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MG Midget and Sprite Technical - Power source for headlights

This is a follow up from an earlier thread where I reported that I had no dipped headlights. I have solved that problem by fitting a new steering column switch (as many on this BB suggested). Now to avoid this problem in the future I am fitting some relays. I have figured out the wiring from the relays to the lighting loom but I'm not sure where to find a power source for the relays (or two sources - one for each relay). It obviously needs to be independent of the ignition switch but from where?

One suggestion is the solenoid. Are those terminals 'live' all the time? My solenoid does not have a spare terminal, so if that is a good source would it be OK to 'piggy back' off one of the existing terminals (there is a 'big one' and a 'small one')? Alternately, should I be looking for a different connection via the fuse box or somewhere else. Any advice?

As you will gather, electrics is not my greatest skill so I hope this query does not seem too stupid and the answer blindingly obvious!
Chris Hasluck

The solenoid is your best bet. I would use a ring terminal on the live post.
Dave O'Neill 2

Ive never thought about the selinoid as a power source...good call

Personally i like the idea of addjng an additional modern fuse box for extrs asscories like this...just remember to mark what is live full time and what is not
1 Paper

piggy-back onto the live side of the fuse box?
David Smith

Another alternative is the output terminal of the alternator. Or in fact any point that has a thick brown wire attached. The nearer to your relay point the better. And also use a thick brown wire - this will maintain the consistency of the colour coding. Have a look at the thickness of the wire from the alternator - this is the gauge you need to match. Secure the wire firmly with clips or fasten it with zip ties along the side of the main loom bundle going forwards along the inner wing. Avoid it contacting any edges - this is a permanently live cable that you do not want to damage the insulation on!

(I used the starter solenoid terminal)
Guy W

My live feed also comes from the solenoid live pole.
Greybeard

Thanks guys. I will go for the solenoid. Guy, I have a dynamo but I get the points you make.
Chris Hasluck

I added extra wires in the harness when i rebuilt and updated the fuse box with new feeds for headlights


mark heyworth

Yep, another one for the solenoid.
Malcolm Le Chevalier

Mark
Your new fuse box is obviously the right way to go but for an electrophobe like me I'll avoid that as too complicated.
Chris Hasluck

Why not take it off the control box into a new fuse box which can then feed the relays? The control box is likely to be very close to where you want to position a new fuse box and that's where the lights are originally powered from anyway.
And beware fuse ratings if you use modern fuses - the fuses are rated differently to the original Lucas glass fuses.
Graeme Williams

id be leary of using the mechanical control regularator box on a dynmo... that thing doent kick in till around 2300 to 2800 rpm

If its off the alt and not a dyno... then that would be cool

As to powering the 2nd fuze box... that depends on the box ideally if the box is easily split to run full time and 1/2 tike then thats the way to go....id say look around the salvqge yard ideally if youcan find one with built in relays then thats the way to go also

But powering will be dependant on the box its self...

I dont think it will be al that complex ... just mark each ciruit and device if its full time or part time powered...and thats the game right there its all gravy train after that

Ifyou get a reallybgood box fromnthe scrap yard you may end up wanting to swich everything over oncr youget going

Before you do anything... take all your grounds and bullets apart fora good cleaning and new coat of dielctric grease ... this will save you alot of greif later on

For extra ideas ... look at the (painless website) there an aftermarket automotive wiring site with great info... whats cool about there system all the wires are marked individully... ifnits the wipers it saysbwopers on the wire ever 9" or so

If you go that dirwction then id rig everything you can into 1 common ground easier to trace and clean

Just some thoughts not that imnanybkind of expert my cliamisim greatat setting fires just so you know of my electeical abilities

Prop


1 Paper

Just a little tip what ever you do make a circuit drawing in workshop manual for your or others future reference helps if you have a fault further down the line.
mark heyworth

Prop: what's the difference? The standard set up is for the lights to be connected to the A1 terminal on the control box, so I added a second wire to power the relays. The original still feeds the light switch but the dip switch now connects to switch the relays.

Mark: quite agree! A month down the line the memory of the details starts to fail. I redrew the complete wiring diagram in AutoCad and had to refer back to it for this thread!
Graeme Williams

Can I just check my calculations. I'm providing a separate power feed to each relay (main and dipped). On main beam the power is 2 x 60 watts = 120 watts (dipped will be less) so at 12 volts the supply will be 10 amps. So each feed wire needs to be rated 15 amps to be safe with a 15 amp fuse in line. Is that OK?

Graeme W - I don't understand you comment that modern fuses are rated differently to Lucas fuses. I have some 'modern' mini blade fuses rated 15 amps. Are they still OK?

Same old apology for dim question - I hate electrics!

Chris
Chris Hasluck

Hi guys,

Best place is off the back of the alternator if the car is so equipped.

Failing that, the battery side of the control box or the starter solenoid is good.

Remember try to keep those current runs short to save voltage drop.

Regards Steve
Steve Smith

But what about my calculations?
Chris Hasluck

Chris, yes I think your calculations are correct.

I use 15A blade fuses - or possibly they are just 10A ones - I forget. The fuses are there to protect the wiring, and especially to protect from a potential dead short which would blow either a 10 or a 15 anyway.

The old type glass fuses have a dual rating - a lower figure for a continuous current flow without overheating and reducing the voltage excessively, and a higher figure at which they blow. Modern ones are designed to minimise any voltage reduction right up to the current at which they blow.
Guy W

When I wired my headlight relays the aim was to reduce current through the various switches, and particularly the flasher stalk unit with its tiny and vulnerable contacts. By by-passing all of these with a heavier duty cable it avoids almost all of the voltage drop that standard systems are prone to.

I also wanted to add the relays as easily as possible with as little alteration to the existing wiring as possible. So I have one new heavier supply cable from the starter solenoid going to the relay position. This is as far forward within the engine bay as possible - in a housing box attached to the off side front splash guard panel. That is the only new supply wire. I unwrapped a section of the loom and cut and connected into the existing blue/white and blue/red wires using these to trigger to the relays. These draw negligible current so the existing electrics are completely safe.

Output from the relays goes back into those same wires going forwards to supply just the nearside lights, with two new wires going the shorter distance to the offside light. All 4 wires are then fused next to the relay (they are 10Amp ones).

There is one oddity that I have found, and for which I have no explanation. On pretty well every night journey, the first time I switch from dipped beam to main beam there is a momentary delay between the dipped going out and the main coming on. Very short but long enough to be slightly unnerving on a dark country road! The odd thing is that it only happens the first time - from then on operating the dip switch up or down is instant. Maybe the relays I have used (scrapyard) need energising for their first switch over. When I remember I do a quick main beam switch whilst still under street lights before leaving town.
Guy W

My understanding of fuses is this (from Rick Astley's book "Classic British Car Electrical Systems":
Modern fuses are rated at their operating current so a modern 15A fuse will run at 15A all day and only fail at around 100% excess.
The Lucas style glass fuses are rated at their failure current and need to be down rated by about a third to a half to arrive at their running current.
I understand that USA fuse ratings (particularly Bussmans) are different.

Please someone put me right if my interpretation of this is wrong!
Graeme Williams

When I went down this route, I went WAY over the top!! Having suffered with a smoking main beam switch, I was petrified that I was going to suffer a small fire etc.

So.... I fitted a 6 way fuse unit and two relays, each relay with two outputs.

I took a supply (from my solenoid) through two separate fuses, one to each relay (one for dipped, one for main beam).

Then... I took each relay output (so four in total, x2 dipped and x2 main) back through four more fuses before actually connecting to the headlights (80/100W halogens).

I also used brand new modern wire (in original colour code) for all of the above, only utilising original wiring for the switch outputs to the relays.

As I say, WAY over the top in terms of fuse protection but I always feel very safe :)
Glynn (1275RWA) Williams

Glen


I want ro see a pic of that set up...your my kind of guy

Graeme

Buy them by the fist full...your going to need them is my interpatation...in thw past few years these will blow just hitting a pothole or breath to heavily kn them

They arw a single filiment... and are rated at various amps...do they blow at the rated amps or before or after good is rhe quesrion....more of a random thing it seems


But i havent seen like you discribe

Prop
1 Paper

Chris you can supply all the relays from ONE large wire (40 amps capable) directly from the battery +ve through a modern circuit breaker (not a fuse) and use fused relays see pic.
But first you really need to renew all the conections to the headlights to get the best result.
Cheers and Merry Xmas to all
Rod





R W Bowers

Well - fitted the relays and wired everything up. Great, everything works well EXCEPT one of the relays is defective. I came to this conclusion by swapping the connections around, with one relay I could have dipped or main depending on the connections while the other relay produced nothing.

This is disappointing! I bought the relay kit from MGOC and, to be fair, they have offered a replacement straight away. However I'm beginning to think that nothing can be relied on to work these days (from a recent fridge purchase, a toaster and lots of other things!!).

Rod. You are right about renewing the connections. I can hardly believe that I had any lights in the first place as the connections were so dirty and corroded. I have supplied the relays with ONE cable from the solenoid rated at 27 amps splitting into two fused feeds rated at 15 amps. I hope that is enough. In any event, my wiring seems heavier duty than that supplied by MGOC although they claim it is rated at 25 amps.
Chris Hasluck

Pic attached for Prop as requested.
In the car, I was quite proud of my wiring - in the picture, it looks very amateurish at best :(
But, it works, and I know I'm way better protected than before. No more smoking mainbeam switch :)


Glynn (1275RWA) Williams

This thread was discussed between 09/12/2016 and 18/12/2016

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