Dim alt/gen light at high rpm

Hi,

1600 type 2, 1972, alternator.

As the title says, when I hit higher rpm (55 mph) the alternator/generator light starts to flicker on and is very dim. A friend suggested this might mean the alternator is reaching the end of its life. Here are some questions I hope you guys can help with, feel free to add any nuggets of wisdom.

1. Do alternators have replaceable parts? Brushes, bearings, and how easy are they to change?

2. I've read replacement parts are hard to come by, as its easier and more profitable for suppliers to sell entire units. Thoughts?

3. Do I need to drop the engine to swap out an alternator?

4. Of course it could be an issue with the wiring or dash, there are issues in that department (e.g. jittery fuel needle, interm

 

Check charging rate , should be 13.5 to 15 volts . Ideally check output at the back of the alternator but checking at the battery is okay .Just replace with good quality replacement unit such as Bosch .

 

Most likely is the brushes are worn, the light flickers as the power to the rotor starts to weaken, so the output of the alternator drops a little.
If it is a stock alternator, there are three screws surrounding the three pole connector on top of the alternator body. Undo these and lift out the assembly.

There will be two carbon brushes spring loaded, they begin life long enough to spring in and out about 15mm or so AFAIR. By the end of life they may be canted over and about to be pushed out of the square tubes - mine were.

If you measure the brushes , they are e.g. 5mm x 7mm by a certain length (measure the length of the rectangular slotted brush holders and allow for the spring at the top), cant remember it was years back) Go on eBay and buy a pair that are the same size . If they are too long, the copper/carbon structure is fairly easy to sand shorter..

You will have to solder in the new brushes, keep the sleeving from the old brushes, copy the installation (take pictures) ..

This will cost you about £10.

 

It could be the other type that does not have the 3pin connector on the top and has a built in regulator rather than the wires going to a box on the firewall or even a dynamo

If you can put up a picture it would help

 

As a starting point, measure the battery voltage with the engine running - as above, should be around 14V.

If you've got an alt with the built-in regulator, worth changing that anyway. Easy half-hour job.

 

The non Bosch aftermarket alternators can hide the regulator inside the alternator body where it's a lot more work to replace it. Like take the whole thing out and dismantle it..

But I would start with checking the brushes as it wont make it to the expected voltage under load even if the regulator is good. In that case the DF voltage will be high if it has an external regulator... have a look at Ratwell.com for alternator checks.

 

But if you've got a late original Bosch alt, the reg will be sitting there nicely, waiting to be replaced.

 

Yes but why bother if it's just the brushes.

 

Because on the later regs, the reg and the brushes are one unit.

 

Thanks for your replies.

So I'm now not sure if I have an alternator or generator. @matty here is a photo. How can I tell? The voltages coming off it are: 14.2 off the larger (red wire) connection and 7,2 off the smaller (green wire) when running on idle.

If I remember correctly, there is, what I assume to be a regulator behind the air filter, and this would suggest a generator.

@mikedjames there is no assembly on the top which looks like it gives access to the brushes. Perhaps this means I will need to take it apart - or maybe best to swap to an alternator?

Thanks again

 

It's a dynamo.

 

Thanks @Zed. Do the same points apply? Can I change the brushes of a dynamo?

 

I'm sure you can. The only dynamo I've had self destructed beyond repair and I replaced it with an alternator.
Alternators are better suited to vehicles with more power demands like we have today. Leisure batteries to charge, more power hungry ICE etc.
But obviously yours has served you fine so... sure, fixing it in situ will be much easier than doing an alternator conversion.

 

Yours seems to have lost its brush covers anyway. Basically the brushes are under the plastic covers over the slots in the casing.

The light coming on dimly is more likely to be the regulator as these if original have electrical contacts inside that wear out.

But heres a summary of changing the brushes (also check out John Muirs book..) https://www.reluctantmechanic.com/step-by-step/change_generator_brushes.php