Jump to content

Dynamo Performance

Recommended Posts

Recently, the battery in my GT6 (Mk1, 1967) gave me some trouble. It ran empty twice soon after I was stuck in slow traffic, in the rain, and at night. Non-ideal conditions for an old dynamo, but I think it should in fact still perform well enough in this setting, at least as long as I don't feel like pushing my luck and turn the heater fan on! So I hooked up the multi-meter and drove around with it on the passenger seat for a couple of days: On normal/good days (no rain, no headlights) it does pretty well, at idle she sits somewhere around 12.5V and the voltage quickly rises to over 15V when I drive off, the cut-off should be set somewhere around 15.5V and I've not seen her go past that yet. However, on rainy days the performance is very different: once warmed up she idles at around 11.5V and she never reaches the cut-off. I took her out on the motorway today just to test and I couldn't get her past 13.2V even at ~4000 rpm. That might just be enough to charge the battery a little, but it's clearly too low.


Is it normal for a dynamo to perform worse in the wet? I can't really see a reason for why that should be, with the possible exception of the belt slipping, but I suppose that should show some audible symptoms as well then. I figure this one's probably more expensive than swapping out the dynamo brushes, right?


Link to comment
Share on other sites

this could be battery related and if some plates are failing , colder temperature reduces its capacity even further.


dynamo brushes are easy to change , do not meddle with the regulator, Ive spent hours with good equipment playing with some of these to no avail

generally ends in disaster charging .


I would take the battery to have it discharge checked first


then have a look at the brushes, or fit a easy swap alternator  they are quite cheap and many fit direct with a little thought and a new wire 



Link to comment
Share on other sites

I did the swap on my  68 Mk1 this year - Pretty easy swap out although if you get a kit, the top bracket you get will be useless as was the fan belt supplied  - just adjust the old top bracket and buy the right length belt - mine needed a 1500 mm one


Anyway -  even though my dynamo was OK, my lights are brighter and I can now fit other electrical ancillaries if and when I want to without any concern 

Link to comment
Share on other sites


 I assume you are measuring voltage at the battery.

" at idle she sits somewhere around 12.5V" This is too low to charge the battery and will result in a low capacity battery.

"the voltage quickly rises to over 15V when I drive off" This suggests that your battery is low capacity.

"once warmed up she idles at around 11.5V" As above, too low and not charging.

"I couldn't get her past 13.2V even at ~4000 rpm." That voltage will not charge very good and will cause battery to have a low charge.


Sorry not much help. Buy a new battery and ensure that there is a minimum of 13.8v on it while the engine is running.

I suspect your voltage readings are not off the battery, as amongst other things, you can still start you car.


Alternator fitment is a good move.




Link to comment
Share on other sites

Batteries loose their capacity with lower temperature, dramatically, below freezing. However, as the weather is unseasonably warm at the moment I wouldn't buy a new battery yet. I would change the dynamo for an alternator. Even if this doesn't cure the problem it's a well worth while upgrade and you've then got something you know works.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...