Flat battery after driving with headlights on

[edhorne]

Morning all

My battery was flat last night after an hours motorway driving. I realise its probably an alternator issue and I will be testing it shortly however the symtoms were strange:

1. Drive 1hr in dark with 10mins of idle in traffic and 70mph most of the rest of it
2. Go to restart at destination no probs but tachometer reads 0rpm
3. Drop mate off after 1hr, 70mph, in dark, car won't start.
a. very sluggish to turn over
b. battery light is on with ignition OFF
c. no battery light with ignition on, just oil light
4. Jump start with another car, after bypassing big fuse, still no tachometer
5. Jump start with big beefy battery charger connected and tachometer works OK (?)

Battery is now on charge so hopefully I can do some testing once I had my 3 Weetabix!

I'll post my findings in a bit but any help would be much appreciated.

Ta

Ed

[RUSSELLM]

100A fuse

[edhorne]

I thought that too but I still had no tachometer after bypassing the 100A fuse.

[RUSSELLM]

Hmmmm.... Is the alternator definately spinning ok, belt not slipping ?

[edhorne]

I've done some testing.

1. Battery fully charged overnight and fairly new
2. Engine not running and ignition off - battery light on
3. Engine not running and ignition on - battery light off
4. Starts easily, battery light off when running

5. Voltage at battery with engine running - 12V
6. Voltage at big fuse (both sides) with engine running - 12V
7. Voltage at alternator terminal with engine running - 12V

8. Stop engine (ignition off) - battery light on

The tachometer stayed at 0 rpm initially and then jumped up to idle. When I revved the engine the tachometer went back to 0 rpm. Very strange.

I reckon the battery is discharging through the alternator as the diode packs have reversed themsleves (read it in another thread).

I will take the alternator off and get it tested.

Bugger

Edited by edhorne on Saturday 25th October 11:34

[itsallyellow]

With the engine running the battery should read about 13.7 i believe. Sounds like the alternator to me.

Mike

[sonnylad]

Ed

b. battery light is on with ignition OFF

This is a classic symptom of faulty diode pack

Russ

[RUSSELLM]

To charge a 12v battery, you'll require between 13.2 and 14.4 volts at the battery

Sounds like your alternator's duff.

[GreenV8S]

sonnylad said:

b. battery light is on with ignition OFF

This is a classic symptom of faulty diode pack

Yes, it's pretty much the only way that's going to happen and will cause exactly these symptoms. (It may also overheat the alternator and could even be a fire risk.)

[edhorne]

Well, I'll whip it off and get it tested.

There are a number fo threads concerning duff alternators. I think mine is a Magnetti Marelli.

Has anyone fitted a Bosch one, perhaps it would be more reliable?

Cheers

Ed