So a few months ago I wired up some external gauge lights to my headlight switch, however switching on the headlights caused the revs to then drop significantly...
I replaced it with a new alternator- no different
And just now I've finished wiring up the gauge lights to a relay activated by the headlight switch, still does it!

Bad earth? The gauge lights are all earthed onto a stack of 4 other devices' earth connections into a screw under the dash.

Any suggestions?

More information is needed...

what sort of lights? Normal headlights or something else?

That the revs drop when the alt is loaded up (switching on lights) is not that surprising, but how much does it drop? Only at idle? What sort of motor?

Are the headlights good & bright? Any other electrical woes?

cheers,

Jacob

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'72 Clubman Van - 1022cc, 295 head, 731 cam - Daily Driver
'69 Morris 1100 S - Dinged by a bus, in shed under repair
'64 Morris 1100 - Early 1100, long term project

It's normal for engine to load up when load (lights) is applied to alternator. As for how much it drops, depends on lots of factors, engine condition/tune, amount of load i.e. high wattage lights etc. I'm assuming it's just a normal Aussie mini, not injected Rover one. A quick test would be to unplug each headlight, preferably high beam, start engine and then whilst idling, switch light switch on. Obviously only parklights will come on but there shouldn't be any significant RPM drop. If there still is it will require more testing. Try it and let us know outcome.

Not trying to be a smartie or anything, but I'm assuming you're not a mechanic or the like. The alternator would normally run everything and have enough reserve to put some back into battery. Basicly for it work, it needs to 'make' power, to do this it draws power (HP) from the engine, the more power you want from it, the more it draws from the engine. .................Basicly

Cheers for the response,
It does make sense to drop as The doc also said its normal that my volts drop 1-2v each time my indicator flashes. Turning on the headlights makes it drop from about 14v to around 11 and almost stalls the car at idle, however revving the car put the volts back up and all is sweet. The headlights don't increase brightness when I rev however, they're really bright even when it's at low voltage, which from my research suggested I have a healthy alternator.

If this is perfectly normal and when I drive at night I just have to rev it up a bit when I stop at the lights, no worries, just don't want to have any potential electrical gremlins lurking in my wiring or be loading up the battery and alternator so much I'm going to kill them

What wattage headlights are they?