Alternator control light

[mirafiori76]

In all those years of being a mechanic i have never encountered the following problem:

I have an alternator control light that switches on and off everytime it feels like it. No matter the surcomtances it lits up and off. Sometimes not for days but sometimes every few minutes.
Anyone ever had this appeirance? Is it a failing diode bridge?

[Thotos]

Is it a Bosch alternator? If so, then the problem is with the regulator. If it's a Magnetti Marelli unit then the problem is with the triode or the triode connections.

[mirafiori76]

I can't tell. This one has no markings whatsoever. But i guess it's better to change it before it really breaks down.

[Thotos]

Bosch units have an all-in-one regulator and brush holder at the back. It'll look something like this:



or might be just a black box. If you have anything like that on the back of the alternator, change it (?10 to ?20) and it'll most probably be fine.

Magnetti Marelli alternators have a separate brush holder and regulator and have a triode within the alternator to control the charging light. The triode is soldered but if the joint is broken and sometimes touches it'll cause the symptoms you described.

The Magneti Marelli regulator will probably look like this:



But it won't be your problem, the problem will be inside the alternator.

Of course if the charging is erratic as well and it's not just the light, you should change the carbon brushes.

[mirafiori76]

Hi Theo,

Thanks. It looks like there is a Marelli mounted. Anyway, i have some alternators in stock so i think it's easier and cheaper to exchange the whole thing. Will take me probably 10 minutes.
Still i think it's strange how the light lits up every now and then.

[Thotos]

I've had the same problem before with a Magneti Marelli alternator and it was the triode connection broken. The triode is pennies to buy but a bit of a pain to change as you have to strip the alternator and you need a very hefty soldering iron to solder it on the rectifier pack. Much easier to change the alternator but I can't do that in 10 minutes, it would take me at least half an hour. I'm getting much slower as I get older.... 

[mirafiori76]

Before raplacing the alternator i decided to check the voltage first. It delivers exactly 14 volts. Switching on all great users it drops down to 12.9 which is still great. Therefor it's not broken at all.
So why does the controll light sometimes say it is broken? And even more strange, why does it only burn half?

I guess there's something else going on. Anyone have a clue?

[fi131]

I had almost the same problem in a 63305100 Marelli unit .The generator light was flickering . The voltage measured gave the same indications with yours .As soon as i changed the whole diode called rectifier  ( it comes as a separate part from different aftermarket suppliers ) the flickering stopped.
The silver unit in the previous photos  with the green & red cable is the regulator not the rectifier .The only problem is that you have to move the generator to install it.