Alternator symptoms

Hi, can anyone tell me what's going on with my alternator?

My car's battery light started coming on dimly, and then went off. Then while driving the next day I had complete electrical failure for a few seconds, which then returned to normal, although again the battery light seemed to be dimly lit. Then I had complete electrical failure again and couldn't restart the car, the battery was completely flat, like it had been shorted out. I recharged the battery and disconnected the alternator, this got me home.

My alternator (while disconnected from the battery) now shows an output of 0VDC but an output of around 3-4vAC, which rises with RPM.

What do you reckon's broken? The rectifier diodes?

If you run the alternator disconnected from the rest of the electrics then you knacker it.

mrcheerful

The message from "mrcheerful" contains these words:

Sometimes. But not always.

Possibly but start at the beginning & check the earth from the alternator to the engine & thence the body. You don't always see any voltage from the alternator without the battery connected.

I agree. In my driving "career" I've had alternators fail to charge four times. Once was a broken belt and the other three times were all due to dirty/corroded/oxidised contacts on the leads. If cleaning all the obvious connections with sandpaper or a file before reconnecting doesn't solve things, a voltmeter should help to find where things go wrong. It sounds like you have already used a voltmeter to confirm the alternator itself is working.

This is a useless test as it proves nothing - the control gear requires a battery connected to make it work.

To check if it is working, you should get a reading across the battery that increases by at about 2 volts between engine stopped and running.

With a fully charged battery this should read about 13.8 volts.

I just added the answer to this question to Section 5.1 in the Car and Deep Cycle Battery on

this morning. Clearly you will need to externally recharge and test your battery before proceeding, but first check the alternator belt and remove in corrosion on your battery connections. Then you can test your charging system with the battery attached.

Kindest regards,

BiLL.......

Sorry I should have mentioned I left the Field Current connection in place so there'd hopefully be an output, and measured the B+ connection relative to the chassis.

Anyway I've done some more experimenting and it turns out I have the unluckiest charging system in the world.

I found three faults, none of which are the alternator! (1) The earth strap from battery to chassis had corroded very badly and was intermittently disconnecting

(2) The connector on the B+ stud on the alternator (the one that carries the current) had also corroded under the plastic sleeve and appeared to have burned out a bit too.

(3) After correcting both of the above, re-connecting the alt, charging my battery manually and putting it back, the alt light is on dimly under high load, which gets brighter as the load increases, implying a battery fault. The battery terminal voltage is 14.5 V when the engine's running, which means the alt is OK. The battery voltage is about 12V when the engine's off.

So I guess the total electrical failure occurred when both connections went open-circuit at the same time.

so hopefully all that's left is to buy a new battery and eveyrthing should be OK.

Thanks for the suggestions anyway!

The corrosion's pretty bad though; that's what comes from living by the sea I guess.

Jim

Probably worth checking the length of the brushes on the alternator as well. Unless it's all happy with happy with the new battery.