I'm by no means a good mechanic, and I'm still very much in the process of learning in general. Take this as a guide through my personal experience, others may pipe in with tricks and tips which I haven't yet learnt.
A short back story, my girl's been struggling to charge her battery over the past month, just a couple of weeks ago I charged the battery fully and the problems seemed to disappear. A couple of days ago before a trip down the M6 I was greeted with a battery warning light, and odd belt/bearing sounds from the bay. I came to the simple conclusion my alternator was shot, more info here.
This guide has some basics on confirming a badly performing alternator, replacing the alternator itself, as well as swapping out the serpentine belt, should you choose to do so.
Before we get stuck in, note: my bay's still filthy from Brands Hatch camping.
- Sockets & Spanners: 10mm, 12mm, 14mm
- Assorted socket extensions recommended.
Testing the Alternator Output:
There's numerous ways to check the alternator output, the simplest is to check the battery voltage with the engine off, and then compare it to when the engine is running. Generally we're looking for 12.6v as the battery's output on it's own, with under 12v implying more than 50% drain already on the battery.
For the voltage with the alternator running, 14.4v is often considered the perfect point - with anywhere from 14.2-14.7v being in the generally accepted tolerance under no load. As long as the alternator's output with lights, stereo and typical use electronics running is 13.8v or above, I'd surmise she's pretty healthy. Others may speak more on the exact figures.
First off, bonnet up, battery terminal cover off, and the multi-meter hooked to the corresponding terminals.
Not a bad starting point considering I ran 80 miles on battery power getting the girl home, probably around 75-80% charge left. Let's start the car:
It appeared like my alternator was indeed putting out a bit of charge, the numbers steadily climbed to just above the figure pictured, before all of a sudden plummeting down:
For my purposes it was pretty clear, the alternator was in fact being naughty and needed to go. @Ichiban explained on the phone the likely component that went, instead of paraphrasing I'll let him fill in the blanks.
I have an alternator from my old '03 Type-S to get me going, it's 11 years old with 89k. Another OEM Denso unit, which tend to be extremely reliable - I've just been unlucky with my '07.
Removing the Belt:
Before we start ripping everything apart, remove the negative battery connection:
The first thing we're going to do is remove the serpentine belt which powers all the accessories, power steering, air conditioning etc. To do so, we need to unlock the tensioner pulley to allow the arm it's sitting on to swing and give us some slack in the belt. One 12mm bolt behind the power steering pulley, deep impact socket recommended.
Now the tensioner can be moved, we're going to use a trick I learnt from @SpeedyGee. The tensioner can be pretty well stuck, or at least, mine was nearly seized. We're going to get a 14mm ring spanner and anchor another spanner on the end to use as leverage. The picture below shows my backwards first attempt, apparently I forgot to get the right shot - you'll get the combination right.
The aim here is to turn clockwise which will allow the tensioner to swing back/away from the belt and give some slack. The initial force required to budge the tensioner seemed to be worse for me than moving it once it was out of it's natural resting position - or more, once you get it going, pop your body weight behind it and you'll be fine.
We just want enough slack of the belt to slip it off the top pulley.
With the belt dislodged, we can move onto the alternator itself. If you're going to be replacing it, feel free to completely remove it at this point - I left mine in till later.
Removing the Alternator:
Starting with the obvious pieces, disconnect the green loom plug and remove the rubber cover from the positive feed.
Next, use a 10mm ring spanner to remove the nut behind the cover and pull off the ring connector. Make sure to put the nut in a safe place if you'll need to re-use it.
Lastly, remove the cable tie for the two sleeves from the bracket.
Next remove the long 12mm bolt securing the alternator from the top.
Underneath there's another two 12mm bolts, it's quite the squeeze but your saving grace is that once they're cracked with a long extension, they are fairly easy to unthread by hand.
The opposing side is even more of a squeeze:
The alternator itself should now be free, getting it out however is extremely tight. I found that removing the power steering bottle from it's holder helped, just pull straight up.
I then also removed the bracket it attached to for further clearance, one 10mm bolt.
I didn't take any pictures of sliding the alternator out, I was too preoccupied with cursing under my breath. It's certainly a very good exercise in patience, you will however manage it with a lot of contortion. Hopefully another member knows a secret easy way!
Replacing with New:
On the left, my "higher mileage", "much older", "working fine" alternator from the '03 looking in better nick than my fairly young '07 alternator.
Like many DIYs, fitting the new components is pretty much just the reversal of removing them, and this is no exception. The point worth covering in more detail is replacing the serpentine belt after the alternator, which takes a bit of getting used to.
As a point of reference, here's a diagram of the belt routing. Suffice to say I got it wrong from memory and had a moan in the club Facebook group.
I replaced the third-party 10 month old belt on my '07 which was replaced just before I brought the car with a 11 year old OEM from my '03. Because I'm nuts obviously, more seriously I wanted to rule that out as the point of failure - and because I intend to go for a lightweight slightly underdriven pulley set-up in the future with new belts.
I didn't take any pictures of the belt replacement as it's fairly straight forward. Feed the belt around the pulleys as illustrated above. Remember smooth surfaces want the back of the belt, and ridged pulleys the perforated side.
Leave off the top power steering pulley and using the same double spanner method, push back the tensioner and loop it over. A second set of hands would make this far easier:
Once the belt and alternator is replaced, it's time to start the engine and check that everything is running smoothly. Any whines, check the belt is seated correctly.
Last but not least, pull the multi-meter back out and check the voltage.
That looks pretty perfect to me.