9.85v at the battery plugs when engine's running

The first step is to try another alternator; if this is not the problem my guess is that there is a heavy current drain somewhere in the ignition supplied components, were it a full short in the harness you should have noticed!

low voltage when engine running sounds like battery is being drained, recovers slightly when switched off. Normally voltage should be around 14.5 running, 12.5 - 12.8 switched off

The first step surely would be to check the wiring to the alternator! No good just fitting a new alternator if it is just a broken wire.

..or make sure that the fanbelt is in place!

Without a fan belt most engines would overheat long before the battery ran flat.

If you're measuring that voltage direct at the battery, the alternator isn't charging.

The message from "Dave Plowman (News)" contains these words:

Depends one which useage of "fan belt" you're using. Many have the water pump driven by the cam belt so there's no problem.

What happens to the voltage with the ignition on but the engine not running?

And what have you got in the way of extra equipment, like ICE for instance?

Steve

thanks to all of you for your answers

yomgui

yomgui wrote: