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[Alex W]

Went to use the car today but the battery was so flat it had trouble opening the doors. The vehicle had been standing for four days.

Charged it up and all OK again - but I need to know what happened so I can stop it happening again.

I have had no sign of any problem with power, no warning lights on the dash or problems starting (since previous posting in July). I do have a faulty central locking motor in the drivers door (and a replacement I am yet to fit).

The battery is about two years old and is a GM battery from the main dealer (£64 at the time). I had a new alternator fitted in March this year when the bearings failed on the old one.

I need to know what steps I should take to try and trace the fault? I haven't got a multimeter but I am prepared to invest in one if required (any recommendations as to which is best for amateur home user).

Thanks in advance for any help offered.

[TheBoy]

Would be good to check battery voltage, with car off, at idle, and at 2.5k rpm...

[Alex W]

Looks like I need a multimeter. Off to Halfords in the morning then! I might get their big socket set while its on special offer too.

[robby30]

Looks like I need a multimeter. Off to Halfords in the morning then! I might get their big socket set while its on special offer too.

screwfix has a good value multimeter for £10 does most things halfrauds i would guess would be much more

[Proz]

Looks like I need a multimeter. Off to Halfords in the morning then! I might get their big socket set while its on special offer too.

I just got the bigger of the two socket sets they have on offer ( £130 ).... dont know about the smaller set but the bigger one has just about everything i could ever use in it ... and if its the Halfords Proffesional one if anything breaks its replaced free  

[Agemo]

I know it's a bit low tech, but are you sure the boot light is going our? One little bulb can sap the battery in four days.
 

[Alex W]

Checked the boot light, its off. Checked the glovebox too while I was at it, thats off too.

[Marks DTM Calib]

Disconnect the power sounder....

[Elite Pete]

Make sure you haven't left the indicator on when you've parked up. This will leave a parking light on

[Alex W]

Power sounder is only two years old, new one fitted by main dealer when he did the battery.

Voltage Readings

Off Car 12.38
On Car engine off 12.4
Engine Idling 13.45-13.48
Engine at 2500rpm 13.45-13.48

[dbug]

Looks ok - would expect voltage with engine not running to be 12.5-12.8 volts.  Wouldn't anticipate issues with your readings though!

[Alex W]

The slightly low voltage at rest is probably because I didn't charge it up for long enough.

I assume that we can rule out a faulty (new) alternator?

If it drains again I will disconnect the power sounder as suggested bit in the meantime has anyone any other ideas?

[TheBoy]

The voltages look OK.

Heated rear window relay is known to stick sometimes, but doubt thats been on this weather.

Door motors sometimes stick on apparently.

Other than that, interior lights, alarm been sounding, any light left on, just the usual...

Worth unplugging power sounder if it repeats, though yours is fairly new.

[Derek_in_Penzance]

I don't know if this applies to Omegas, but it might just....

...be the main interior courtesy lamp. On late Monzas, Senator Bs and Carltons the circuitry above the bulbs can fail, drawing a tiny, tiny almost unmeasurable current. Over a period of say 10 days, this can flatten a battery.

It's worth thinking about. The same fault may afflict Omegas. It took me ten years to diagnose this fault on my Monza GSE, and then it only happened by accident.

I recently advised a Senny B owner about this, and it cured his problem of battery going flat in ten days or so, with no apparent reason, and having checked everything under the sun.

[Alex W]

So, could my faulty driver's door central locking motor be the culprit?

It does the usual, works sometimes, sometimes not. As I said I have a second-hand replacement just haven't got round to fitting it yet. I'll try and swap it this weekend.