Mercedes-Benz Belt Tensioner Replacement Guide
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A Mercedes belt tensioner is a 30 to 60 minute job with basic tools, against $400 to $650 at a dealer and $280 to $450 at an independent shop. The reason not to leave it running is that a cracked tensioner can let go completely, and when it does it takes the belt with it, which means losing the alternator, the water pump and the power steering at the same moment.
At a Glance
- Symptoms: belt squeal or chirp, a loose or flapping belt, noise from the tensioner pulley.
- Tools: 17mm socket, E10 or E12 Torx socket, extension, a 5mm pin to lock the tensioner.
- Steps: rotate the tensioner with 17mm, pin it, remove the belt, unbolt and replace the tensioner.
- Sketch the belt route before removal and check it sits correctly on every pulley.
- Applies to many engines, including M112, M113 and M273; replace the belt at the same time if worn.
Replacing the belt tensioner on a Mercedes-Benz is a straightforward job that takes 30 to 60 minutes. Follow the video and steps below to learn how to replace a Mercedes-Benz serpentine belt tensioner.
How to Tell the Tensioner Is Bad
- With the engine idling, watch the tensioner arm. A little movement is normal; an arm that bounces or chatters means the damper is worn.
- With the belt off, spin the tensioner pulley by hand. It should turn smoothly and quietly with no wobble or rough spots.
- Look for a cracked tensioner body or a pulley that sits at an angle, which wears the edge of the belt.
- A belt that squeals even though it is new usually means the tensioner is not holding enough tension.
Work Safely
- Do this with the engine off and the key out of the car.
- The tensioner spring is strong. Keep fingers clear of the belt and pulleys while the tensioner is under load, and make sure the locking pin is fully inserted.
- Do not run the engine with the belt off; on these engines the water pump is belt driven.
What will you need
- Mercedes-Benz Belt Tensioner
- 17mm socket
- Female Torx Socket Set
- (E10 or E12)
- Ratchet Wrench
- 2″ Ratchet Extension
- Metal Pin
- Use a nail or 5mm Allen key
Optional
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- Mercedes Serpentine Drive Belt
- While replacing the drive belt is not required, it makes sense to do so simultaneously with the tensioner. It is okay to reuse the old belt if it does not show wear, cracking, degradation, or drying.
Procedure
- Park the Mercedes-Benz, set the parking brake and allow the engine to cool down.
- Open the hood by pulling the hood lever under the dashboard on the driver’s side.
- Use a 17mm socket to move the tensioner up (service) position.
- Insert a metal pin in the tensioner hole to keep the tensioner up.
A 5mm Allen wrench or bolt can be used as a pin. The pin needs to be strong enough to withhold shear force. Do not use any item made of plastic as a pin.
- Slide the poly V belt away from the tensioner.
- Remove the top Torx bolt using an E12 Torx socket and a 2″ extension.
- Remove the lower Torx bolt.
- Some models also have a shock attached to the tensioner. Remove the upper bolt from the shock to disconnect it from the engine block.
- Install the new belt tensioner. Start threading the tensioner bolts by hand until a few threads have gone in.
- Torque the tensioner bolts to the recommended manufacturer specifications. Take the figure for your engine from the service manual, since it varies with bolt size.
- Install the drive belt. Make sure it is routed properly on all the pulleys. If the belt does not reach, ensure it is not sitting on top of one of the pulleys.
- Put pressure on the tensioner’s 17mm head to move it up and remove the locking pin.
- Start the engine and ensure the belt and tensioner are working properly.
A 5mm Allen wrench or bolt can be used as a pin. The pin needs to be strong enough to withhold shear force. Do not use any item made of plastic as a pin.
Over time, the tensioner pulley can start to crack. If not replaced on time, it can fail while the engine is running. The drive belt will come off the various pulleys, possibly causing damage. The tensioner pulley bearing may also fail. When this happens, you normally hear noise from the front of the engine.
Replacing the drive belt tensioner on the Mercedes-Benz engine is relatively easy. It requires a few basic tools and 30 to 60 minutes. This guide applies to the majority of Mercedes-Benz cars. The procedure is the same for many engines, including M112, M113, and M273.
The average price to change the belt tensioner at the dealer ranges from $400 to $650. Local repair shops will charge between $280 to $450. The DIY cost to change the Mercedes-Benz drive belt tensioner ranges between $50 and $120, even if you have to buy the tools needed for this job and a new serpentine belt.
| PART NUMBER | FITMENT | |||
|---|---|---|---|---|
|
1122000970
| Interchange Part Numbers | Interchange Part Numbers |
| |
| Interchange Part Numbers | ||||
| Interchange Part Numbers | ||||
|
2722000270 (272 200 02 70) |
| |||
| 2712000270 |
|
Why a cracked tensioner is more serious than it sounds
On many Mercedes engines the tensioner body is aluminum and it fatigues and cracks around the pulley boss rather than simply going weak. When it lets go, the serpentine belt comes off at speed, and because that single belt drives everything, you lose the water pump, the alternator and the power steering pump in the same instant.
What that means in practice is that the engine starts overheating immediately with no coolant circulating, the battery is now running the whole car, and the steering goes heavy without warning. A belt that departs at highway speed can also wrap itself into the crank pulley area and take out the crankshaft position sensor wiring or damage the radiator. This is why a tensioner making a noise is a repair for this week rather than next month.
The warning signs are a rattle or chirp from the front of the engine that changes with revs, a belt that looks glazed or has started to fray at one edge, visible cracking around the tensioner arm, and a pulley that wobbles or feels rough when turned by hand with the belt off.
Replace the belt, the pulleys and the bolt together
Three points save people from doing this job twice. Fit a new belt at the same time, because an old belt that has been running against a failing tensioner is already glazed and will chirp on a new one. Check the idler pulleys while everything is apart, spinning each by hand and listening for roughness, since they fail from the same cause and replacing one without the other is a false economy given the labor overlap.
And use a new tensioner bolt where the manufacturer specifies one, torqued to the correct figure. Some of these are stretch bolts designed for single use, and a reused or under tightened bolt on a component under constant load is exactly the wrong place to economise.
Before you refit the belt, photograph the routing or sketch it. Mercedes serpentine belt paths are not intuitive and a belt routed incorrectly will either shred or drive the water pump backwards. Many engines have a routing diagram on a sticker under the hood, and it is worth finding before the old belt comes off rather than after.
If the noise persists afterwards
A chirp or squeal that survives a new tensioner and belt is usually coming from something else the belt drives. Check the alternator, the air conditioning compressor clutch, the water pump and the power steering pump by spinning each pulley with the belt removed and feeling for play or roughness. A failing bearing in any of them makes a noise that is easy to attribute to the belt.
Also check pulley alignment. If one pulley sits even slightly out of line with the others, the belt tracks against a flange and squeals, and it will wear out quickly. A straight edge laid across the pulley faces will show it. And if a charging or overheating warning accompanied the original fault, read the stored codes afterwards rather than assuming they cleared, since a full system scanner that supports Mercedes will show whether anything was logged while the belt was slipping.
Replacing the belt at the same time? See our step by step Mercedes serpentine belt replacement guide.
Frequently Asked Questions
Should I replace the belt with the tensioner?
If the belt has cracks, glazing or more than a few years of use, yes. A worn tensioner often damages the belt edge, and fitting both together avoids doing the job twice.
Can I drive with a bad belt tensioner?
For a short time, but a failing tensioner can let the belt slip or come off, and then you lose charging and engine cooling. Replace it soon.
What causes the noise to continue after a new tensioner?
Usually a worn idler pulley or a belt that is glazed or routed wrong. Check each pulley by hand with the belt off.
More guides: browse all of our Mercedes-Benz repair and troubleshooting guides.
Thank you for the instruction and it help me replaced my 2010 GLK350 idler puley, belt and tensioner as my car had chirping sound from the hood. Though I have a question, I noticed that tensioner on the mount side have two markers as well as one marker on the tensioner side, I saw some video said that the marker on tensioner side should falls between that two markers on the mount side, but mine doesn’t it’s a little bit on left side wider marker and it’s almost the same position before I replaced belt and tensioner (that was the reason I was thinking I should replace the belt and tensioner). Please let me know if this is normal or some other things I need to check out.
Thanks in advance.