When I bought my car a few weeks ago, it had a brand new alternator put on for purchase. Unclear exactly what the problem with the old one was, but the one on there is definitely new. A few days after purchase I had some belt squeal on startup along with the voltage rising and falling fairly significantly with rpm (12-14v between idle and on throttle) and realised that the new belt was pretty loose - twistable nearly 180 degrees. Tightened it up, the squeal went away, and if I'm honest I didn't pay much more attention to the dial having assumed the fluctuation was down to the loose belt.

That was until a whine developed a couple of days ago that sounded as though the car had spontaneously developed a supercharger. Audible at low revs just listening to the engine it rises and falls with revs, and upon a little investigation the very same noise comes through the speakers with a little volume. And sure enough, the voltage dial fluctuates significantly with rpm, sitting at a terrible 12v at idle and getting up to 14v when on throttle.

It seems unlikely (although I'm sure completely possible) for the brand new alternator to be faulty already, and I am assuming that the two symptoms - voltage fluctuation and whine - are probably somehow related. The question is how? I'm not very good with electrics so am not sure if this is an obvious case of faulty alternator, or if a ground or connection issue somewhere is the issue.

Thank you in advance!

 

the whine would almost certainly be a faulty noise suppressor.
they exist to stop exactly what you are saying.

can't help you on the voltage fluctuations as i also have that.
alt came from a known working car. seems great when cold, potentially starts to fail a little when warm.
one day i will get a rebuild done for it

 

There are 2 components to alternator noise suppression , the 4 diode package inside the alternator and the external noise suppressor . You should hear the noise better on an AM stereo station . You can remove the pigtail connector plug on the noise suppressor ( about a foot maybe from the alternator and see if the noise changes ) . Apparently not all X300s have the external noise suppressor from a contributor that had his missing ( ? ) in England , One of the wires to the noise suppressor is attached to the large B + alternator post , the other wire may have been mis place

A replacement alternator can be run on the auto parts store spin up machine in the back room and the machine will give you a fault readout before installing , new and rebuilt alternators and batteries junk these days

X300 alternator as 120 amp Denso brand original equipment







Parts here as well as the IN435 Voltage regulator on the X300 , should be this specific VR as a fast response vs. the other number as IN225 ( ? ) E - bay venders would sent me ( stamped on VR )

# IN435 Voltage Regulator for Denso Alternators, 12 Volt,On: Asuna (1993-1993) , Geo (1990-1993) , Isuzu (1990-1993) , Jaguar (1993-1997) , Jaguar - Europe (1994-1997)

The alternator can be run on car at the local auto parts store and will give you a test of not pure enough DC alternator output as it is converted from AC generation , How it would discern if the external noise suppressor at fault is a question

The alternator belt on a professional level should be checked for tension by a 3 fingered tensiometer but is normally done by feel or deflection amount

The tension level may be contained in the shop manual forthcoming

See post # 4

Repair Manual - Jaguar Forums - Jaguar Enthusiasts Forum

One thing to consider is if alternator pully bearing loaded too much ( pully warm to touch .........On A Stopped Engine ) or a slipping harmonic balancer ........more on that simple test later

No belts removal for test



There is an alternator / battery cable loose tie point on the starter solenoid that has tripped up a few . This can be seen through the engine intake pipes from above that someone made a pic of

 

Here’s a story to add in to your mix.



About 3 years ago, a friend of a friend brought her car to me. It had a brand new alternator fitted (by someone else). 2 days after fitting, the battery had gone dead. She bought a new battery and had it fitted. 2 days later it was dead again.

I did some diagnosisis. The alternator was faulty. It was replaced under warranty by the supplier with an identical unit from same company. I fitted it for her.

6 months later, she came back. Same symptoms. I did diagnosis – alternator faulty again.



Alternator was replaced by me. This time with a Denso unit. No more problems since.



Moral of the story is – just because a unit is new doesn’t mean it is good. If a cheap Chinese crap part has been fitted (as in this case) then be suspicious ……

 

Cross street neighbor going through long pains on a newer Jeep with Batt / Alt issues that may come down to today's parts quality that doesn't help owners anger level and many tows

On the X300 in recent years seem to be cheaper than past alternator new builds available different than Denso , wonder how they by brand are holding up , Kept my Denso ( 2 ) and rebuilt by alternator shop and myself

 

I got an Alt from A-premium.com. I'm sure it's Chinese but it's been in for a year and so far so good. I had a real hard time getting the plug connected at first. I had to pull it back out and realized that the case was preventing if from going in. Bent back the case a bit and it went in just fine. I do plan on giving a rebuild of the original a shot this winter. Here's a video I found.

 

Thank you so much for the detailed information all. I'm fairly sure the belt tension is about right, being *just* twistable to 90 degrees by hand. Everything certainly looks OK while running. I will check the posts and connections. I do have the noise suppressor.

The thing that gives me pause from immediately swapping the alternator is the question of whether it was replaced to try fix a different problem that I am now facing. But this morning things were worse with it really not wanting to budge from 12v at idle in traffic with lights on, to the point where I sat with a bit of throttle to keep things going. All of the dash lights were dimming and brightening with this.

So an immediate replacement may have to be the first port of call.

 

Check the harmonic balancer test as an easy look at before pulling alternator , Alt only way comes out from top but the long pivot bolt ( 17 mm ) from bottem . The clevis may pinch and not let the alternator remove , keep track of small wire attached to the alternator housing back ( not round plug ) as noise suppressor ground

You would have to load up alternator and A / C on to stress the HB and then check the lines line back together as you turn engine off

Contributor Al made the pic

 

Checked the harmonic balancer with some chalk and no problem there so I swapped over the alternator and sure enough we’re back in business! There are no obvious identifiers on the old (new!?) alternator that I can use to name and shame the poor quality, but I’ll take another look at some point.

Incidentally and contrary to what I’ve read, I had no difficulty at all in removing and refitting entirely from the top i.e. under the bonnet. Other than the lock nut on the alternator tension adjuster which is a strange size - possibly 9/16”.