So today I got for a drive after my Xkr has sat maybe for a week. Right away I notice a whining noise on lower rpms that gets louder as rpms go up. I check parameters and Voltage shows a drop to 12v at idle from the usual 13.5-13.8. however when rpms go up so does the voltage up to normal 14ish .
The voltage drop definitely indicates an issue with charging , and The corresponding noise is most likely a failing Alternator? Anything else it can be?
Anyone experienced a similar issue?
97k miles btw on the car

 

So I fully charged the battery, started the car the noise is still there, went to drive for 5 minutes the noise has disappeared and the voltage is back to normal. So obviously it was the alternator. Question is is it going out or is it because the battery wasn't fully charged

 

Hmmmmm........ a whining alternator is no bueno. Did you isolate the sound TO the alternator yet? Stethescope or hunk of wood does the trick.
ALSO, maybe the whine isn't the alternator and the voltage showed so low because the battery was TOTALLY charged and the alternator knew enough to not OVERcharge is? Just me spitballing here.
I think you should have a stick or something handy next time so you can more pinpoint the sound. I've always had a power steering pump problem when I got the whineys.

 

Well in a way it kind of sounded like the steering pump but it did not change with turning , and the correlation with the voltage drop leads me to think it was the alternator.

 

You could remove the belt from the alternator, but with the voltage change you noted, I'd go with the alternator in need of replacing.

 

Alternator does seem likely. The suggestion to use a stethoscope or wood stick (or a couple foot piece of heater hose) to isolate the noise source is a good one.

 

I don't recall if the early 4.2 cars had the clutch type alternator. If it does that can be the whine issue. If you have a low battery the alternator has to work harder and it buts more strain on the belts. So if it is the pully clutch type the the bearing in it is going. Easy to replace and not as costly as an alternator. If you don't have that type of pully the the strain of a low battery may cause the whine . Has the noise gone away with a fully charged battery

 

Your next step is to find the source of the whine. Mechanics stethoscope is dirt cheap online, and the right tool at this point. Piece of wood can work, but a stethoscope probe is far more precise, and having your ear on a piece of wood while it's probing near moving parts is concerning.

Alternators are simple things. Bearing wear and excessive brush wear are the most probable whine sources on the alternator. BUT other items on the belt path are just as likely. You could easily find it's a idler pulley (for example) on the belt path that's initially loud. Obviously, be really careful with any tool near moving parts.

If the scope hears more sound at the front of the alternator, it's likely bearing. If at the back, at the regulator part, it's likely brushes. Depending on how easy it is to replace only the regulator, at 100K it's worth checking. The brushes are likely >50% worn by now, anyway. IMHO, if the regulator is hard to remove (if for example, you have to pull the alternator to get to it) replace the entire thing. If you do, labor costs will eat up any savings on just replacing the regulator. But listen for the failure first.

 

XK alternators never fail. They are "sealed for life". That's why they decided to remove the voltmeter/gauge from the dashboard - you don't even need it!!

Just kidding...

 

Alex, if you end up doing an R&R on the alternator, consider a step by step with pics so we can add it to the DIY section.

 

Guy's don't you have shops that rebuild and service alternators?

Ask as on my X100 and my Audi A6 my local mobile mechanic removed them, took them to a specialist where they replaced the regulator and brushes and then refitted.

Average cost for both including removal/service/refit was £80, turnaround was 2 days.

Usually places that specialise in batteries do this here in the UK

 

Looks like a pain to remove. Have to remove one of the engine mounts also. Ouch.

 

Have you ever run the car with a discharged battery before? If so, did it whine then?
If the battery was low on voltage, it takes a while for the alt to get it boosted back up, and it produces less amperage at idle. Nothing unusual about the 12V at idle right after starting with a low voltage condition. After it was charged up, what it the voltage at idle?
When an alternator is under heavy load, like pushing a lot of amperage, they can make a whining noise.
If you are concerned, take it to a shop and have it load tested to see if the noise can be replicated.

 

So to answer some of the questions.
I I have never seen voltage drop below 12.9 with a car idling before this, and now it was around 12 and even slightly under, this is when the alternator was making the noise. Battery couldn't have been that low since it started perfectly fine but it was drained I'm sure. Now the voltages 13.8 ish with the car idling. And no more noise so go figure ...
Really not looking forward to replacing it if I don't have to

 

Replacement isn't a 30 minute job, but really isn't that bad.