Alternator MERCEDES-BENZ A-Class (W169)

This has been discussed a few times, usually the search function helps. But since you're asking, it's not exactly a walk in the park on the W169. You need specific tools and a lot of patience because the engine bay is incredibly tight on these sandwich floors.

Right, the space is the main issue. The A-Class is notorious for this. You don't necessarily have to remove the whole alternator, which is a blessing because getting the alternator out is even worse. You can change the pulley in situ if you have nimble fingers. You definitively need the special 33-spline socket tool (usually T50 or M10 bit combined with the sliding outer sleeve). If you try to do it with pliers or a standard socket, you'll just strip it.

I did this job last November on my 2006 A180 CDI with about 180k miles on the clock. Honest advice? If you aren't confident with spanners, take it to a pro. It took me nearly an entire Saturday and I have been working on cars for years. The problem started with a chirping noise on cold mornings, which eventually turned into a nasty metallic rattle at idle. It sounded like a bag of bolts.
To get to it, you need to go in through the right-hand wheel arch. Remove the wheel and the plastic liner. Even then, you are working largely by feel. I didn't drop the engine, though the official WIS manual suggests lowering the subframe slightly to get better access. I managed without it but my knuckles were absolutely shredded by the end of it. The gap between the alternator and the chassis rail is tiny. I bought a laser tool set for the pulley, the one with the short bit because a standard length bit won't fit in that gap.
One thing to check while you are struggling in there is the tensioner. Mine was bouncing around like crazy because the seized pulley wasn't dampening the pulses anymore. I replaced the pulley with an INA part (they make the genuine ones for Mercedes anyway) and also swapped the tensioner assembly and the belt for a Contitech kit. It is false economy to leave the old tensioner because if that fails later, you have to do all this work again. The hardest part for me was actually getting the new belt routed correctly around all those pulleys blindly. Take a photo of the belt path before you take the old one off!

@Archie Adams Is the tool a standard size? I see some kits on Amazon with different lengths. You mentioned a short one?

@a_evans Yes, get the shortest one you can find. The clearance to the side member is the limiting factor. If the bit is too long, you can't get the ratchet head on it. I think the total length of my tool was about 60mm.

Also verify if it is actually the pulley. Remove the belt (15mm spanner on the tensioner to release it) and spin the alternator by hand. It should spin freely one way and lock the other way. If it locks both ways, it's seized. If it feels gritty or makes a noise, the bearings are shot. On my A170, the noise was so bad I thought the timing chain was rattling.

I did this last month. One tip that saved me: access from below is actually better for the tensioner bolt. I put the car on ramps. You still need to work through the wheel well for the pulley itself, but for releasing the belt and changing the tensioner, coming from underneath gives you more leverage.

I had a complete nightmare with this on my wife's A200 CDI last year. I ignored the rattling sound for too long, thinking it was just a loose heat shield or something trivial. Big mistake. The pulley actually sheared off while she was driving, threw the belt, and she lost power steering and water pump drive instantly. Had to get it towed.
When I got into it, the alternator shaft threads were damaged because the pulley had been wobbling before it let go. I ended up having to replace the entire alternator, which on the W169 involves lowering the engine subframe because it physically won't fit out through the top or bottom gaps otherwise. That turned a £40 pulley job into a massive operation.
If you are doing just the pulley, make sure you clean the threads on the alternator shaft really well before screwing the new one on. I used a tiny bit of thread locker just to be safe, though the rotation naturally tightens it. I went with a Bosch alternator in the end because the Valeo one that came off was toast. For the belt, I always use Gates, never had one snap. Also, inspect the idler pulleys while the belt is off. They are cheap plastic things and if they feel dry, swap them out. It’s crazy how much noise a bad £15 pulley can make.

@Ethan_Scott Ouch, that sounds expensive. Lowering the subframe is definitely DIY territory limits for most people without a lift.

Just a quick note on the plastic cap that covers the pulley face. Don't forget to put the new one on! It keeps road grime and salt out of the clutch mechanism. The new INA pulley should come with one. My old one was missing, which probably explains why it seized up so early.

My local mechanic quoted me 3 hours labor for this. Seeing these comments, it seems fair. He uses Luk or INA parts.

@Archie Adams thanks for the tip on the belt routing. I spent 20 mins trying to figure it out upside down.

Make sure you disconnect the battery negative terminal before you stick any metal tools down there. The main B+ cable on the alternator is live and very close to where you are working. You don't want to weld your ratchet to the chassis rail.