Alternator overcharging *SOLVED*

I've had a search of the forum but can't seem to find anything so here goes.

My alternator is outputting 18.5v. Measured across the battery terminals at idle and also measured from the alternator output and a ground.

I thought its regulator was duff so got hold of a new alternator and fitted it only to find it did exactly the same, it output around 16v and slowly increased to 18v again. I've replaced the battery with a new one too.

There is a plug on the back of the alternator with the terminals IG and L according to my manual. I believe IG turns the alternator regulator on with the ignition and L feeds the battery light in the dash. Now i've measured across these terminals - IG seems to output the same as the battery so around 15v and L outputs just over 13v. Does this sound right?

I've ordered another alternator just in case the replacement was at fault so just waiting for that to arrive.

Has anyone experienced this before? I have no idea what to try next if the 2nd replacement alt still does the same.

Thanks very much.

These alternators are internally sensed, so it should output 14V or so (depends on temperature) between the big terminal and the case of the alternator.
I suppose it is possible somehow for a bad earth between alternator and battery to cause a problem, but I can't see it raising the battery voltage by 4V or more, unless the battery is dead, or not connected and the alternator is having trouble regulating open circuit - check the main fuse.

The L terminal rises to battery voltage when it is charging, and turns the dash light off, the IG terminal has to go to ignition live to run the alternator, I'm not sure why, as the good old British ones would start up from the tickle through the warning light, and then self excited, you'd have thought modern stuff was better.

Are you sure the voltage is really that high, and it isn't your voltmeter that has a flat battery or something, at 18V I'd expect the battery to start venting and stink like rotten eggs. What is the voltage between the battery terminals?

facade wrote: the good old British ones would start up from the tickle through the warning light, and then self excited, you'd have thought modern stuff was better.

Yes and when the warning light blew the battery went flat without any warning; the modern alternators have better regulators.
Is the battery good?

Thank you Facade, it was my voltmeter. The battery was low, how embarrassing :blush: That was the one thing I didn't think to check!

All sorted.