Is It A Short Circuit Or Bad Ground, Bad Alternator Or A Weak Battery

[waqar_lionheart]

Hi All,

I drive a 1.8 vvti gs avensis. I have a slight problem. My cars lights have been dimming ever so slightly when I press the brake pedal. The dash lights also flicker with bass in the car. I have bought a multi meter to check the voltage and stuff but dont know how to use it. I am not sure whether this is a weak alternator, Battery or a bad ground somewhere. When I have driving around for a while, the dimming isnt so bad. This makes me lean towards a weak Battery. I would like to check things over and I need help on as how to check them.

I welcome your support.

Regards,

Waqar

[gazza1286]

First thing to do is check the voltage across the Battery terminals - should read 12v (minimum) Anything less then the Battery is either failing or the alternator isn't charging. Do the same with the engine running - should read 14.5v (approx). Anything less than say 14v means the alternator isn't charging - this could be loose cables or a slack belt - but probably the alternator itself.

[battytrebor]

Hi Waqar,

If the Battery is weak, you will have difficulty starting the car. After a run, and with everything turned off, set your multimeter to read 0-20Vdc which is the usual low voltage range on a low cost multimeter. Across the Battery terminals, you should get very close to 12.7 Volts dc, and this is a sign of a healthy, fully charged Battery. If the voltage is significantly lower, say 12.2 Volts, it would point to a charging or battery problem, and 12.0 Vdc points to a ccompletely flat battery If that was the case the car wouldn't start.

As mentioned in the previous post, with the engine running at just above tickover, you would expect in excess of 14 Vdc which would indicate that your alternator and charging circuits are ok.

If lights are dimming when extra current is being drawn (ie the voltage is falling) it is because the extra current is being drawn through a "resistance". That resistance could be inside the battery ie a duff battery, or as a result of a bad connection or earthing somewhere in the system. If the car starts ok, and the starter motor spins the engine quickly and easily, it is unlikely that the battery is defective. In which case, you have to hunt for the bad connection, which is no easy thing!

Always try the main battery connections first. They must be clean and free from corrosion. Vaseline helps. Start with the main earth connections as well. If you are not confident about working on car electrics, don't touch it!!! You can get some very big flashes from a 12V car battery if you accidentally earth the live terminal with your metal watch strap, a gold ring or a screwdriver.

Good luck

Bob

[Mooly]

Just to add a further point... the alternator output voltage is temperature dependent by design. A cold start in cold weather will see the Battery voltage rise to as high as 16 volts or so for a good few seconds before it reduces slightly. A hot engine on a hot day could see the voltage not much more than around 13v.

Filament bulbs (headlamps etc) show a dramatic increase in brightness for a very small increase in voltage. As long as the alternator maintains at least 13 volts across the Battery at idle with headlamps on you should be fine. In fact it should cope with much more, heated window and wipers etc too.

Is your idle speed OK and does it increase slightly to compensate for the extra load of electrical items as they are switched on ?

You can do a quick rough and ready test on the Battery by switching headlights and heated window on for 10 minutes or so without the engine running. That will pull around 25 to 30 amps. The battery voltage should hold up quite well. If it starts heading down quickly (particularly if you can also see/hear any cell in the battery fizzing and gassing) then the battery is faulty.

(This reminds of the Corolla TSport "issue" where reports of headlights "going out" and dimming and flickering were all found to be just the auxilliary air pump cutting in and dipping the voltage slightly. Owners were being told by electrical automotive engineers that the battery was faulty, the alternator faulty/weak/intermitent/underated when there was in fact no problem at all)