Headlight Relay

[Ciaran]

Evening everyone

Just a quickie, does anyone know if the headlight relay needs to be a specific ampage?
Mines been acting up lately, and I've tried cleaning the contacts etc many times. This works for a while, but the flickering lights problem soon returns a few weeks later. Its got to the point where people change lanes when I'm driving behind them, as with the flashing lights they think I'm some kind of impatient twat flashing them out of the way!

Anyway, the legs of the relay really have seen better days, and I have a spare Maplins one upstairs (40A I think), which I could stick in. Will this do the job, or do I need something more hefty?

Cheers.

Ciarán.

[bigjohnh]

Hi Ciarán,

Working on the fact that

Watts = Volts x Amps

and 1 headlight is 55Watts

Battery voltage is 12v

Amps = Watts / Volts

Amps (one lamp) = 55/ 12 = 5 and a bit (say 6 to be safe)

So even if the relay is carrying all 4 bulbs main and dipped beam this is still only 24 amps.

So a 40 amp relay should be OK so long as the pins are all the same it should just plug in.

Best of luck

John

[onthecut]

Hi Ciaran,

While you're at it, don't forget to have a check around the contact areas for the earth cables on the inner wings. If they've become corroded and high resistance, it all ups the load on the relevant relays.

Mike.

[Ciaran]

Hi guys.

John, many thanks for that detailed explanation, makes sense now. It brings memories of Physics class flooding back.. wish I'd paid more attention. Given that I only left school 8 years ago though, I don't really have as much excuse as some people!

Mike, cheers for the point about the earthing points. They're something I've been meaning to check for a while now, so that was a timely reminder. I haven't had too many electrical issues (touch wood), but theres the odd irritating thing, such as the dot matrix and clock dimming signifigantly in time with the indicators flashing, etc, so a good cleanup of the wing earthing points wouldn't be a bad place to start really! Not to mention, its probably the reason the headlight relay's legs corrode again so soon..

Cheers!

Ciarán.

[xmexclusive]

Hi All

I did not realise just how poor the main earthing points were BY DESIGN until I took a Mk2 to bits. The wires end in crimped rings but the mounting stud is part of the chassis and painted when the body is sprayed so it is an insulating layer. This needs to be taken back to shiny metal before the connections are reassembled otherwise the earth path is through the crimped rings to the bolt head then back into the chassis via the contacting parts of the bolt threads. Many people do not realise how small the contact area of a threaded bolt is and that as you tighten the bolt the area reduces.

Regards

XMexc

[Peter.N.]

For any electrical calculations I use the easily remembered formula that it takes ' one volt to push one amp through one ohm' and 'Watts = amps X volts'. If you are even only moderatly good with mental arithmatic you shuld be able to roughly work out most calculations in your head. This is the system I have always used and I was a TV engineer! But I cant remember formulae. I passed my amateur radio exam using this method!

Peter.N.

[bigjohnh]

Peter,

What do you mean you can't remember formulae?

By your simple method you have remembered the two most important formulae in Electrical Engineering including Ohm's Law.

John

[Ciaran]

Hello again everyone.

Well, I swapped over the headlight relay earlier. Turns out the old one I removed was only a 22A... so according to John's calculations it was actually slightly under rated

I gave the relay holder contacts a good clean up and stuck in the new 40A one, so far so good. The lights actually appear a good bit brighter, but perhaps thats just placebo effect.
I didn't get the chance to clean up the wing earthing bolts, as it turns out the central locking had plenty to keep me busy! They did look ok, though I'll give them a look this weekend.

Thanks for the help guys.

Cheers.

Ciarán.

This post has been edited by Ciaran on January 23, 2007 12:17 am