Voltmeter pegging

[DCONNELL]

New out of the blue problem...

 

Driving and noticed the voltmeter pegged, never seen this before.

 

New-ish / strong battery, tested at rest = 12 volts

 

Start the car, voltmeter rises slowly, pauses just after 11, then moves smoothly to red, barely, but red

 

Running, battery tests at about 18 volts.

 

Checked the levels, all low... so I topped them off.

 

Battery tests at about 14 volts at rest, running, about 18 volts

 

Looks like I have a relatively new Lucas 43 AMP alternator - in reading the next step is to test output while running to see if the voltage regulator is behaving... so I started her, measured volts from the neg batt term and the large lead off the alternator, 18. something volts.

Edited May 26, 2016 by DCONNELL

[Steves_TR6]

If the alternator is putting out 18v it is faulty.

 

Steve

[Bodiam]

If you are lucky, something like this might do the trick more cheaply than a replacement alternator -

 

http://www.autoelectricalspares.co.uk/130639-voltage-regulator-14tr-for-lucas-15acr-16acr-18acr-alternator-4-wire-1575-p.asp

 

(I'm guessing you have a Lucas 18 ACR alternator at the moment)

 

John

[TRTerry]

Overcharging creates gassing which is potentially dangerous. Anything over 14.46 volts will produce gas. Battery explosions are nasty!

 

 

Excessive charging causes electrolysis, emitting hydrogen and oxygen. This process is known as "gassing". Wet cells have open vents to release any gas produced, and VRLA batteries rely on valves fitted to each cell. Catalytic caps are available for flooded cells to recombine hydrogen and oxygen. A VRLA cell normally recombines any hydrogen and oxygen produced inside the cell, but malfunction or overheating may cause gas to build up. If this happens (for example, on overcharging) the valve vents the gas and normalizes the pressure, producing a characteristic acid smell. However, valves can fail, such as if dirt and debris accumulate, allowing pressure to build up.

Accumulated hydrogen and oxygen sometimes ignite in an internal explosion. The force of the explosion can cause the battery's casing to burst, or cause its top to fly off, spraying acid and casing fragments. An explosion in one cell may ignite any combustible gas mixture in the remaining cells. Similarly, in a poorly ventilated area, connecting or disconnecting a closed circuit (such as a load or a charger) to the battery terminals can also cause sparks and an explosion, if any gas was vented from the cells.

[Icarus60]

Either your regulator has failed ( which may or may not part of the alternator) or it is not sensing the system voltage for some reason like a broken or disconnected wire.

Try to borrow an induction ammeter and measure the current flowing into a fully charged battery, (But not at 18v level!) it should be zero but as the alternator has potential to push out around 40 volts it could well be continue to stoke the battery even though the battery is about to explode.

You need to fix it as stated above as the next stages could well be an acidic smell, bulging of the battery casing and ........... See above.

If it were me I would leave the headlights on to drop the unloaded voltage with car not running, inspect all cable leads and plugs and test as above.

Regards