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MG Midget and Sprite Technical - Dynamo to alt control box conversion

Long time no speak.

I bought all the bits to swap from a dynamo to alternator many years ago, including a spare control box thing in order to keep the existing loom plugs intact.
In the meantime the link I saved of a webpage that showed the necessary connections has vanished, and hunting through the archive here has not enlightened me much.

Today I gutted the box and made the connections I believe to be required.
Yes all the through bolts are insulated from the metal plate, and I've tested for continuity just in case.

So basically have I got it right? Having just got back into fiddling with my car I really don't want to set it alight!


And having just looked at the pictures, no, I have no idea why I decided to solder that one joint where a tab existed rather than drill for another bolt????






G Hawkins

Basically, for the battery to charge, the larger terminal on the alternator should now connect directly to the battery and the smaller terminal should connect directly to the ignition light.
Bill B

I also disable (cut through) the (resistor?) link across the back. From memory, without one with me to study, they link to the coils. They certainly serve no purpose when the unit is being used just as an anchor point to connect the various cables to the alternator.

But from the look of it you have removed the coils?
Paul Walbran

Spridget should have an RB106 Lucas control box.

Gut it.
Join A1 to A to D………..I have a 30 amp inline fuse fitted.
Move the charge warning wire from D to F




Alan Anstead

Yes it's completely gutted apart from the (slightly reconfigured) spades and the one resistor thing which attaches to nothing.
G Hawkins

Here is my take on alternator conversion (MK1 midget 1962)

New loom, no changes to it.

https://www.metricrat.co.uk/mg-midget/mg-midget-alternator-conversion
Tim Carter

“ Spridget should have an RB106 Lucas control box.”

Later 1275s had an RB340.
Dave O'Neill 2

Tim your page was one I came across. Nice simple instructions.

But yes I do have the RB340 control box as Dave points out.

I have finally come across another image of this type converted for alternator connections. Would appear mine is correct.

Three big spades join together and likewise the two small ones.

Now to put mine all together and find out what length belt I need. Another area where people experience differs.


G Hawkins

I've kept mine simple too. 🤣🤣🤣

At least 40 years since converting.


anamnesis

Yep, why make things complicated!


GuyW

Not dissimilar to my loom, which was supplied by Autosparks.


Dave O'Neill 2

Apologies re error on later cars.
I retained the RB106 on both of my cars to maintain the ‘look’ under the bonnet. I run Dynalite alternators that look like dynamos but are actually alternators. The deceit continues with a false Revcounter drive on one that actually has an electronic revcounter.


Alan Anstead

I'd love a Dynalite alternator, but can't run to it. I need four tyres!
G Hawkins

The Dynalites I use are from Bad Boy Classics not the mega expensive ones. A friend trialled one for a couple of years on some of my tours proving their reliability before I purchased mine.
I also found an article from the States where someone had stripped one and commented on the quality of its internals.
BBC usually have a stand at NEC Classic Show showcasing their wares
Alan Anstead

Never really seen the point of these 'Dynalternators' other than perhaps for near concours original cars?
GuyW

Nostalgia Guy.

The aesthetics of yore and function of the moment.

Ain't we all a bit like that?

anamnesis

Yes, I get that. But when much of the other visible stuff is already changed then what's the aversion to an alternator that looks like one. They aren't that far out of period date as it is.
Not having a go at anyone's personal choice. It's just that I don't get it!
GuyW

True. And I'm no 'purist' either.

Just remembering why I changed to an Alt'.

Pretty sure it was because my rb340 was acting up, and my alt' conversion was free; courtesy of a mini dumped in a road near to where I lived. Lol

Bugger originality, when more 'modern' replacements were free. 🤣.



anamnesis

Is my ‘Healey’ rocker box cover a step too far?


Alan Anstead

That's nice. Where did you get that Alan?
anamnesis

It is fake just like the Dynamater, the gutted RB106 Regulator, and electronic ignition hidden in the Dizzy. Even the Revcounter drive terminates out of sight under the heater snail as the Tacho is electronic.
There are apparently real Healey covers but mega rare so rare that with little chance of getting one I made my own.
The cover was gifted to me-one of many parting gifts received, when Masc Area Rep, when a member left.
I ground out the recess in the fluting; copied and cut the signature into a bit of ally and superglued it in.
Alan Anstead

Brilliant.

There could be a market for those too Alan. Maybe you should churn a few out. 😉 👍.

Assuming you could find a 'cheap' supply to modify.

anamnesis

Getting back to alternators, I've never seen the need to change from a dynamo on older Spridgets.

Trev
T Mason

I don't have the time. The 1275 release bearings are still popular and our Spridgeteers Group is quite active, at home and abroad, having taken over where Kent Area Midget & Sprite Club finished, so with other things I am well occupied.





Alan Anstead

That reminds me Alan, I must contact you about one of your release bearings.
G Hawkins

G.Hawkins, Wiltshire
alan dot anstead at btopenworld dot com.
Alan Anstead

Alan Anstead, Kent, United Kingdom

Is my ‘Healey’ rocker box cover a step too far?




It is when your running a Midget :)))
A de Best

I agree Trev. My Lucas C39 seems fine to me and I don't have to change the rev counter!
Bob Beaumont

Nothing intrinsically wrong with dynamos (J. Davis would agree I'm sure) - they were used successfully for a long time.

But technology moves on. If you're happy dealing with a failing or out of adjustment Lucas regulator (or you're going for originality or even concours) then fine. The 1275 engine I bought for the Frog came with a Lucas ACR alternator so the temptation was too great to use it. Fitted a new internal electronic regulator and bought a spare rectifier pack and it works fine. They have more output so charge the battery more quickly. I didnt have the tach drive or the mechanical tach anyway, so I made a decision, as you do. Just like fitting front discs etc.
Bill B

Bill, I'm not sure your comment about them charging quicker is correct. I've never checked but seem to remember reading somewhere years ago that an alternator charges at a higher rate at tick over or low revs but a dynamo charges more at normal driving or higher revs.

Trev
T Mason

Trev
I think that, if the regulator is set correctly, the dynamo output at higher revs should be very similar to alternator output I.e. about 14v. If that is the case they will both. theoretically, charge at the same rate. However, the dynamo commutator (where the charging current comes from) could have losses at higher load so its output would reduce. It may be that at very high revs the dynamo output voltage rises more and, potentially, charges more quickly for the period of high revs. But the commutator losses may increase as well. Alternator charge current comes from the fixed stator. so no losses.

An alternator has more output at lower revs so I think that, overall, the alternator will charge more quickly.
Bill B

https://mossmotors.com/media/instructions/130-068.pdf

The page I'm currently (pun intended) working with for to do the job.
Richard Reeves

You're a bright spark. 😉
anamnesis

Is an electrician known as a sparky in the US, as in the UK?
Dave O'Neill 2

No David. Lower volts in USA so they are called Fizzys.They just get a slight fizz, not a spark
GuyW

Just finished wrapping the new loom today.
I ditched the VR box.
Some old bits flagged for later attention, some new bits added to the fusebox.


Richard Reeves

Tidy. 👍.
anamnesis

Anyone that doesn't run 160W or even 130W main beam headlights may well not see a need to convert to an alternator (not sure about 100W).

A like what I call 'period correct' modifications though have a modern alternator on my car's engine.

When I first saw the 'Healey' rocker cover I was surprised because the only time I saw one for an A-series engine it certainly wasn't as good as Alan's. My spare rocker cover is a Janspeed. However, someone once told me that the Mini Spares, Janspeed etc rocker covers were all the same casting except for an insert that was swapped for the name of the company that was to appear on the casting: Janspeed, Mini Spares etc. So it seems even Alan's approach is 'period correct'.
Daniel

This thread was discussed between 15/05/2025 and 08/07/2025

MG Midget and Sprite Technical index

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