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MG Midget and Sprite Technical - Have I Blown my Control Box?

On my first test run after changing the radiator, water pump and various other bits, my ignition light did not go out.

I had had the dynamo off while I did the plumbing and when I tested that as per the Haynes manual I couldn't get a voltage reading. So that is suspect number one. Fortunately I have an old one that seems to be OK so I'm hoping that will solve the problem.

However, while I was working on the cooling system I managed to drop a spanner across the fuse box - It took me a moment or two to work out why the fuel pump had suddenly started running! (I know, I know, I should have disconnected the battery first - I will do next time I promise!)I didn't see which contacts had been bridged as I whipped the spanner off as quick as I could. Could this have done something nasty to the control box - and how would I tell?

Incidentally, if I have read the Haynes dynamo testing instructions correctly, I disconnected the electrical leads, connected the two terminals together and then, with the engine ticking over, took a voltage reading between the connection and earth. There was no voltage. When I tried the same arrangement on the spare by driving it at below tickover speed with my electric drill I got a low voltage. One method for testing dynamos that I found on Youtube involved putting a voltage in through the linked terminals to make it 'motor' - mine motored nicely, which is why I think it should be OK.

Hoping someone can put my mind at rest before I go splashing out on a new control box!

Cheers

Colin
Colin Mee

Hi,


Simple test for a dynamo is as follows.......


Detach the wire from the D terminal (thick brown/yellow) at the control box, remove the wire from the F terminal(brown/green).

Link these wires with a short piece of wire.

Get a 12v test lamp, connect one side to earth and the other to your linked wire.


Start the vehicle and let it idle, no need to rev it up.


The light should be nice and bright as you're running the dynamo unregulated, they can go up to 70/80 volts if you rev them! All you'll do is blow the bulb in the test lamp.


If the dynamo passes that test it's ok.


Check the earth at the control box and the battery lives at A/A1. If all seem ok the control box is probably knackered.


Time for an alternator conversion?



SR Smith 1

Thanks SR.

That's basically the test I did on the two dynamos except with a multimeter, so that suggests it's the dynamo.

I will take a good look at the Control box connections as soon as I can once the obligatory Easter visitations are completed!!

C
Colin Mee

Is the dynamo earthing to the mounting bracket and water pump OK?
Dave O'Neill 2

Did you remember to flash ( reenergize) the dyno...

I cant remember the procedure... but on generators they have to be delt with or wont work

Prop
Prop and the Blackhole Midget

Here colin,

I did a quick google (how to re energize a lucas generator) and this was the 1st of several 1000 responses...also called polarizing

Hope this helps

http://www.britishcarweek.org/polarizing.html

Prop
Prop and the Blackhole Midget

The answer to my original question is: No!

After replacing the control box with a new one, and swapping the dynamo for an old one that I had on the shelf, it ran fine for a couple of days then the light came on again.

I have just had the original dynamo rebuilt and all seems to be O.K. now. So far I've only run it briefly in the garage but the ignition light has gone out readily enough each time I've tried it.

I took the dynamo to R. Jones of Snailbeach, which is in the back of beyond and was, in days gone by, the centre of Shropshire's lead mining industry. Telephone 01743 791428. I don't think email has reached that part of Shropshire yet!

He was a bit surprised when he tested it as the dynamo 'motored' when he applied a voltage, which is usually a sign that it it O.K. but it wouldn't produce any voltage when asked to.

He also tested and passed the original control box, so now I've got a good spare! He charged £60 for the work which included new brushes, new brush springs and new rear bearing, the front one was O.K. Dropped off on Wednesday and picked up on Friday, though it was ready Thursday P.M.

Although most of his work is with alternators, starter motors etc. he says he rebuilds dynamos from all over the country for folk who want to retain their original equipment instead of replacing with cheap foreign imports!

Colin Mee

Good job

Im glad it worked out for you... yepp, every town needs an electric motor dude

Prop
Prop and the Blackhole Midget

This thread was discussed between 04/04/2015 and 03/05/2015

MG Midget and Sprite Technical index

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