Alternator driving me crazy!

[mrlex]

Hey guys,
I need your mechanical minds to help me solve a problem...

Battery check with multimeter reads 12.
Start the car and it drops to 11. Then I think Alternator is stuffed!
Give it a rev, let it come back down to idle and it stays reading high 13's. Alternator is working..NO? WTF?

Cheers
Alex

[h100vw]

Alternator theory.

When you start the car the alternator needs a small amount of voltage in the blue wire, the field.

This starts the alternator working. The blue wire comes from the warning light on the dash. When the voltage out of the alternator matches that on the other side of the warning light. It goes out.

Your alternator is working fine. In the old days before LED warning lights, you could change the wattage of the warning light to control when the alternator kicked in. Essentially it was related to engine speed.

You might find that a new regulator pack and brushes will put the light out without revving the engine but don't count on it.

The reason the voltage drops of is the juice required to start the car.

Gavin

[Jarred]

I was going to mention before, and Gavin has confirmed, that's pretty standard alternator behaviour. I wouldn't be concerned.

[Golf Loon]

Does the car start? Id be expecting low 14s to be charging the battery.

[mrlex]

Cheers for the replies people.
The car starts, drives for about a week..maybe more, then breaks down as its driving then wont start. Jump start it, drive home..Charge the battery and goes for about the same period.
But the alternator must be charging a little to enable it to drive for that long..no?

[Stan]

A half stuffed battery will give you all sorts of wierd electrical gremlins, including ones like you are suffering.
You may have a cell on the way out. It holds charge fine when driven and then after a while gives up until you give it a big recharge - quite possible, especially if a few years old.
Try the same readings with a multimeter across the battery and not just relying on the voltmeter in the car (i presume that's what you are doing...)

[mrlex]

Yeh..didnt think of that, was wondering if it could be the battery.
Ive been using the multimeter on both the alternator and the battery as my car has no meter gauges in the car.
Couldnt say how old the battery is..looks relatively new as does the alternator which is a pain.

[Jarred]

Go down to a battery place or even repco and have them test the battery for you, should be for free really. this should give you a quick indication of the health of the battery.

Then you can start pointing fingers and getting things reco'd or replaced etc/

[Golf Loon]

Or take the battery out of the car, test it, leave overnight and see if it has lost charge by the morning.

[mrlex]

Jarred,
Been down to a battery place and all they said was that it has charge, which i obviously knew as it was charged!
I thought they would do a more thorough check by loading it etc...

Might try another place.
Get a second opinion.

Cheers