D2 td5 Alternator voltage

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tom1979

tom1979

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my understanding of a 14v alternator is that it should be putting out at least 14v?

Lately I have seen voltages with engine idling as low as 13.2 up to 13.5v and doesn’t seem to increase with revs. My alternator is a Denso one and only around 16 months old. The readings were via the 12v socket, I’m away at the moment and don’t have a multimeter but I know it’s below 13.5 as that’s when my relay switches for the caravan fridge!
 
I did bring a power probe though!

13.1v at the battery ?? Voltage was 12.6v with engine and ignition off

1EC6963D-385D-4DC7-9033-39F0AFDF5B92.jpeg
 
Hi tom,

Battery voltage is a moving target! I depends on temperature, the higher the battery temperature, the lower the charging voltage needs/must be, but over 14V and less than 15V would be the safe normal range. To maintain battery life, once the battery is fully charged the charging voltage need to drop nearer to 13.5V, but I don't know of any alternators that implement this. Your 13.1V is below the charging or maintaining voltage, so something is not right! Confirmation with a known good multimeter would be useful.

You only show the +ve lead of your PowerScope. I assume the -ve lead was connected to the battery -ve connection? If so, the voltage is definitely low. With a working system the voltage shouldn't increase much with revs or load, such as headlights or heated screens. The voltage at the 12V socket is to some extent dependant on load taken elsewhere, the 12V socket does not have a dedicated feed so other loads can affect the measured voltage due to lead resistance.

If the measured voltage at the battery changes little with load this really rules high resistance wiring from the alternator (bad connections etc) as the voltage would then fall with increasing load.

Can you connect directly to the alternator output? If so, are the voltages similar? If the voltage at the alternator output is of the same order the only real suspect is the regulator in the alternator. Worn brushes would tent to produce a revs and load dependant output voltage.

Cheers.
 
PopPops said:
Hi tom,

Battery voltage is a moving target! I depends on temperature, the higher the battery temperature, the lower the charging voltage needs/must be, but over 14V and less than 15V would be the safe normal range. To maintain battery life, once the battery is fully charged the charging voltage need to drop nearer to 13.5V, but I don't know of any alternators that implement this. Your 13.1V is below the charging or maintaining voltage, so something is not right! Confirmation with a known good multimeter would be useful.

You only show the +ve lead of your PowerScope. I assume the -ve lead was connected to the battery -ve connection? If so, the voltage is definitely low. With a working system the voltage shouldn't increase much with revs or load, such as headlights or heated screens. The voltage at the 12V socket is to some extent dependant on load taken elsewhere, the 12V socket does not have a dedicated feed so other loads can affect the measured voltage due to lead resistance.

If the measured voltage at the battery changes little with load this really rules high resistance wiring from the alternator (bad connections etc) as the voltage would then fall with increasing load.

Can you connect directly to the alternator output? If so, are the voltages similar? If the voltage at the alternator output is of the same order the only real suspect is the regulator in the alternator. Worn brushes would tent to produce a revs and load dependant output voltage.

Cheers.
Click to expand...

Many thanks! Yes the negative lead was on the battery. I will check the alternator output shortly if I can get to it.

It’s a Denso alternator no more than 16 months old so would be disappointing to say the least if the rectifier needs replacing already.
 
repairkitsuk.com do brushes, regulators and rectifiers for considerably less money than a new alternator. I have bought brushes from them (not fitted, I keep them as a "just in case"). 16 months would be disappointing for an alternator, if it has failed.

Good luck measuring the voltage at the alternator. For reference, on my 2004 Disco TD5, original alternator I think, at cold startup, the battery voltage goes to around 14.8V. After a good run on a warm day it has fallen to just over 14V and is pretty much unaffected by load or engine revs.

Cheers.
 
Put the probe on the alternator and it’s varying wildly between 14-14.5. Not sure what that means?

The voltages all match with the engine off so I think that rules out any resistance?

 
Last edited:
If the voltage on the battery doesn't match the alternator when the engine is running, which means current flowing out of the alternator, the volts are being "lost" in a high resistance cable/connector etc somewhere. Current across a resistance equals a voltage (drop). With the engine off, nothing comes out of the alternator so no current across a resistance equals no voltage (drop) so the battery and alternator voltages will match. There will always be a small voltage drop across all cables and connectors when a current flows because there is no such thing as zero resistance, but it seems you are loosing a volt, maybe 1.5V between the alternator and battery which is too much. The alternator is probably OK, so I would look for a bad connection starting with the connection onto the alternator, then back to the engine compartment fuse box (looking at RAVE) at "Link 1" which is 150 amp and anything you can see inbetween.

My Disco is quite new to me and I haven't had to mess about with that area just yet, so I'm not completely familiar with the cable runs etc, but I have to go out now and do an oil change on mine so I'll have a look at that area.

Good luck!
 
tom1979 said:
All of a sudden Nanocom and 12v socket is showing 14.1v . Strange
Click to expand...
This is the correct voltage when running. A possible clue here is that you have very slightly disturbed the connection to the alternator. As the alternator was replaced recently it may be possible that this connection wasn't done up tight enough or is in some other way not good. Just putting the meter probe on it could have disturbed it just enough to make a good connection, so you now see the correct voltage.

My oil change took much longer than expected, sump plug seized in and still is, so I didn't have as much time to look around as I wanted, but the feed from the alternator, which is brown and would be a thick wire, connects to link 1, FL1 in RAVE. The voltage on FL1, both ends, should be the same as at the alternator and the battery with the engine running. I know it's a nightmare getting to the alternator connection, but if the voltage drops again, I would check at FL1 and the alternator. Trying to find a fault that isn't there isn't easy......

Cheers and good luck.
 
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