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MG Midget and Sprite Technical - Headlamp relay question

Hope you guys are having a long lunch or quiet afternoon?

I am about to put in an order and want to buy relays for a halogen headlamp conversion. The list shows Headlamp 2 relay kit and Headlamp 4 relay kit. Which do I need? I have a standard good old fashioned Frog with side lights and about to have 55/60 halogen main lights.

Is the four relay for controlling spots as well as headlights or is it two dipped and two main? I can fit electrical parts it’s just understanding how they work I don’t get!

What do I need to buy?

Mark
Mark '59 Frog

... need details of the kit, since:

2 relay kit: each relay might run both sides,; one for dip and one for main

4 relay kit: to switch each side, dip and main separately

My guess if the 2-relay kit, wired as above, provided the relay can switch/pass 10A min.

Don't forget to put ~20A continuous fuse on the main power feed.

A
Anthony

A relay?

Wow

I haven't done any more than ensure good earths and clean power connections to the bulb holders, in my fibreglass wings. 55/60W units

Should I do more?

I have a relay for the brake light switch and one for the electric fan, isn't that enough?

Mark the "four or two" options are probably for twin or single headlight circuits.

how do we get to look at the info you have, is there a website we can look at?

this should spark some debate I hope
Bill

It's a (whisper) moss thing! They have hardly any words in their catalogue, just some stuff about you need it or your car will melt! It's in the front section of the last mag so you can't download it and post.

"early electrical systems did not use relays for high current components such as headlamps (they can't mean my dim old lamps), meaning all the current has to run through the switchgear, putting high loads on these components especially if fitting halogen headlamps."

Then it just says it is east to fit and runs the high current circuit through the relay to protect your circuits and reduces the resistance to allow thing full power to make your lamps brighter. The same logic as the few articles in the archive refer to.

I decided to phone Moss and a chap told me that the 2 relay would be fine and his logic seemed to be that the Frog had hardly any load so two would be adequate and not strain the rest of the circuit (a lot of assumption from me there).

A mate told me he has been running halogens in his midget for years without a relay of any sort, although he has been advised to. My reading of this is as Anthony suggests; one for main, one for dip and if you have a later car with more electrickery in it you may want to do one for each side main and dip.
Mark '59 Frog @ work but just going home!

I assume that the four relay kit allows you to have a left/right split for fuses, where as the two relay kit allows you to fuse only dip and full seperatly.


So wiring would look like this

Fuse L ---> relay1 ---> left dip
........-> relay2 ---> left full


Fuse R ---> relay3 ---> right dip
........-> relay4 ---> right full


or

Fuse dip ---> relay 1 ---> left dip
.......................-> right dip

Fuse full ---> relay 2 ---> left full
........................-> right full
Will Munns

The loading on the switch is identical for Halogen bulbs when compared to tungsten bulbs WHEN BOTH ARE THE SAME WATTAGE.

Daniel Thirteen-Twelve

http://www.mgb-stuff.org.uk/lights10.htm

Hope this helps..
K Harris

K Harris,
that's really helpful so I need to get a couple of relays.
Bill,
why a relay for the brake light swirch - do the new ones fail or what?

Jeremy
Jeremy 3

Yep the new ones are not very good!!
K Harris

Oh yes

recent history shows that these things are as weak internally as a weak thing and the contacts fail very quickly in service.

Don't ask me how many new ones I have fitted since the car went back on the road 2000 AD ish

'cos it isnt a secret!

I am on number three but number three went in with a relay and shows no sign yet of failure, handling relay switching current instead of brake lamp loading.

My halogens seem to be perfectly OK and have not melted the front of Lara like chocolate at all

But I see two ways of looking at it

Switch loads may cause failure &
Relay fitment gives many more cable connections that may fail, both coming from the extra power supply and the revised switching circuits

I know the relays make sense but I remember many breakdowns attended due to failed relays.

Your money
Your choice
Bill

Only other point to consider is if the wiring loom is old then having 12V (ish) at the battery almost certainly means a lot less by the time you go through an assortment of switches and connections. A nice new shiny wire from battery to relay to lights (and finally to earth if fglass)will mean that you get the benefit of all the volts.

Old connections may also start to heat up more with higher current being passed with risk of melting - though I agree that it doesn't happen very often though it did to a motorbike of mine.....

Invest in a relay unless you have rewired recently, its not a lot of cash.
robint

Invest in relays even with a new loom, its a bad idea switching high current around the car.

Vibration, wet, age, and poor design is the lot of the BMC loom..

Trust nothing is my motto:)
K Harris

Well that was very helpful chaps. Particularly the wiring diagram K!

I will be investing in relays and hope that next time I am caught in a storm, in the middle of the night, in a dark country lane, I will be able to see the bank at the side of the road before SWMBO shouts that I am about to hit it!

I am having a week at home next week and for the first time ever (on any car!) I will be taking out the engine and gearbox. Expect to hear a lot from me...

Thanks guys I knew I could rely on you!

Mark
Mark '59 Frog

This thread was discussed between 14/04/2008 and 15/04/2008

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.