Hi There, I have a Fiat 500F (1970) with a 126 652cc engine and Magneti Marelli alternator. The alternator was reconditioned unit fitted when I rebuilt the engine.

All has been fine for the first 1500 miles but yesterday the battery started emitting a rotten egg smell and the battery became quite hot,. I understand that this could be a dying battery or the alternator is suppling too much voltage.

To be safe I have replaced the battery (Yuasa) and when I measure the voltage from the alternator with the engine idling it reads 14.85 volts which seems too high but I have seen somewhere on a forum that a 126 alternator should give between 14.4 and 14.8 volts? HOWEVER my regular and trusted mechanic says that modern batteries should only see between 13.5 and 14.3 volts, any more and things will go wrong!

I am trying to find out what the correct voltage from the Magneti Marelli alternator should be (as its a modern battery I am assuming it should be the lower range 13.5 and 14.3 volts)

My Scenarios are;
A. 14.85 volts is correct and the battery has just died.
B. 14.85 volts is too high and has fried the battery (voltage regulator died)

Im not sure if the voltage regulator can be easily adjusted to lower the charging voltage? It may be just bust but before I get a replacement voltage regulator I thought id ask the people who know... ie you guys and girls!

any ideas gratefully recieved!
Many thanks.

Hi, I'm sure the experts will be along shortly with a more accurate answer. My car has a dynamo and I had 15.5V at the battery when on rpm. One dead battery later. . . .Replaced the mechanical voltage regulator with an electronic one. Sorted.

Rob

Hi Thanks for that, I have ordered a new voltage regulator and will measure the alternator voltage after installing it then, finger crossed!

I would expect 13.8v certainly not 14.8v.

Good call on the regulator.

D

As far as I know it should be 14.4 volt (2.4volt per cell) to be perfect. Below it will not get funny charges and above it will kill the battery with time. 14.8 at idle sounds a lot. Is that on the battery or the alternator?

You dont use the voltage regulator with an alternator. Your alternator should have its own regulator bolted to the side. I stripped out the internals of my voltage regulator and used it just as a junction box. If you are still using the voltage regulator this is probably why you are having issues

Hi thank you everyone for your replies ! I have just replaced the what I thought was the voltage regulator (sorry Paolo! im not too hot on these things) mounted to the magnet Marelli alternator and after a 20 minute run when idling I am now getting 14.3 volts across the battery and the alternator output. so no more rotten eggs smell !

The '650' 126 engine (Mk 2 126) used 2 types of alternator--Marelli or Polmont. The alternators were first fitted in August 1977 and had a separate regulator, this was changed in March 1979 to a alternator with an integral voltage regulator. In all cases, maximum voltage is 14.5 volts.