2008 Buick Lucerne Belt Diagram and Routing Guide
The 2008 Buick Lucerne came with a few different engines, but the vast majority of them used the 3.8L V6 (VIN code E, the L77). That engine has one serpentine belt and a timing chain underneath the plastic covers. There is no timing belt to worry about, which is one reason this engine has a reputation for being nearly indestructible if you keep oil in it. Here is the serpentine belt routing for the 3.8L V6. The belt path runs around six main components plus the tensioner. Starting at the bottom with the crankshaft pulley: Crankshaft pulley (bottom center) goes up the front side to the A/C compressor pulley (driver's side, lower middle) then back down and around the water pump (behind the crank pulley area, not directly in the belt path but the belt tensions against the water pump shaft housing) actually the correct path is simpler than that description implies. Let me lay it out cleanly.
The belt routes like this on the 3.8L L77: Crankshaft pulley (bottom) ——> Power steering pump (top left, passenger side of engine) —> Idler bracket/pulley (center, below the power steering pump) —> Alternator (top right, driver side of engine) —> A/C compressor (lower left, driver side) —> Tensioner pulley (lower center, spring-loaded) —> back to the crankshaft pulley. The exact sequence depends on whether you have the A/C option or not. Non-A/C cars skip the A/C compressor pulley and the belt takes a slightly shorter path. Most Lucernes had A/C, so if yours doesn't, the belt is about two inches shorter and routes around the alternator then directly back to the tensioner and crank.
I've done this belt swap on at least a dozen of these cars. The tensioner is a spring-loaded unit located at the lower center of the engine, and it has a 15mm bolt head where you slide a ratchet or breaker bar to compress it. You rotate the tensioner clockwise (toward the engine) to release belt slack, then slip the belt off. Reinstall is the reverse — compress the tensioner, slide the belt on, and double-check that every rib of the belt is seated in every pulley groove before you let the tensioner spring back. One thing people mess up: the idler pulley between the power steering pump and the alternator. It looks like it just sits there doing nothing, but it's actually the pivot point that keeps proper tension on the long run of belt. If you skip routing the belt around it, the belt will fall off the moment you start the engine. I've seen this happen more than once when someone tried to "eyeball" the routing instead of following a diagram. The standard belt part number for the 3.8L L77 is GM part 12635799 or the aftermarket equivalent Gates K060062. Dorman also makes one under part number 10570. They all cost roughly the same — about eighteen to twenty-five dollars at any auto parts store. Don't buy the cheap no-name belts from the gas station. The rubber compound on these things matters because the engine bay runs hot and the belt is right above the exhaust manifold on the passenger side.
Get the Full Details
If you're trying to figure out the routing on a car that already has the belt on it and it's covered in grease, take a photo before you loosen the tensioner. I know that sounds obvious, but these belts get caked in road grime and the grooves blend together. One time I was working on a Lucerne at a friend's house with no internet access and no diagram printed out. I took a picture with my phone, but the angle made it look like the belt was routed wrong. I installed it the way the photo showed, started the engine, and heard a loud squeal within three seconds. The belt was on every pulley except the A/C compressor — it had slipped off during the photo because the tensioner wasn't fully engaged. I redid it using the tensioner release bolt as a reference point instead, and it was fine. Taking a photo of the routing while the tensioner is relaxed is misleading. Take the photo with the belt fully tensioned and sitting properly in every groove. Here's a rough ASCII-style diagram to help visualize it. This is the A/C equipped 3.8L:
[PS Pump] [Alternator]
o---------------o
\ /
\ /
\ /
\ /
[Idler]---[Tensioner]
| |
| |
o-------o
[Crank] [A/C Comp]
The crank is at the bottom. The tensioner is spring-loaded and pushes the belt tight against the idler pulley. The idler pulley is the one most people miss when routing by memory. Make sure the belt goes around it. A few practical things to know. The tensioner bearing on these engines typically lasts between 60,000 and 90,000 miles. If you hear a rumbling or grinding noise from the front of the engine that gets louder with RPM, the tensioner pulley bearing is going bad. Replacing just the belt won't fix it — the noise will come back within a few thousand miles. The tensioner assembly itself is GM part 12644511 and runs about forty to fifty dollars. It's a straightforward bolt-on: two 15mm bolts hold it to the engine block. The whole job takes about twenty minutes if you already have the belt off and another twenty to put it back on. The power steering pump pulley is another failure point. It's a small pressed-on pulley that sits on the end of the power steering pump shaft. Over time the belt friction heats the hub, the press fit loosens, and the pulley starts spinning on the shaft independently of the pump. You'll notice the power steering warning light coming on intermittently and a squealing noise that doesn't go away after the belt is replaced. The fix is either replacing the pulley (part number 12643386, about fifteen bucks) or replacing the entire power steering pump if the shaft splines are worn. I've seen both. The pulley-only fix is fine if you catch it early, but if the shaft is glazed or the splines are rounded out, you'll be doing the job again in six months.
For the non-A/C version of this engine, the belt is shorter and the routing skips the A/C compressor entirely. The belt goes from the alternator directly to the tensioner, then down to the crank. It's easy to confuse the two layouts because the pulleys are in the same positions — only the belt path differs. If you put an A/C belt on a non-A/C engine, it will be too long and will flop around. If you put a non-A/C belt on an A/C engine, it will be two inches too short and won't reach. If you need a downloadable reference, General Motors doesn't publish official belt routing diagrams on their public website anymore. The best free sources are the Haynes or Chilton manuals for the 2006-2011 Lucerne, which you can find as PDFs through various automotive forums. The Buick-Cadillac registry (buickregistry.org) also has scanned factory service manual pages with the exact routing. I usually screenshot those pages and save them to my phone before going into the engine bay. Phone screens get greasy fast, but a quick reference image is better than guessing. One last thing that isn't obvious: the 3.8L V6 is an interference engine. If the timing chain jumps a tooth — which is rare but happens when the chain stretches beyond specification — you will bend valves. The timing chain itself is rated for the life of the engine under normal conditions, but if you've been running low on oil or extending oil change intervals past 7,500 miles, the chain tensioners can wear out. A stretched chain on this engine makes a ticking sound from the front of the engine at idle that disappears when you rev it. That's the tensioner trying to take up slack. If you ignore it, the chain will skip and you'll be looking at a valve job or a head replacement. Keep the oil changed on time and the chain will outlast the car.
The serpentine belt should be replaced every 45,000 to 60,000 miles as a preventive measure. Check it visually during oil changes — look for cracks in the ribbed side, fraying at the edges, or any glazing that makes the belt look shiny. A shiny belt is a slipping belt, and a slipping belt will fail without warning. I've had belts snap on these cars at 55,000 miles that looked fine from the top. The cracks were on the inside where you couldn't see them without removing it. That's why preventive replacement is worth the twenty dollars and twenty minutes of labor.
