Alternator voltage

Turbo-James · Post by **Turbo-James** » Tue Mar 08, 2011 2:21 pm

I have a tzd turbo that has a problem with its charging system which results in the battery going flat. I have tested the alternator with a multimetre and the voltage is spot on 14.5v after starting the engine from cold. If however you leave the engine to warm up for a few minutes and then test it again it is making around 11.5v.

I,m quite sure there is no problem with the battery as it had a brand new one fitted at the end of last year. Please could anyone offer any clues to what it might me other than the alternator. I was thinking maybe a bad earth, but where do you find this? I would like to check every other possibility to make sure it is definatly the alternator.

thanks for any help

sdelasal · Post by **sdelasal** » Tue Mar 08, 2011 7:02 pm

It could be the voltage regulator playing up - it's the bit that looks like this.
http://www.thetoolboxshop.com/0-812-00- ... 4381-p.asp

These do have a temperature compensation in them which drops voltage as temperature rises. Below 11.v is too low though. steve

electrokid · Post by **electrokid** » Tue Mar 08, 2011 11:44 pm

First check all connections on the battery and alternator are good.

11.6v on the battery - it's flat - fully charged it should settle close to 14v after the engine has run and charged properly (or after using, then disconnecting a charger).

If you hook up the voltmeter on the battery and then use a mains battery charger the voltage should settle within a minute or so and then rise very slowly - if it rises quite quickly - and if it keeps rising after reaching 14v - gets to 14.5v and looks as though it's heading towards 15v then the battery has failed prematurely.

If you are able to monitor the battery voltage (measured directly on the terminals) during cranking it should go no lower than 9v.

If you have one cell of the battery U/S then it may still start the engine though somewhat sluggishly - battery volts will be 11.6 after being charged (which looks the same as an uncharged battery ! ) - but the volts during cranking will fall much lower to around 6 or 7v.

If the battery is ok then the regulator is suspect - if there's just one large connection to the alternator make sure it is tight - if there's a second, smaller connector as well (that's the 'sense' connection for the regulator) that should be good too - this connection being loose may cause overcharging and may be the reason for you seeing 14.5v which is much too high if the battery is healthy.

Good luck.