On Friday, while driving up to Roger Bray's near Exeter (about 80 miles), as I had to back off the throttle and change down when a car pulled out to overtake someone right in front of me, there was a slight mechanical sound from the back of the car – a sort of metallic whir. It was only very brief and stopped the instant I dipped the clutch (which allowed the engine to go to idle). I gingerly opened the throttle again to see if the noise would return, but it didn't. I thought maybe I'd not quite selected fourth properly and it had tried to pop out of gear on the overrun. Carried on...
As I shifted down through the gears ready to turn into Roger's place, the sound manifested itself again, but barely audibly. In fact, there was a guy strimming the grass verge and I wasn't even sure it wasn't his petrol strimmer I heard(!). On the way home, it happened once more, again on the overrun and only for a brief instant. Now, I'd just had the cam chain tensioners refurbished and replaced the day before, so my instant thought when the sound first occurred was that a tensioner had failed – but I've had that happen (on a Porsche 911SC press car on the M4!) and boy does that make a noise. It certainly isn't a brief sound that goes away, unless you bring the revs right up to make sure the chain goes tight. And it's not a sound you forget in a hurry! So I discounted a tensioner failure as paranoia and started scratching my head what the sound might have been. On the way home, I also noticed the rev counter was very slow to read and struggling to get above 3500rpm...
The next morning, I went back to the garage to dig the car out to have a nose around only to discover that the battery was dead – so dead the gauge needles wouldn't even flicker. This started me thinking: maybe the sound I heard was an alternator problem of some mechanical kind? I charged the battery overnight and put it back on the car today. It started instantly. Time to take some voltage readings.
Across battery terminals (no load): 12.8-12.9 volts ie 100 per cent charge
Fuel pump on: drops to 12.7 volts (to be expected)
At idle: 12.5 - 13.5 volts
Engine revved: 13.9 volts (suggests alternator putting out charge at least)
OK, with engine NOT running (didn't want to screw up a good alternator), removed positive terminal and connected voltmeter between battery +ve and the +ve battery cable: this showed a 12.5 volt drain!
Removed alternator altogether and carried out the same test: no battery drain. So alternator appears to be draining back to earth when connected up. Yet it's charging when running (or so it seemed).
The alternator (which was overhauled before fitting) spins freely, with no odd noises. The commutator looks pretty worn, and the brushes are worn to different lengths (or were they always like that?). It's an early SEV Marchal/Motorola unit with external regulator.
I now appear to have found the reason why the battery went dead (and maybe also for the reluctant rev counter – I was driving on the battery?) but what about the noise? I just feel it's too much of a coincidence hearing that brief sound (think of 'a stick in the fan' for a millisecond) and then suffering a dead battery at the end of the journey.
I'd welcome any serious thoughts on this as I am at a loss. Alternator appeared to be charging when tested at battery terminals. Alternator appears to allow a major drain to earth when engine off. Odd noise.