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MG Midget and Sprite Technical - Headlight Relays

Another day of *retirement* another project!

For those who struggle with bad headlight performance and are considering fitting relays, I have just done so today. Just in time for the summer! :-) Some thoughts:

- It is easy, it took me about two hours, including making up all the extra wiring etc. and realising I had wired it up wrong and having to make corrections!
- It is cheap, the relays were about £3.50 each, I had wire and connectors already.
- I used fused relays as an added feature, as the headlights aren't normally fused.
- None of the original wiring needs changing, it just some reconfiguring of the wiring at the lamps adding in a couple of jumper wires, an extra wire for +12v, and an extra earth (per relay).

Before relays: battery voltage measured at 12.7 v before test, voltage at headlights measured at 11.3 v, an 11% drop.

After relays: battery voltage measured at 12.6 v before test, voltage at headlights measured at 12.1 v, a 4% drop.

Scale this up to when the car is running and the alternator is working (assuming 14 volts) without relays = 12.46 v at the lamps, with relays = 13.44 v.

Pic of before/after, not sure it really shows up much difference when tested in my garage, although I have maybe convinced myself the shadows on the after picture are sharper!

And it prolongs the life of your indicator stalk switch too.

Cheers,
Malcolm


Malcolm Le Chevalier

Picture maybe uploads better this way.


Malcolm Le Chevalier

Just in the middle of this on a Frogeye in preperation for this years Le Mans.

I'm converting from BMC glow worms to H4 search lights and was worried about the dash switch overheating due to extra current.

I'm putting in a single relay to take the load off this switch but retaining the foot operated dip switch. One relay under the dash somewhere hidden. I know it won't be as efficient as a relay at the headlights but it should still result in much better lighting.

Rob
Rob aka MG Moneypit

Good shout Rob for those with early wiring.

An addendum, the above readings were taken for the low beam. I have just tested the high beam setting:

No relays: 12.7 v / 10.9 v / 86%
Relays: 12.6 v / 12.0 v / 95%

Scaled up to 14v, no relays main beam voltage ~ 12.0 v, with relays ~ 13.3 v

Malcolm
Malcolm Le Chevalier

Good project Malcolm. I certainly noticed a big improvement in my lights after I fitted relays and fuses, though I didn't do the V drop testing you did. That was interesting reading. And as you say the stalk switchset gets a much easier life.

On that note I've been pondering about installing another relay for the horn. As I understand they typically draw around 5 amps and anything up to 30 amps for the big scary aftermarket air horns. Yikes! That'll be why moderns have horn relays, then.

I reckon it's probably time to buy another relay, what do you think?

If my car were an earlier one with the horn push in the wheel boss I would defo do it.
Greybeard

The only time l ever use the horn is to test it for the MOT. On the occasions l might have used it for real l am always too bust taking avoiding action!
GuyW

"Just in the middle of this on a Frogeye in preparation for this years Le Mans."

Be sure to keep to the right as Mark Webber comes past you in his Porsche LMP1😂
Bernie Higginson

Hey Greybeard,

Yeah, from the info I could find/measure the horn is second highest current draw after the headlights, around 5 - 6 amps. Although the only reference I could find was Wikipedia, I don't fancy physically measuring it with my multimeter. I am sure my neighbours will be thankful too.

Next up I measured the blower fan at ~3.5 amps (working current, it spiked at a bit more at start up). Everything else should be less than two amps.

Like Guy, my horn has only been used for MOT purposes, the one time I wanted to use it I couldn't find the button!

I am planning a big re-wire at some point, might include it then. But depends if you are one of these people that enjoys peeping at everyone and everything!

Malcolm
Malcolm Le Chevalier

I should be more honest and say too busy panicking, rather than"taking avoiding action" !!
GuyW

Sometimes, the use of the horn can obviate the need to take avoiding action ;o)

For those with floor mounted dipswitches, there is a very useful Volkswagen relay available which replaces the dipswitch and is operated with a single momentary action flick switch. With headlamps on, one flick of the switch will switch between dip and main, or vice-versa. With headlamps off, it acts as a headlamp flash switch.

Details on the MGAGuru website...

http://mgaguru.com/mgtech/accessories/at113a.htm
Dave O'Neill 2

I nearly used one of those VW relays when I had the hiatus with the wonky stalk switchset Dave. I looked like a really good option at the time.

My electric fan has it's own relay so I think I'll just leave the two of them to get on with it :-)

I've just been given a relay that I'm going to use for the hooter. I have a couple of spare slots in my supplementary fusebox as well. I wonder if the horn will be louder or change it's pitch. (Not that I GAS TBH).

Next up - the blower fan. I hadn't thought of that. I don't have a meter capable of measuring more than 2A since my AVO got stolen :-( But it's a good point bout the transient starting current. Same must apply to airhorns which are essentially a motor in stall mode, unlike conventional diaphragm horns.

Just because I almost never use it (like the horn) doesn't mean I want it to cause a problem when I do switch it on.

I still think it's a must for older cars with the centre horn push. Those contacts were never the best as I recall.

But anyway - well done Malc. A good bit of work and a useful reminder to me revisit my electrical systems. Cheers.
Greybeard

Rob,

I did that same change on our Minor with foot dipswitch, and there was still a difference in main beam brightness when using headlamp flash. The flash triggered a relay that connected directly to the main beam connector AFTER the dip switch. If I switched on main beam and pulled the headlight flash, there was a noticeable increase in brightness.

My conclusion was that there was a voltdrop across the dip switch itself.

After the recent complete disassembly, respray and reassembly, I have fitted relays for both dip and main beam, so that all the dip switch does now is to trigger the relays.

Richard
Richard Wale

Richard.
It may be something for the future if the dipswitch shows signs of deteriation.

Even with the BMC glow worms fitted, at night the headlight switch gets noticably warm indicating some voltage drop.

Last time I went to Le Mans was in my yellow midget and on return when taking the headlight deflectors off I realised I had put them on upside down so making the headlights more annoying to French drivers. There's always an upside to everything.

Rob
Rob aka MG Moneypit

I've been fixing my horn today (which broke on the way home from Saturdays MOT!) and given the 'unusual' nature of the contacts at horn push end (centre of wheel, old 3rd party boss and DIY slip ring & contact) I was thinking that taking some strain off it by adding a relay might not be a bad idea.

When fitting relays is it better to site them nearer the switch or the accessory (lights, horn etc..) or does it make not a blind bit of difference?


G Hawkins

Hiya,


It's good practice to fit any relay as close to the consumer as possible and take the feed to the relay from a live supply as close as possible as well.



All my headlamps, horn and cooling fan relays draw their current from the rear of the alternator for example.



The relay actuating circuit draws a very small current, typically .25amp so a long wire run from the switch is no problem.



Remember the shorter the wire run and the the thicker the wire, the less volt drop you have.
SR Smith 1

I burnt out a dip switch on the stalk before I went to relays. Losing all lights when you go from dip to full on a back road late at night certainly gets the adrenalin pumping.

Relays on the bulkhead by the fusebox and fed from the main live post on the back of the solenoid.

Dean Smith ('73 RWA)

My relays are just on the inner wing half way between the solenoid and headlamps. Main feed taken from solenoid also, the rest tied into the existing loom.

Not tidy at the moment, but functional. A re-wire of the whole car is on the cards. I will bet everyone a tenner I never actually get round to it...
Malcolm Le Chevalier

Malcolm,

While the voltage increase is 10% from your figures.

Power is V squared / Resistance.

Now the resistance will change with filament temperature but we do not have these figures, assuming a constant resistance, your set up will give a 21% increase in power and a great increase in light output not the 10% inferred by your voltage difference.

Eddie Cairns

Thanks Eddie. Did not know/remember that. Even better.
Malcolm Le Chevalier

This thread was discussed between 05/05/2016 and 15/05/2016

MG Midget and Sprite Technical index

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