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testing the alternator


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I recently had a starting problem. Usually my 95 300Tdi starts instantly. On leaving home recently I noticed she wasnt quite as quick, but thought nothing of it. After driving about 12Km with heater and lights on and stopping to drop off my young son, she started but possibly needed slightly more effort again. After a further 20 odd Km she was worse still when I stopped for diesel and drove straight to my local garage. With a battery checker across the the battery even when revved she would not give more than around 13V, certainly nowhere near 14V.

When I got home I put the battery on charge and recondition and since then have had no more problems with starting. However I never get more than around 13V across the battery when the engine is running/revved. Shouldn't it go higher? to 14v or a bit more?

The alternator was replaced last year, with a Denso; and the battery is a recent Bosch S5. I have bought a new alternator but don't want to send the old one back until I am sure it is faulty.

How do I test it? The garage insists the output should be higher.

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Is it good enough for me to put a voltmeter across the battery and rev the engine or is it more complicated than that? Does the state of the charge of the battery affect how high the voltage output from the alternator is?

I'd have to send the alternator back to Denso for testing from Italy which will be quite expensive, but it is still under warranty.

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4 hours ago, pgrbff said:

Is it good enough for me to put a voltmeter across the battery and rev the engine or is it more complicated than that? Does the state of the charge of the battery affect how high the voltage output from the alternator is?

There are other reasons for things not to charge.  Poor wiring connection on power or ground,  failure in the sense circuit...    Most likely it is the alternator, but without a test outside of the vehicle you can't be completely sure.  I thought the Denso branded alternators are Marrelli, i.e. Italy head office...

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10 hours ago, Red90 said:

There are other reasons for things not to charge.  Poor wiring connection on power or ground,  failure in the sense circuit...    Most likely it is the alternator, but without a test outside of the vehicle you can't be completely sure.  I thought the Denso branded alternators are Marrelli, i.e. Italy head office...

They are made in Italy but far easier to buy from the UK, and far cheaper, unless you have a problem and have to send it back.

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I always measure the voltage with load and no load at idle and about 1500prm, i.e. no load would be engine running everything electrical switched off where possible, load would be lights on( including main bean) etc.

As above 13.8 - 14.4 volts, especially under load is ideal however I personally wouldn't worry if there were 13.4v under load.

I was once told by a chap who rebuilt alternators 13v under load is ok .

 

 

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