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Alternator Voltage At Small Exciter Terminal But Not Large Positive Terminals [Solved]


Best Answer WiredbyWilson , 03 September 2015 - 09:00 PM

The smaller terminal is simply the wire that tells the alternator to charge the battery, and it will illuminate if the windings in the alternator are not working. As you state the light is not illuminated with the engine running you can initially assume it is charging - sadly the voltages say otherwise.

 

The two bigger terminals are the ones that supply power to the starter solenoid connection and from there the battery.

 

As the battery is discharging when you use the car - the battery is definitely not receceiving charge from the alternator.

 

My recommendations:

 

1. Clean the terminals in the alternator - high amperage WILL NOT pass through rust effectively.

2. Crimp the terminals within the alternator plug to ensure a tight connection

3. Remove and clean ALL terminals connected to the starter solenoid (13mm nut). they all need to be shiny brass.

4. Clean both battery terminals and connections.

5. Clean and refit all earths (main one in the boot and usually a braided one under the bonnet between exhaust and bulkhead).

 

Then retest the alternator - do this but using a multimeter set to 20V and connecting to the battery - not the alternator.

 

Let us know the results.

 

David

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#1 l_bratch

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Posted 03 September 2015 - 08:30 PM

Good evening

 

I have a 1995 Mini Cooper Si (an SPI model).

 

My alternator stopping charging the battery a couple of weeks ago, and after reading every relevant thread on TMF I can find and testing things, I need to post to ask!

 

The battery seems good - it charges to ~12.5 V off an external charger, and runs the car for a couple of hours before dropping down to ~11 V.  If the car isn't used, the battery holds its charge well - certainly more than a couple of days, but I haven't left it any longer to test.

 

With the ignition off, I get voltages as follows:

 

At alternator small exciter wire lucar connector: 0 V

At both alternator large positive lucar connectors: ~12.5 V

At solenoid end of positive wires to alternator: ~12.5 V

Battery indicator light is off as expected.

 

With the ignition on but engine off, I get voltages as follows:

 

At alternator small exciter wire lucar connector: ~1.5 V

At both alternator large positive lucar connectors: ~12.5 V

At solenoid end of positive wires to alternator: ~12.5 V

Battery indicator light is on as expected.

 

With the ignition on and the engine running, I get voltages as follows:

 

At alternator small exciter wire lucar connector: ~14 V

At both alternator large positive lucar connectors: ~12.3 V

At solenoid end of positive wires to alternator: ~12.3 V

Battery indicator light goes off again as expected.

 

With my multimeter set to resistance mode, I get a current between the alternator body and the car body (earth) even with the alternator connectors disconnected, which I think means the alternator is earthed well.  Is this right?

 

As you can see, the battery light is behaving normally, and the small alternator terminal is reading ~14 V with the engine running which I *think* means the alternator is working.  Is this right?

 

I hope the format above is useful, it's all the light/voltage information that was ultimately posted in other threads.  WIth all that in mind, does anybody have any idea why I'm not seeing alternator voltage at the positive terminal?

 

Of note: The positive terminals on the alternator look quite rusty, but the exciter terminal is quite clean.  Might it be as simple as rusty connectors?  I have been assuming not, as the connectors are quite large and the voltage should reasonably high which I think will deal with a rusty connector.  I have tried to clean off the rust using a solution, a small wire brush, and sand paper, but couldn't shift any of it.

 

I have attached a photograph of the terminals so you can see their condition.

 

Thanks very much

Luke

 

Edit:
Apologies for any incorrect terminology, I've learnt most of what I now know about alternators in the last couple of weeks!

Attached Files


Edited by l_bratch, 03 September 2015 - 09:57 PM.


#2 WiredbyWilson

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Posted 03 September 2015 - 09:00 PM   Best Answer

The smaller terminal is simply the wire that tells the alternator to charge the battery, and it will illuminate if the windings in the alternator are not working. As you state the light is not illuminated with the engine running you can initially assume it is charging - sadly the voltages say otherwise.

 

The two bigger terminals are the ones that supply power to the starter solenoid connection and from there the battery.

 

As the battery is discharging when you use the car - the battery is definitely not receceiving charge from the alternator.

 

My recommendations:

 

1. Clean the terminals in the alternator - high amperage WILL NOT pass through rust effectively.

2. Crimp the terminals within the alternator plug to ensure a tight connection

3. Remove and clean ALL terminals connected to the starter solenoid (13mm nut). they all need to be shiny brass.

4. Clean both battery terminals and connections.

5. Clean and refit all earths (main one in the boot and usually a braided one under the bonnet between exhaust and bulkhead).

 

Then retest the alternator - do this but using a multimeter set to 20V and connecting to the battery - not the alternator.

 

Let us know the results.

 

David


Edited by WiredbyWilson, 03 September 2015 - 09:00 PM.


#3 l_bratch

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Posted 03 September 2015 - 09:55 PM

Thank you very much David - sorted by recommendation number 1!  It was the rusty terminals.  I had convinced myself that I couldn't get any more rust off them, but you gave me the required boost to think otherwise!

 

The solution was a mixture of scraping with screwdriver ends to make some scratches in the rust, then a round parallel file to grind away at it.  My other attempts at removing the rust were virtually useless in comparison.

 

Back up to ~14 V at the battery now.

 

Thanks again

Luke






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