Battery drain and odd noise(!)

[KS Carrera]

On Friday, while driving up to a friend's workshop about 80 miles from home, 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 my friend'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 (weed-whacker to you guys) 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 road test car back in 1978!) 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 tacho was very slow to read and struggling to get above 3500rpm...

The next morning, I went back to the garage to have a look 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. To recap: Alternator appeared to be charging when tested at battery terminals. Alternator appears to allow a major drain to earth when engine off. Odd noise.

Damned cars...

[NeunElf]

I can think of maybe two paths to ground through your alternator:

1. The alternator produces three-phase alternating current. There are some diodes to turn that alternating current into direct current. Maybe one of the diodes shorted internally allowing current to flow back through the windings. That happened once to me a long time ago with an alternator from the Lord of Darkness in a Land Rover. But! That left the alternator light illuminated with the car not running.
2. Maybe something could cause your voltage regulator to pump current through the alternator field coils with the car not running. I'm not so sure about this; I can't see a connection from the regulator to the ignition switch on the L 57 wiring diagram so the regulator should "know" not to do this...

One of your trouble-shooting checks was done in error:

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!

That's not a drain--drains are measured in amperes. 12.5 volts between the battery and the battery connector just means the battery is charged to 12.5 volts as it should be (note that electrical engineers use the prepositions "between" or "across" for voltage).

You need to use an ammeter to look for a current drain. You can start with the same measurement, positive battery terminal to positive battery connector. A current more than a couple milliamps indicates a drain somewhere.

To try to track the drain current down, you could check for problem # 2 buy disconnecting the wire from the DF terminal of the voltage regulator and checking with an ammeter. If current flows from the DF terminal to the DF wire that's probably excessive field current with the car off.

You might check for problem # 1 by looking for current between the alternator's B+ connector and its wire with everything connected. At least on my '65 that requires disassembling the fan-alternator assembly.

[KS Carrera]

Thanks Jim - being a journalist (with a degree in Architecture...), auto electrics are not my strongest point! I did wonder about the drain and checking it. I just went up to the garage (1/4 mile away up a steep hill – good for exercise...) and checked for a drain in milliamps but none registered. I'm now even more confused as to why the battery went flat...

[304065]

Shorted alternator diode. This will show up on the oscilloscope

[raspy2point2]

Alternator problems are one thing. What about the " metallic whir " noise. I wonder if a baring is going bad in the transmission? Chris

[KS Carrera]

Alternator problems are one thing. What about the " metallic whir " noise. I wonder if a baring is going bad in the transmission? Chris

One of my thoughts, too. Just odd that it had never happened before on a trans that has 3000 miles on it since rebuild. BUT we've all got used to taking nothing for granted, so time to drain some trans oil and see if there's anything grim in there.