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Nippon Denso alternator woes

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Hi all. We have a 3 pin Denso type alternator on an XE. When running at idle there is 13.5v at the battery and very similar at the alternator. As soon as a load is applied, such as the lights or the cooling fan, this drops to just over 12.3v at both the battery and alternator. This causes the instruments to go haywire on the dash.

The battery seems healthy and is fully charged so does this indicate a fault with the alternator? Is there a test I can do? I assume it would be an issue with the voltage regulator?

Also worth noting is the battery charge light doesn't work and never has since this alternator was installed. 

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If the battery charge light is not working, then the alternator won't be charging, if u remove the battery warning light wire off the alternator, put ignition on and earth the wire the battery warning light should could on, if it does check the battery positive is 12volts and check u have a good earth on the alternator body and engine earth, post Yr results. 

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What's the model no of the alternator, I will look it up in my alty book

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I’ve just had altenator problems ,I took it to Greens starter motor and altenator repairs in Leicester .They had it stripped fixed and ready for collection within the day all for £25 .

Domt know where you are but there’s bound to be someone similar near by .

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When you say three pin one do you mean 3 pins in the mutiplug,or are you counting the main output as a pin?

If it's got 3 pins in the multiplug one of them needs to be connected to the battery I'm sure

But as above,without the light working it won't be charging

Alternator excitation needs to occur for it to charge,this is threw the warning light.

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Thanks for the replies! I'll get the model number tomorrow. It has a positive post and then two connections arranged in a 'T' shape. One is for the charging light and one is a switched live. The switched live is connected and working. The battery light is connected and not working. Engine off battery measures 12.7V, idling battery measures 13.5V and above, idling under electrical load battery measures 12.3V. This increases if I rev it up to approx 12.9V allowing the instruments to recover slightly but they don't fully recover until the load is removed and the voltage returns to normal.

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Earth the warning light wire and see if the battery light comes on if it does, remove alty and take to reconditioners unless you can replace regulator yourself that is. 

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Thanks for the help! So I grounded the battery light wire and sure enough it lit up on the dash so I've gone ahead and ordered a new regulator. Old one was easy enough to remove so fingers crossed it cures the problems!

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The new regulator arrived today so I fitted it and started her up. The battery light now works correctly so on with ignition on and then off once running. However the voltages still aren't right. 14V with the engine running and back down to 12.4V when the lights are turned on causing the dashlights to dim and, water temp to over read by 20 degrees and oil pressure to drop by over 1 bar.

I've tried another much larger battery but the effect is still the same. Engine earth is connected to the side of the head and is in excellent condition. My readings are also the same whether taken from the battery posts or between the alternator output and it's body.

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Try also jumper lead from negative of battery to chassis, and earth's are tight for dash etc. 

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I've just been back into the garage and connected a jump lead between the alternator body and battery negative terminal but it had no effect at all on the voltage readings.

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I've taken a few resistance readings from the alternator body, earth point and behind the dash and they're all about the same. Nothing way out anyway.

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