When I start my car the alternator light stays on until the car is reved, then it goes out and starts charging.....

The alternator was reconditioned, but don't want to rev the car each time to get it to charge...

Any idea what's wrong or can something be done to stop this.

Cheers tim

 

Have a look at the small wire to the alternator. This is the excitation feed to the alternator. It is fed with 12 volts through the ignition warning light when the ignition is on. This voltage creates a magnetic field through the field windings, allowing the alternator to charge at low rpm. Above a certain rpm, the alternator will self excite and does not need this voltage. This then feeds 12 volt onto the excitation wire and thus the ignition warning light goes out. A dodgy connection will still allow the bulb to light but may not give enough power to excite the field windings.

 

cheers i will take a look, but its been freshly rewired.....could something on the alternator be broke

 

It could be the rectifier has partially blown, this converts the ac current the alternator produces into dc current. There are four diodes in it. If one blows then it will only convert half of the ac current to dc. This can cause the warning light to come on at low revs. I discounted this as you said the alternator was recently reconditioned but it is possible. You wouldn't be able to tell unless you have access to an oscilloscope to check the voltage waveform though.

 

The alternator is gone already, The only reason the light is going off when you rev it is because you are giving it enough charge but it will eventually come back even after revving. Happened to mine and then power just cut out, luckily I could just pull into a car park, I'd suggest changing it for a new one before anything happens.

 

You don't mention which car it is, clearly it's an older model (3 DR?) and this is a known issue, the diodes can be a little lazy, I wouldn't worry as clearly it's fine!

Martin

 

Its a MK1 escort running a YB, the engine was from a 1990 car, it seems all ok when you rev it a bit.
Will run it as its a new build and see how things go.

Thanks for the help

 

don't mean to hijack the thread but ive got a question on the same subject. wats the chances of the ignition bulb blowning in the dash? and if this has gone would it stop the car charging?

 

It's not out the realms of possibility, but unlikely, more likely there is a break in the wire and yes this will prevent it from charging, unless you rev hard which will initiate a charge.

Martin