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MG Midget and Sprite Technical - Alternator or not?

I've just converted my MKII midget to negative earth and just about to order a Simonbbc electronic ignition and dissy.

I will now refit the cigarette lighter as I managed to toast an iPod when it was wired in before the conversion!

Would I need to convert to an alternator if I wanted to fit halogen headlights or would they be ok with the dynamo?


Tony Brough

It's fine with the dynamo. I have had H4s in my A for ten years without any other electrical changes
dominic clancy

I can't see why you'd need an alternator for the extra wattage on the headlight conversion - or the need to fit relays on this conversion unless your switches and wiring are old and not in the best of health

of course you're always best with a good battery in good condition and kept well charged with all the main connections, leads and earths clean, secure and protected

if you are going to put a lot of other extra electrical load on the car and use it at night or during rain then you might want/need an alternator

if you're buying a Powerspark kit including HT leads, etc. do consider how many parts you're getting for such little cost, something usually gives, I understand that the HT leads are not the best made

ETA: I was typing as Dominic posted, so there's proof you shouldn't need to

and that you don't have to have relays for H4 conversions (subject to condition of wiring and switches)
Nigel Atkins

Id go the alt over dyno

If your doing hologens then your the type of person to do other elctrical additions

I would do relays with hologens... as any long drive time at night can cook some then zorry

I like modern single wire alts with built in electronic regulators...much easier

Prop and the Blackhole Midget

Thanks for the replies.

I don't intend driving much at night and I can't think of anything else electrical that I would want to add, so I will stick with the dynamo for now.

The kit I'm looking at is the dissy, red rotor and coil for about £61 which I appreciate is cheap but at least I will be able to have a go at rebuilding the original dissy and still have the car on the road :-)

Tony Brough

good call

I was thinking of anything power hungry or constant perhaps plugged into a lighter socket or three but if not then yes still with the alternator (and good battery and connections regime) and you'll find out for yourself if you want or need an alternator

with the electronic dissy kit fitted you might find you never get bother to get around to rebuilding the original

let us know what you think of the dissy kit

you may also be able to widen the gap on your plugs, don't keep old plugs put fresh ones in to feel the benefit more
Nigel Atkins

OK current draw thru the switch is:
Headlights (110 watts H4 Low beam) 9.5 amps (Hi is 120)
tail number and parking (5X15 = 75) 6.5 amps
total 16 amps coupled with old dodgy resistive contacts = heat and subsequent failure.
Fit relays and an alternator
PS at switch on you can X by 1.5 for a millisecond or so
R W Bowers

I fitted 123 ignition. Changed the headlamps to H4 for more visebelity at night. Used relays as well (which really improved the light!) Still had problems to keep the battery charged after long drives with the lights on. Changed the dynamo twice, but converted to alternator in the end and had no problems anymore. (Had to convert the rev counter to electronic as well)
J.W. Vlaanderen


Id definatly go to relays... they are alot cheaper then the head light switch... and the new repoduction switches are considard dodgy at best

Something else to consider, the alt starts charging at around 600 to 800 rpm... basically idle the dyno dosnt kick in till round 1800 to 2200 rpm

So you will need balance that out with your night time driving road course.... if its slow and windy, all night long may want to consider packing an extra full charged batt in the boot just in case

But definatly use relays as a minume to protect your switches...ive seen photos of them melting before, its not pretty

Prop and the Blackhole Midget


I fitted Simon BBC dizzy and electronic ignition three years ago and touch wood it's been fine. I also put on an alternator which I feel was worth doing as it has much more in hand than the dynamo. Nothing fancy, just standard issue from Sussex. The standard tacho kept on working just fine.
Remember that the wiring from the alternator into the system ought to be uprated (in my opinion anyway) as the alternator produces more amps.
Aparently you should also take steps to protect the alternator if you use a master-switch which might be operated with the engine running.
G Williams

I can't remember what wattage "ordinary" headlamp bulbs are (can't be a*sed to look it up) but if they are around the 50 - 55 watts each, then they will put no extra load on than halogens. Halogens just give more light per watt, they don't use more watts!
Dynamo is fine unless you have lots of very short journeys with lots of electrics switched on.
Dominic Excell

Mk2 has lower wattage headlights IIRC so halogen being higher wattage will put a bit extra load, but not a lot

so as I put before >>or the need to fit relays on this conversion unless your switches and wiring are old and not in the best of health<<


>>you don't have to have relays for H4 conversions (subject to condition of wiring and switches)<<

but if you want relays have them, seems more wires and boxes to me

I've had H4 conversion without relays for over 4 years now with a very loose column switch, I keep waiting for the switch to break or fail as I have its replacement in the shed but even at a charity cheque presentation where a young disabled lad repeatedly treated my column stalk like an old stiff column gear change lever and loved putting the stalk switch through so much that he insisted on returning to my car for a further three times, even after that still no smoke or failure of lights or switches and the stalk switch remained interacted, as much as it was before, and still works
Nigel Atkins

IIRC original sealed beams were 60/45w, whereas standard H4 bulbs are 60/55w.
Dave O'Neill 2

Just driven back from the classic car do at the Coopers Arms, I had xenon/halogen headlamps on, heater on, dashboard lights on.

Main beam also switches my 7 inch replica Lucas spots with halogen bulbs in and the dynamo seemed to do ok.

Malc Gilliver


Car battery flat this morning , so scratch my previous rambling…..

Malc Gilliver

I was think of those bulbs on the Mk2 which I see are 45/40w, until someone pointed it out to me on here I didn't realise they were that low

was your battery reasonable well charged before, perhaps you need the bobbins adjusting in the control box to get full charge through (I think that's right but been many years since I had to play around with such things but I know there's a John Twist video about it)

heater on? blower maybe but heater, really, at this time of year?
Nigel Atkins

I did adjust the RB340 regulator on my last Midget, as it didn't do much mileage and mainly short trips.

ISTR that the WSM advised increasing the output for short trips.
Dave O'Neill 2

Has anyone fitted the Accuspark alternator? It's like a Dynalite, packaging an alternator in a dynamo body, but its a great deal cheaper.
dominic clancy

Mark: I bet if you add up all the amps you were using it will well exceed the output of the dynamo (19 amps output?). You are then depending on the battery to keep it all going which is only around 45 amp if in good nick. Could be a bit like p**sing in the wind!

I have an ammeter fitted and it is quite an eye opener how much the needle initially swings into the "-" when something like the cooling fan goes on.
Graeme W

I had side lights on, dashboard lights, dip beam - occasional main beam = 4 x 55w halogens, and heater blower.

so 5+5+5+5 = 20w on the dash, 5+5+5+5 for the side lights gives another 20w - dip is 110w

so 150w/12v = 12.5amp plus motor

when on main beam it's 260w / 12v = 21 amp plus motor.

I don't know what the coil takes out.


The blower was on because 70mph with the roof down at 11:00pm with just a fleece is tad chilly.


Malc Gilliver

dash lights aren't 5w (are they on earlier cars?) if only they were, they're 2.2w IIRC on my late 1275

you need to add 42w for occasional brake lights and 42w for occasional indicators (plus warning lamp for them 2.2w?) - number plate lights 10w, mainbeam warning light - perhaps the use of use of windscreen wipers, radio, cassette, ancillaries/accessories (courtesy and boot light?), starter motor

all adds up

if you have the fresh air flap fully open and blower off you'll get more heated air, certainly with the roof down, roof up you wouldn't need the blower on either - the blower is to compensate for the lack of air passing over the matrix at lower speeds (about 30mph or less) - try it next time and see, fresh air intake flap fully open blower off

it must be a lot colder in Staffordshire than Northants or your heater matrix and heating and cooling system isn't as clean and well function as mine as my wife, coldest mortal in Britain who doesn't decide whether to wear a coat but how many (seriously) won't often have the footwell flap open even in winter if the heater tap is open and very rarely. when asked by me, wants the heater tap to be reopened once I've closed it for the warmer weather seasons
Nigel Atkins


70mph is the legal speed limit……

I didn't check the dash lights, so thanks for the correction, as to all these new fangled radio, cassette, ancillary lights, boot lights etc - not fitted

I'll have a look at the heater routing as the fan motor on mine makes a massive difference - so maybe I have a blockage.

I just had a quick look at heater installs my Sprite has the old box type heater which looks different to the later one so maybe that contributes - but I'll have look inside first.

Malc Gilliver

I meant you don't need the blower on at about less that 30, perhaps 35-40 if you want - surely you'd only be doing 70 on dual and motorways and what Spridget owner wants to be on them

check that the long fresh air trunk is fully seated to the panel at the front (side of grille) and not resting before it between it and the other panel with aperture as this means it's not sealed and probably missing out on full air flow (or the trunking might go to the fresh air flap attachment at the front so check it's secured properly there)

also check for blockages or partial blockages at the entrance of the trunk, along its passage and at the end

on the fresh air aperture on my heater box is a mesh grill and it's surprising how a couple of big dried out leaves can be pressed against it having got into the trunking

also see if the fresh air flap (valve, butterfly) is fully open and not closed or only partially open when the heater switch pull is pushed in to the dash

apart from having the matrix clean inside and out the presence, position and condition of the foam seal that wraps around the matrix inside the heater box is important as is the presence and condition of the seal between the heater box and car body

as you have the separate blower to heater box you also need to check the trunking there for blockage or debris and that the trunking is secure both ends and without hidden splits or holes

I also found that although my blower worked well it was wired the wrong way round, not a big difference but noticeable

you've probably seen me bang on about my simple thorough coolant/heating systems cleaning so I won't repeat here
Nigel Atkins

Alternator or not

Found this site

In summary a good Lucas C40 will do 22 amps. It also make the point that the more power you take out, the more fuel you use.

Malc Gilliver

Very true.

You don't get owt for nowt, as they say.
Dave O'Neill 2

This thread was discussed between 01/06/2014 and 08/06/2014

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