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Alternator

Started by laspy , Apr 06 2006 06:25 PM

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5 replies to this topic

#1 laspy

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Posted 06 April 2006 - 06:25 PM

I think my alternator is dying ive had problems with voltage dropping for a while now. After driving the car and turning it off sometimes the car wont start becoz of a flat battery. Ive been told its most likely the little black box on the back of the alternator. I have a spare alternator in the shed but have no idea of its condition. Is there any tests i can do to find out its condition without installing it?

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#2 rodomo

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Posted 06 April 2006 - 07:14 PM

Check your battery voltage with the motor running, headlights on and fan on full. Should read about 13.5 volts. Any less check voltage where big wire bolts on to back of alternator. Should be close to voltage at battery, give or take .2 - .5 volts. Any more voltage drop between the 2 readings and the wiring/connections are faulty between the 2. If your readings are lower, say around 12.5 your alternator is tired. If your readings are O.K chances are your battery is stuffed. Having said that a stuffed battery can also cause low charge readings. If you suspect the battery, hook a known good battery up to your car, by-passing your battery, and see if this lifts the charging voltage to at least 13.5 with lights and fan on. If it does chuck the battery, if it doesn't, your alternator is not coping. Others may argue this diagnosis but it works for me.

With regard to "the little black box" it has the brushes and regulator inbuilt. Replacing it may be only a short term fix as if the "slip rings" that the brushes run on are worn and/or oval they will wear again prematurely.

Edited by rodomo, 06 April 2006 - 07:20 PM.

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#3 _J&S Racing_

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Posted 06 April 2006 - 07:39 PM

I agree with your diagnosis method rodomo.

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#4 laspy

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Posted 07 April 2006 - 02:51 PM

thanks for the advice rodomo... i had a play around and found that:
- the voltage at the battery with car running is 13.46
- the voltage at the thick wire (back of alternator) is 13.96

problem is untill the car is warm the revs sit low (no choke) and the voltage at the battery is just over 12. Other thing is when the lights are on and the fan is on high the voltage at the battery drops to 13 volts is this normal?

cheers

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#5 _devilsadvocate_

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Posted 07 April 2006 - 03:38 PM

thanks for the advice rodomo... i had a play around and found that:
- the voltage at the battery with car running is 13.46
- the voltage at the thick wire (back of alternator) is 13.96

problem is untill the car is warm the revs sit low (no choke) and the voltage at the battery is just over 12. Other thing is when the lights are on and the fan is on high the voltage at the battery drops to 13 volts is this normal?

cheers

What car is it, does the alternator wire go direct to the battery?
See if you can see where you are losing the majority of the 0.5v. Is it across the terminals connecting the wire to the battery? and alternator or through the length of the wire itself.
If you are getting 13v at idle with the lights and fans on etc, thats fine for a standard alternator, but not for higher rpm.