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Dynamo issue


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Hi.  I seem to have developed a potentially dangerous problem with my dynamo that I can't explain. 

I've had some problems with starting and running recently, which I initially thought was fuel delivery related, but this afternoon I noticed an odd (not fuel) smell in the engine bay.

The dynamo was hot and smoking lightly, so I immediately stopped the engine.  It continued to make a bit of a crackling sound and produce smoke (had the fire extinguisher on standby at this point) before I realised I'd left the ignition on.  I turned that off, and disconnected battery.

After it cooled, I removed the dynamo and replaced it with an old one I had on hand.  I only got as far as turning the ignition on before a little bit of smoke started coming out.  Again, I disconnected the battery and left it to cool. 

To me, it seems like the battery might be somehow overloading the alternator in some way?

Dynamo is connected the only way it can, with large spade connector (brown/yellow) to large 'D' connector, and smaller one (brown/green) to smaller 'F' connection. 

In terms of changes I have made recently which could have affected things, I've done the following;

- Replaced coil with Bosch one about a week ago. Wired connected as per old one. 

-Replaced control box (after testing this and dynamo using method described in WSM) about a month ago.  Wired up as per old control box (see pictures)

The only other thing I've done is charge the battery (off the vehicle) which was showing 3/4 charge when I put it back on. 

It's very odd as I've had the car running fine and well with the wiring in this configuration (which i believe is correct) for a while, and this seems like a new fault.  I haven't made any significant changes to the wiring or anything like that. 

Does anybody have any ideas? It's on a negative earth mk3 Spitfire 1300.





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I think the issue is with the control box, despite your statement of work done.

Having replaced the dynamo with another unit negates that component.

Additionally your wiring to the control box terminals, especially far right, needs a little TLC to say the least.

Control boxes do require delicate adjustment and are very sensitive to incorrect settings which can lead to charging issues...……………………. such as smoke !!

I'd go back to the drawing board and get another control box or certainly check the one you have to ensure all is good.

Wiring connections play a big factor, so you need to ensure yours are up to spec.

Good luck.


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Thanks Richard, 

To be honest, I did not know there was any adjustment needed with the control box. I just bought a new one from Rimmers, bolted it straight on, and connected it up in the same way as the old one.  I did clean the connectors up with a wire brush, but that's it.  I'll consult the WSM and see if any adjustments are required. 

I believe all the wiring is original.  I can't see any breaks, but it's possible one has occurred causing a short somewhere within the loom. I might try rewiring the connections from the control box to see if that solves things if it turns out not to be the new control box itself.


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The control box will have bobbins and these need to be gapped correctly, I have attached a very informative document (thanks MG Club) which may assist you when read in conjunction with your own WSM.

I do not know what CB type you have, but hopefully it will be covered via the attachment.




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Having read that article and looked at those tests, I think you are right, it's the control box (or at least the settings). I have the RB340 cb, or part number GEU6605.

I think it is the 'cut in' operating too soon, and the battery is then discharging through the dynamo which I assume is causing the heat/smoking.

I hadn't noticed this happening before as the battery I had on there was flat (has been since before I replaced the control box) until I charged it over the weekend. So now that there is some charge in there to discharge through the dynamo, the fault/incorrect setting has revealed itself! 

I will investigate tomorrow to confirm.

Once again, many thanks for your help. Would have taken me forever to figure out.



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What 7s the addition piggy backed wire on the end terminal ???

I have with tim hunt CT spent ages checking settings and readings on his tr4a rb340  with all the correct results 

It blew all headlamp bulbs on a test run, in the trade these were always  considered un twiddleable

And best replaced even in my warranty days testing these on our Octopus  alternator dynamo test machine proved unreliable results we never bothered with them ..    notorious orrible fussy bits of kit , best left to Lucas


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I agree Pete, factory set & fitted - best left at that and purchase new if an issue.

The red taped wire far right did cross my mind as to its function, perhaps citybreeze will be able to confirm.

All the best.


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I have ordered a replacement control box from Canleys, for less than I paid for the one from Rimmers.  A little annoyed that it's failed after only a month, during which time the car has been used only about 5 times for short journeys.  I'll speak to them about sending it back.

The wires going into connector B on the right, with the piggyback connector, are how it was connected when I bought the car.  The wire with red insulation tape over it is brown underneath that, though I can't remember what it's connected to. I will investigate this evening to see what it does.




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Keep us posted how you get on with Rimmers ,  - not related but ive just had a Dynamator (alternator ) fail after 1 month though id did have it a year before fitting - I contacted Accuspark and as it was more than a year I paid for postage to send it back and 3 days later a replacement arrived 


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Your control box contacts may be seized closed in which case the current from the battery is reversing when the engine is turned off and the battery now drives the dynamo, instead of the other way round, and the strain of fighting against the resistance from the fan belt is causing overheating since it can't spin freely. If your replacement control box is old stock it may have seized through lack of use; a good clean of the coils and contacts may get it working.

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