A new one - battery voltage drops when car starts.

**Pinelli** · 3rd June 2019, 04:18 PM

2006 V8 D3. Has been ticking along pretty well until this afternoon. I noticed the engine seemed to be 'roaring', perhaps the radiator fan? And then the battery light started to come on and off at strange intervals, and the voltmeter on the main battery reading 11.9V while driving.

Managed to get home ok. Popped the bonnet. Battery reads 12.3V with the engine off. With the engine running, drops to 11.9V. Belts seem fine. Cables to the alternator & battery seem fine.

One code on the nanocom that's relevant P0626 Generator Field Terminal - circuit high (Intermittent).

I mean, seems like the alternator to me, but I never get tired of discovering the ways in which modern cars can do odd things.

Any other thoughts as to what it might be? The only thing I haven't tried yet is reading the voltage directly off the alternator post.

**Eevo** · 3rd June 2019, 04:31 PM

sounds like alternator.
what makes you think this is odd?

**Pinelli** · 3rd June 2019, 04:39 PM

I don't think this is odd if it's just the alternator. Just saying I've seen some strange causes of symptoms form modern cars, and not to necessarily trust my first thoughts.

DiscoMick · 3rd June 2019, 05:04 PM

If your vehicle has a variable output alternator, then it will vary it's output depending on the needs of the vehicle and battery. If they have plenty of charge, the alternator will reduce its output.
I'm not an auto-electrician, but I think if the battery is up around 80%, the alternator may cut it's output. 12.3 is about that level.

**alien** · 3rd June 2019, 05:04 PM

Originally Posted by Eevo

sounds like alternator.
what makes you think this is odd?

Probably read the Discovery2 posts about blown brake light fuses causing charging issues. It never hurts to ask those who know. Unfortunately I don’t know much about the D3’s.

Markus1 · 3rd June 2019, 05:11 PM

With volts readings like that I'd say it's the alternator. I'm no expert in LR but I've never owned a car that measured less than 13.4 volts regardless of battery state of charge.

The reason it reads less when running is because you car engine is now pulling current out of the battery. That's a net loss.

It won't be long before you have a flat battery.

**LRD414** · 3rd June 2019, 05:28 PM

Originally Posted by Pinelli

One code on the nanocom that's relevant P0626 Generator Field Terminal - circuit high (Intermittent).
I mean, seems like the alternator to me, but I never get tired of discovering the ways in which modern cars can do odd things.

Have a read of this thread:
Alternator - Various Fault Codes

Cheers,
Scott

**trout1105** · 3rd June 2019, 05:43 PM

Originally Posted by Markus1

With volts readings like that I'd say it's the alternator. I'm no expert in LR but I've never owned a car that measured less than 13.4 volts regardless of battery state of charge.

The reason it reads less when running is because you car engine is now pulling current out of the battery. That's a net loss.

It won't be long before you have a flat battery.

+1
The alternator is just about toast, It may just be worn brushes.

**SeanC** · 4th June 2019, 03:41 PM

Have a look at this. First couple of minutes. Similar symptoms, due to the diode failing.

YouTube

**drivesafe** · 4th June 2019, 04:05 PM

Hi Pinelli, D3 alternators will operate somewhere around 13.5v to 15.5v, but 15.5v is usually only in very cold conditions.

While D4 alternators can drop to 12.2v, this is normally only for a very short time and is something that only happens periodically, when the BMS wants to test the cranking battery’s condition.

So to be as low as 11.9v is well and truely below normal operating voltages.

As others have posted, I would start with getting your alternator tested.

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