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Main / dipped beam operation/ voltages at h4 plug


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Hi all,

My headlights were rubbish, I never expected them to be great but they were yellowy and dim and I thought it must be due to the headlight units being so rusted inside onthe reflectors etc.

I upgraded my headlights and discovered and removed some atrocious scotch locks and skinny wires powering some additional spots...

I have a relay switching the headlights now, fed from the passenger side H4 plug, switching a fused wire direct from the battery. but it's operation has been a little temperamental when switching to main beam with the column stalk. With a multimeter I'm seeing voltage of say 9-10 volts on the pins for the dipped beam, and when I switch to main I see anywhere between 6 and 8 volts across BOTH dip to ground and main to ground. pins - so for testing I plug a h4 bulb into the socket and i get a bright dip, then switch to main and both the the main and dip filaments are lit but dimmer. Doesn't matter if I pull the lever for flash or push forward for main.

This issue with the test bulb seems to be cured if I also plug a bulb into the drivers side h4 socket. Both flash and dip accordingly.

I don't have a dim-dip setup, or at least it might have been bypassed if I did. Could the indicator/main beam stalk be at fault? I am assuming this behaviour isn't normal for the headlight switching?



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i wired 2 40amp relays [ for dip and main beam ]direct from the alternator ouput terminal, ie the terminal that the thick cable goes to for charging the battery , and then ran 25amp cable feeds from the relay outputs to the headlight units and connected to the back of the H4 bulb sockets as close as possible to the socket and with some bullet connectors .

i switched the relays for main and dipped beam using just the nearside original headlight wiring which activated the relays via their coils .

this then removed the heavy current load on the headlight switch which is normally the cause of the switch contacts melting into the plastic of the lighting switch on the steering column .

i blanked off the offside drivers side headlight wiring and it isnt used anymore .

now i have 14.5v at the headlights with the engine running on tickover and 55w bulbs are just fine and nice and bright .

before i wired direct feed from the alternator output i had similar voltage drop to you and really dim headlights .

unfortunately landrover used some seriously under rated wiring which resulted in high resistance and a lot of voltage drop at the headlights compared to the voltage input , but also the headlight switch on the landy would get that hot it would make the switch contacts melt into the insulation of the bottom of the switch , i had it happen to me one night at 60mph down a country road and my lights all of a sudden went out , not for the faint hearted trying to brake and hope you stop without hitting anything !! .

i am using wipac crystal headlights , much better pattern from these , and 60/55w standard halogen bulbs and they work fine for me .

i did have 80/100w bulbs in but decided to go back to the 55w ones because of the current reqd to power them versus diesel economy and trying to save myself some fuel .

i do find that the standard halogen bulbs give me better visibility on dark tarmac and off road , for some reason the yellower light shows up better than if using those "white" type of replacement bulbs .

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I really appreciate the advice, I didn't think to tap off the alternator feed so I might change what I've done to copy m0bcg, but isn't it strange that I'm seeing voltage on both main and dip circuits simultaneously when it should be all on the main circuit? I'm not great with electrics, is that sign of poor switching or could that be a bad earth causing some kind of leakage or back feed?

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I don't know the voltages at the H4 plug but I do know that a QI or Halogen lamp needs over 13 volts to work properly.

I've tried to copy and past the full write up but it doesn't work so please have a look at


It's old hat now but that's how I would wire my rally car headlights up.

Good headlight units are also a must. At the least HELLA

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All the above is good stuff, but also one point to not overlook is what sort of voltage is your alternator giving you when you have everything on? I found an alternator was only giving about 28A max, even though it was a 60 amp unit (I'm guessing at diode failure). End result was the voltage was hovering about 12.5 rather than the 13.5+ it should be. Wipers and flashers were slow, lights were dim etc, but it all worked.

You need 2 relays to perform the switching normally; can't see how you are doing off, main and dip with one. Can you put up a diagram?

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