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Strange voltage fluctuations


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Hi all

strange problem with the charging on my 300 Tdi. It is often left standing for 3 weeks at a time, and this time home struggled to start. It has a twin battery system and a 100A alternator.

Ran it for a few days without any issues, then one evening after using head lights, it struggled to start after a 20 minute stop.

Checked the voltage this morning before starting, and both batteries were a 12.7v which is a bit low compared to normal. Fired up no problem, and voltage rose to 14.02v. Again a bit low compared to the normal figure of 14.2v. Adding electrical load dropped it to 13.3v. On one occasion. Trying the same test it dropped to 12.5v with headlights and spotlights. Strangely starting the heater blower seemed to poke it in to life and he voltage then cam up to 13.6v

the alternator is about a year old, and was a genuine parts replacement not from a blue box.

Does this sound like the diode pack giving up, or should I be looking elsewhere?

Both batteries have been charged up overnight so will refit hem today, and monitor what is happening.

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I'd start by getting your batteries drop tested, that way you can easily rule them out. How do have them connected? Split charge or just wired in parallel?

Then go about and clean all your earth's, and alternator connections. It seems intermittent, so I'm not convinced it is alternator related.

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Thanks Bowie

Batteries are connected via an x-eng split charge relay, nothing complicated. The earths are tied together, and one has an earth direct to the winch, the other direct to chassis. Will get underneath while the rain has stopped and check the terminations.

Should have added that while maintaining a steady load and rpm, the voltage fluctuates by about 0.7v. It used to be rock steady as long as the load didn't change.

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Cool, and yup, remove, clean and refit the terminations, don't just check they are tight ;):)

IIRC the charge wires for the alternator come via the starter terminal? If so check here as well, yes, I know it is awkward...

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Thanks gents

You hit the nail on the head. The main earth runs from the battery to the gearbox with a clamp half way along to give the chassis earth. Looks like corrosion here. I ran another earth from the body to the engine block, and now back up to 14.2v at tick over no load. Drops to 13.9v at tick over with every conceivable electric load on. Climbs back to 14v at a fast idle, so all looking like back to normal.

in incidentally I picked up on the varying voltage while driving because of the Sparkbright LED voltage monitor I had fitted, so thank you Simon As well!

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