here, and I'm very late to bed.

Rusty Wrench

Like dd says you have to go by process now, start at lights and work your way back at each terminal or plug ect try measure volts at fuse just in case, also as many ppl find out that many of the older cars run an earth circuit id also be looking at earths as well as it may just be something simple your not looking at! I've mad many mistakes with thing so simple :) good luck chris

Sounds like you are on the right track. Gotta find that short and unshortify it.

Start with elimination. Remove the headlights and flip the High Beams on. Still blow the fuse?

Pull the wiring connectors off (if the B10 has connectors) where the dash wiring goes through the firewall. Etc. Until you isolate it.

Because it's only the high beams, that means
* Headlights
* high-beam wires (RW red/white wires)
* Turn-signal/dimmer combo switch
* high beam indicator lamp in dash
There's not much else involved. One of those is likely to be shorted. It's unlikely to be the light switch, since the high beams don't work off that. It only feeds the dimmer switch. But there is a possibility it's shorting under heavier loads.

We could nut it out but fir you png/sa types, that doesn't mean getting your nuts out or touching others nuts.... Mr t didn't mean get sum nuts like that.... Yours are slightly dark and chrmewy though

In 1ks do the headlights form part of the ground in the circuit? So the projector beams are not conductive so forming a short?

Just an idea. Chris im as talented with elecs as you... Let our tard powers unite!

I removed the projector lights that were in there, the side of the globe said 100/90w and the ones I have replaced them with are 55/60w. The second fuse down is still blowing.

This is the stock wiring, no relays installed yet.

If I join the circuit with something conductive in place of the fuse the lights will not illuminate and the spark is quite severe, it is almost certainly shorting out.

Looks like B10 combines headlamps and all other lights on the same circuit, thus the 30A fuse. B110 more sensibly splits them into two smaller circuits so that if one blows, at least some lights are still working on your car and blokes won't crash into your dark car...

Switch is fine and clean as a whistle no signs of a potential short.

The problem exists with the factory wiring but I'll have a look at the wattage on the globes now.

Original fuse box and it's the second fuse down from the top of the box.

It sounds like a dogs breakfast. How many relays are there? What is the wattage of the high beam? Which fuse is it blowing? In the orig little box or an external one?