Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Battery warning light

HI,
On my +4 1991 Rover M16 engine my battery warning light is often on ( not bright but a weak yellow light) and then the read out voltage is about 10Volt or lower.
I checked the very thin (brown) wiring coming from alternator and goes behind the starter, behind the starter there is a connection.
checked all connections and these will be OK,
sometimes after starting the engine there is no problem (no yellow warning light and the read out voltage on the Voltmeter is above 13V.
So this is a intermittend fault.
Can someone help ( it it a fault in the Voltmeter or is it a fault in the ECU) ?
best regards,
Bart

It sounds as though the alternator is on the way out.

That is what I thought too

Might be worth just double checking the belt tension

You also need to check the output on start up and then under load with a multimeter which will eliminate a faulty instrument

Arwyn

The warning light is not lying judging by those figures and the 10 volts suggests a pancake flat battery. Suggest sorting before any further journeys. In order of likelihood I would check for a rectifier diode down (any one of three main pairs), faulty regulator or even brushes. If you remove alternator you should be able to check diodes on diode or resistance range of multimeter.

Richard, if the battery is old & shot with dead cells, would the alternator refuse to charge and hence you get a flickering charge light ? even if the alternator is healthy

When the ignition switch is turned on, the warning light is supplied from the battery and hence becomes illuminated. When you start the engine, the alternator voltage produced is applied to the light also on the other side of it, and largely balances out the battery voltage and as there is then little voltage across the light,(maybe only a volt or so difference) it goes out.
If your battery is bad, the car probably wouldn't start in the first place, so we look towards the alternator.
A loose alternator belt is a strong possibility as immediately after starting, the alternator has to work hard to restore charge to the battery so a lot more force is applied to the belt which can slip if not tight enough. Another possibility is if your tick over speed is set very low, the alternator may not quite manage to produce enough voltage at such low revs.
Other possibilities of course include a faulty alternator or poor/ loose connections.

If it's a loose belt you would normally hear it screeching.

Alternators should give a good voltage from very low rpm if it's in good working order. (That is one of the benefits of alternator v dynamo)

I would suspect either loose or corroded alternator connections or the alternator itself.

There are lots of companies who will test and recon an alternator for you.

BTW can't see how this could be an ECU fault.

On older designed (Lucas) alternators, the warning light is on a separate diode circuit from the charging circuit.

I've had this happen a couple of times in the past. Bart, does switching the headlamps on make the warning light brighter????

That happened with mine.

As Doug suggests the warning light sits between alternator field circuit and battery which in turn is permanently connected to alternator output. Once the two are at similar voltage little or no current flows through warning light so it extinguishes.

Generally alternators do require a kickstart in the first place unless there is residual magnetism in the field rotor. This is supplied to the rotor to create a magnetic field via the current (0.25 amp) flowing through the illuminated (3 watt) warning light bulb when ignition first turned on. The battery is therefore required to provide this but would be in a very poor state not to do so. Once the alternator comes online, sometimes with help of a few revs above idle, it becomes self sustaining via a trio of dedicated diodes bleeding back from the output to maintain the field, with the output voltage controlled by the field regulator.

Once working with some load, it technically doesn't need the battery therefore, although if it presented by same with an open circuit and load dropped, volts would surge, regulator wouldn't be able to cope and something would blow, probably diodes - Phew!