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[Kevin Wood]

I wouldn't rely on the dashboard light as an indication that all is well. I have seen alternators that have failed either totally or which produce insifficient output with no warning light on.

I have also seen alternators that drain batteries when left idle. If a diode breaks down in the rectifier this happens so this is also a common failure mode.

However, if it's draining fresh batteries it could also be any other circuit in the car so it's important that some investigation is carried out to determine if there is excessive current drain when idle and where it's going to.

Kevin

[TheBoy]

doing some work on TheBoys car

Calling it a car is the nicest thing anyone has ever said about the tractor....

[dave abra]

I now have an multimetre but i am not sure what the reading should be on a battery which is normal. Can anyone tell me.

Thanks for invite to Brackley on Sunday. I had other commitments so was not able to make it.

[gofwb]

Hi, In another life (nearly 20 years ago) I was an Auto Electrician.

I found the easiest way to find shorts that drain the battery was to connect a brake light or head lamp bulb (not a halogen) between the positive battery post and the battery lead, if it lights up brightly without the ignition on you have a short, you could use an ammeter, but if you do have a short then a bulb is much easier to see, you then need to disconnect various circuits to find the short, try Alternator first then if the short is still there then start pulling out the fuses one at a time until the bulb goes out the short will be on the circuit that the fuse covers

Frank.

[Kevin Wood]

I found the easiest way to find shorts that drain the battery was to connect a brake light or head lamp bulb...

This is an excellent method. If the bulb doesn't light up you don't have a short but you may still have a higher than normal current drain. In this case, add a multimeter on the amps range in series with the bulb as well, so that the bulb protects the multimeter in case anything tries to draw an excessive current.

Leave the car for a good few minutes after connecting this, as the Omega has electrical items that take a few minutes to shut down. The current reading should settle down to a few tens of milliamps. Certainly current draw of over 100 ma is cause for concern as it will cause the battery to flatten when left idle.

If you find excessive current draw, it's a case of removing fuses until you find the circuit at fault.

Regarding checking the charging, you need the multimeter on the volts range (double check it's NOT on amps) and connect it across the battery terminals. You should see 12.5 volts or more with the car off and no electrical items on with a healthy battery. Start the car and the voltage should build up to 14 volts or more at a fast idle. More than 14.5 volts indicates an overcharging situation due to a faulty regulator in the alternator.

Once the voltage has stabilised, turn on a few electrical loads (e.g. headlights, heated rear window) and check the voltage doesn't fall significantly (say below 13.5 volts).

If the charging voltages look low, your problem is probably that the battery is not charging sufficiently when the car is being used. Suspect the alternator or the wiring between it and the battery.

Kevin

[dave abra]

Hi

The car is now working. It turned out that the battery drain was a courtesy light in the back. I feel a bit of a plank but nevermind.
The gear box problem was the auto gearbox selector switch. It has been changed now and the car is as good as new.

Thanks for all your help

Dave

[VX1]

Well done for getting it all sorted out   . I had the same problem with the rear curtosey light it drain the battery quickly and since then the battery would turn the car over but had to have the jump leads to hand, at the same time the alternator went as well but because I was only travelling 6 miles (in total) to work I didn't change the alternator for a while. When I did the car was fine. So again well done for sorting out