Technical Diagnosing a dodgy alternator

[kitch88]

Hi all,

I'm hoping you can help me diagnose a potentially dodgy alternator on my girlfriends 2007 grande punto 1.2 (non aircon).

She reported yesterday that the power steering failed to work at the end of a 30-minute drive. I've just had a look at the car and, and tested the battery and found the following:

1) Battery was at 12v when car had been left off (after a separate drive home to the one where the power steering failed).
2) When the car started the voltage rose to 14.3v.
3) With the engine running I turned on the heater to full, lights and rear windscreen demister... The voltage soon dropped to 12v and was continuing to drop... I therefore turned off the heater etc. and the voltage rose to 14v again.

Before I go and book it in for the alternator to be replaced:

• Is it clear cut that it's the alternator that's the problem here?

• Are there any recommended garages for this work in South Gloucestershire/Bristol?

• Is there a rough cost I should expect to pay?

Thanks v much, I always find these forums really useful.

 

[cmosfxx]

It could be bad cables (corroded on the inside), corroded ground points etc

 

[Bernievarian]

It could be bad cables (corroded on the inside), corroded ground points etc

Don't think so, voltage @ battery terminals wouldn't drop with corroded cables / earthpoints !

GP's alternator are rated from 105 to 120 amps, which is about 1500 watts, still more than all lights, heater, defrost etc can potentially draw when turned ON, so the voltage shouln't drop and the battery should keep on charging.

Unless the alternator can't provide this output anymore … This can be as simple and cheap as worn-out brushes: the magnetic field needed to initiate the charging current doesn't reach a sufficent level so the delivered amps are not enough to power the accessories, therefore the voltage drops …

BRs, Bernie

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[g8rpi]

Normally I agree with Bernie, but the battery voltage measured at the terminals can be caused by bad cables for example if the alternator cable or engine "ground " is high resistance the alternator voltage will drop across this resistance ane the lights etc will drain the battery causing the voltage to drop. What is important with this test is engine speed. If the on load battery voltage is dropping with the engine idling, does it rise as the speed is increased to 1500-2000 RPM? If it does the alternator is probably OK. If voltage stays low connect the meter between the battery negtive post and emgine block with the load on. If it is more than 1V the connection is bad.


Robert G8RPI.

 

[Bernievarian]

Normally I agree with Bernie ...

So do I with Robert !

BUT: the voltage @ battery terminals is correct with engine running, which means there is NO voltage drop... until high power accessories are connected. We all know (supposed to) that U=R x I, that gives us, for 10 amps current, a voltage drop of 1 volts with a 0.1 ohm "resisting" connection, or 2 V for 0.2 ohm etc.
This voltage drop is measured ACROSS the resisting point, NOT @ the terminals !!


So far so good, but there is another player on the field: the battery. It has its own internal resistance so when current is drawn from it there is a voltage drop across the battery: the lower the resistance (good battery), the lowest the drop. A dodgy battery would have an higher internal resistance, which will lead to higher voltage drop when solicitated (with engine off). In normal condition (engine running) the alternator would compensate that (current will be drawn from the alternator, not from the battery) but if the alternator is dying it will not deliver enough current so the missing power will be collected from the battery, with voltage drop @ terminals !


It is not rocket science to check for good battery / alternator / grounding / cables, this has already been explained many times in the past, a set of jumper cables is all you need ...


BTW, I was wrong about the alternator outputs, here is the correct table...

BRs, Bernie

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[kitch88]

Thanks v much for the replies so far all. Sounds like I can't say with certainty that it's the alternator just yet.

Bernie: I'll do some searches on the forum to check whether the alternator/grounding/cables are still good and report back.

It's also worth noting that since the first time this issue arose it has not happened again... and the car has remained in regular use on the same journey.

 

[g8rpi]

So do I with Robert !

BUT: the voltage @ battery terminals is correct with engine running, which means there is NO voltage drop... until high power accessories are connected. We all know (supposed to) that U=R x I, that gives us, for 10 amps current, a voltage drop of 1 volts with a 0.1 ohm "resisting" connection, or 2 V for 0.2 ohm etc.
This voltage drop is measured ACROSS the resisting point, NOT @ the terminals !!


So far so good, but there is another player on the field: the battery. It has its own internal resistance so when current is drawn from it there is a voltage drop across the battery: the lower the resistance (good battery), the lowest the drop. A dodgy battery would have an higher internal resistance, which will lead to higher voltage drop when solicitated (with engine off). In normal condition (engine running) the alternator would compensate that (current will be drawn from the alternator, not from the battery) but if the alternator is dying it will not deliver enough current so the missing power will be collected from the battery, with voltage drop @ terminals !


It is not rocket science to check for good battery / alternator / grounding / cables, this has already been explained many times in the past, a set of jumper cables is all you need ...


BTW, I was wrong about the alternator outputs, here is the correct table...

BRs, Bernie

If someone here helped You fix -or better, understand- your issue, hit the thanks icon @ bottom right corner, it's free and makes us feel helpy ;-)

Correct, but the OP was trying to diagnose an alternator fault by measuring the voltage at the battery which is inconclusive. I probably confused things by saying " the battery voltage measured at the terminals" "the voltage measured at the battery terminals" would have been better. The engine speed during his test needs to be addressed as well.



Robert G8RPI.

 

[Bernievarian]

Hmmm, well, the voltage test @ battery with engine stopped vs running is the very first to check wether the alternator charges or not …
Next one would be to check how much it charges, either by monitoring the voltage drop when big consumers are switched ON, or with an Ameter used in the same condition.

Seems to me that's what the OP did, no ?

Beside that, have you ever faced a positive power cable getting rotten as the ground one does ?? It's path being alternator > starter main relay > battery terminal, it could potentially cause charging issues …


BRs, Bernie

 

[cmosfxx]

Positive cable is very hard to corrode like the ground cable.

On these cars I've seen:

1) Bad connection at the starter
2) Corroded ground cable (engine or chassis side)
3) Corroded ground point (chassis side)
4) Broken D+ cable (there is an obd error but no battery light, also the alternator is not charging if you start the engine without revving it more than 2k)
5) Bad regulator (but still a working alternator at low/medium loads and high rpms)
6) Water in and under the fusebox.

I think it's easier (and smarter) checking the cables before you spend money and time on a new alternator.

 

[Bernievarian]

I understand from the OP that the alternator does actualy charge the battery (14.3 V) but collapses when load is applied.
For me it clearly is a sign of defective excitation circuit:
- bad regulator
- worn brushes
- bad signal from BCM

BRs, Bernie

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