P38A Voltage drop?

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plumbfisher

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'morning all, excuse long post - trying to give all the info.
I have a second battery and split charger fitted. When the second battery was charging the voltmeter (on the second battery) used to show 14.75 v and when the main battery was charging a meter showed 14.75 too. All good. Then I noticed both these readings had dropped to 13.8 v. Not wanting to chance it, I changed the alternator. No difference! I've since had the old alternator tested and apparently it is fine, putting out 14.75 on the bench. So what's going on? What could be causing a voltage drop (or loss) ? I've disconnected the split charger and tried a different (new) battery and still only 13.8 at terminals. Everything off and each battery shows 12.5 v. Help please!
 
When the batteries are full, the alternator will back off to a lower value, I've seen it fluctuate 13.5-8v during a journey
Leave your headlights on for 10 mins then start the engine, you should see 14.7 then.
If not, measure the village between the positive post of the alternator and the alternator body for a true reading
 
When the batteries are full, the alternator will back off to a lower value, I've seen it fluctuate 13.5-8v during a journey
Leave your headlights on for 10 mins then start the engine, you should see 14.7 then.
If not, measure the village between the positive post of the alternator and the alternator body for a true reading
Thanks I'll try that when the rain stops ;)
 
'morning all, excuse long post - trying to give all the info.
I have a second battery and split charger fitted. When the second battery was charging the voltmeter (on the second battery) used to show 14.75 v and when the main battery was charging a meter showed 14.75 too. All good. Then I noticed both these readings had dropped to 13.8 v. Not wanting to chance it, I changed the alternator. No difference! I've since had the old alternator tested and apparently it is fine, putting out 14.75 on the bench. So what's going on? What could be causing a voltage drop (or loss) ? I've disconnected the split charger and tried a different (new) battery and still only 13.8 at terminals. Everything off and each battery shows 12.5 v. Help please!
A common cause of low charging volts is a broken earth strap engine to chassis. As Kermit said, the alternator will back off to 13.8 volts when the battery is charged, this can also happen with a duff battery. I don't know your split charging system but the charge voltage to the second battery is likely to be dictated by the state of charge of the main battery.
 
Thanks for your replies. I've done what you suggested Kermit. Yup! 14.7 very briefly, then back to 13.8. I think this is a classic case of me suspecting that there's something wrong when there's not! Comes of years of having a P38 I suppose :oops: Both batteries are fully charged most of the time it seems...
 
Thanks for your replies. I've done what you suggested Kermit. Yup! 14.7 very briefly, then back to 13.8. I think this is a classic case of me suspecting that there's something wrong when there's not! Comes of years of having a P38 I suppose :oops: Both batteries are fully charged most of the time it seems...
That's why I have voltmeters in both my P38's:)
 
Thanks for your replies. I've done what you suggested Kermit. Yup! 14.7 very briefly, then back to 13.8. I think this is a classic case of me suspecting that there's something wrong when there's not! Comes of years of having a P38 I suppose :oops: Both batteries are fully charged most of the time it seems...

13.8 does sound bang on for just keeping the batterries topped off. Surprised a P38 alternator is that clever but given you're above 14.5V when it is charging I guess you've replaced the gubbins in the alternator.
 
13.8 does sound bang on for just keeping the batterries topped off. Surprised a P38 alternator is that clever but given you're above 14.5V when it is charging I guess you've replaced the gubbins in the alternator.
The Mobiltron regulator charges at up to 14.8 volts immediately after starting in cold weather, a little less in summer, then gradually folds back to as low as 13.5 volts on a long run.
 
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