M44NKY said:

Hey guys,

On My V8 3.9i Disco 1

Found that when I had alot of electrics on the stereo would switch off!. Checked the Battery an it is fine, But when engine running battery voltage went down to 11.6 volts.

At 11.6 volts the alternator is barely charging, and I guess the volts went down far enough that the stereo switched itself off to prevent damage caused by running on low voltage.

Suspect Alternator! Fitted a new one. Ran engine and voltage was the same, left vehicle idling and charge rose to 13.1 volts. I then noticed some smoke comming from new alternator pulley area. When I switched off and checked it, it was F**king boiling hot! To try and prevent any serious damage to it.

That is absolutely not right. What was smoking? Was the belt burning on a stuck pulley? Take the belt off and check ALL the pulleys are free to rotate apart from the crankshaft pulley of course.

Put the old one back on! vehicle still runs and runs at about 12.6 volts, starts ok in morning too!

12.6 volts is still too low if that's an accurate measurement (using a good meter) with the engine running about 2,000 rpm. It should hold more volts than that even with EVERYTHING switched on.


Can anyone shine a very bright torch on this problem. Charge fuse is ok!

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M44NKY said:

Many thanks for getting back CharlesY.

I have put the old one back on! The vehicle still runs fine at the moment. Should I Just carry out a continuity check from the alternator to the Batt poz!

You must be absolutely sure the main cable from Alternator to battery + is perfect all the way, both ends, and the fuse. A continuity check will not prove a lot - you may have continuity in a very dodgy circuit that may carry upwards of 100 amps occasionally. That big cable must be perfect.

It was the alternator shaft that was hot all others were fine!

It was new! why did it heat up? Was the drive belt loose? If it was, as soon as you started the engine the alternator would "see" a low battery voltage and try to hoick a lot of current into the battery. In doing so the grip-strain between drive belt and pulley would be HEAPS, and if the drive belt wasn't tight enough or was oily, it would slip, and friction would make heat and smoke damed fast.

Can the Alternator be run without these leads conected to check it's output?

NO! NEVER run an alternator not connected to a battery. It WILL burn itself out, minutes or seconds, anyone's guess.

If the Battery is bad, would this give me a false reading for the charge output?

Yes, to an extent, but you say it starts the engine?

There are 3 wires, 2 small one large. Large one goes via the charging fuse to the Battery Positive, small ones are rev counter and god knows what the other one is but it is on the correct pole.

Probably for the wee red light known as the IGNITION light ...

I will of course take your lead and check once more the connections.

Yes, and drive belt tension. I suspect it possible your hot pulley and smoke were caused by a slack belt. Keep that in mind as a possibility. Refit the new alternator and get the belt TIGHT, and try again.


CharlesY

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Dave 2000 said:

It might be an idea to bench charge the battery for 24 hours and do a specific gravity test, don't get too anal about the numbers just look for discolouration of the water and/or a low reading on one cell clearly lower than other cells. If all good then putting the battery on fully charged will take the initial load of the alternator.

This should be done on all alternator swaps, the reason being if you look at the brushes on a new or reconditioned alternator you will see they do not match the curve of the armature perfectly and will take a few 'miles' to bed in, during this 'running in' period high loads cause premature burning of the brushes/armature as the contact area is reduced.

regards

Dave

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