P38A Voltage drop?

kermit_rr said:

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

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plumbfisher said:

Thanks I'll try that when the rain stops

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plumbfisher said:

'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!

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plumbfisher said:

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 Both batteries are fully charged most of the time it seems...

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plumbfisher said:

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 Both batteries are fully charged most of the time it seems...

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Grrrrrr said:

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.

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Datatek said:

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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kermit_rr said:

this is the regulator fitted to my new alternator when I replaced it a couple of years ago

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