Home > DIY > SLK > R170 (1996-2004) > How to Replace the Alternator on a Mercedes-Benz M112 and M113 Engine: C-Class, E-Class, CLK and SLK 1998 to 2007

How to Replace the Alternator on a Mercedes-Benz M112 and M113 Engine: C-Class, E-Class, CLK and SLK 1998 to 2007

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In this guide (8 sections)
  1. How to replace the alternator on a Mercedes-Benz.
  2. Step-by-step instructions on how to replace the alternator on a Mercedes-Benz
  3. Torque Specifications
  4. How much does a Mercedes-Benz alternator replacement cost?
  5. Applicable Models
  6. Related Part Numbers
  7. Related Reading
  8. What Scanner Can I Use to Troubleshoot Mercedes-Benz Alternator Problems?

How to replace the alternator on a Mercedes-Benz.

If your instrument cluster displays a warning such as ‘Battery/Alternator Defective. Visit the workshop,’ do not assume that the alternator needs to be replaced immediately. First, test the battery and charging system to confirm the cause of the warning: here is how to tell a failing battery from a failing alternator. In this article, we will show you how to replace the alternator on a Mercedes-Benz. Replacing the alternator at the dealer can cost you over $1000, but if you do it yourself, you can do it for as little as $90, so keep reading. Alternator replacement cost varies significantly depending on the vehicle, alternator output, brand, labor rates, and whether the unit is new, remanufactured, or OEM. It’s not a complicated job, and we know you can do it. This guide demonstrates alternator replacement on a Mercedes-Benz W203 C-Class equipped with an M112 engine. The general procedure is similar on other M112/M113 applications, but access, component removal, alternator type, and electrical connections may vary by model and engine.

Before buying an alternator, I’d recommend:

  • Inspect the serpentine belt and tensioner.
  • Check battery condition.
  • Check battery terminals and grounds.
  • Measure battery voltage with engine off.
  • Measure charging voltage with engine running.
  • Scan for relevant fault codes.
  • Check alternator output/current if appropriate.
  • Confirm the alternator is actually defective.

For this job, you will need a female E-Torx tool set, which you will need to remove the two bolts that hold the alternator in place. Note the position of the spacer sleeve/bushing at the alternator mounting bolt and make sure it is correctly positioned before tightening the bolts. Below is a picture of these sockets, and if you need a set, you can find them on Amazon for under $20.

female torx socket setamazon check price

Step-by-step instructions on how to replace the alternator on a Mercedes-Benz

Applies to the C-Class, E-Class, CLK-Class, and many other models with the V6 M112 and V8 M113 engines.
    1. Open the hood.
    2. Switch the ignition off and disconnect the negative (-) battery cable before working on the alternator. Make sure the disconnected cable cannot accidentally contact the battery terminal. FINDING THE BATTERY FIRST. Battery location varies by model and chassis, so confirm where yours is before you go looking for a terminal. On the C-Class W203 shown in this guide it is in the engine bay, behind the cabin air filter housing. On the 1996 to 2002 W210 E-Class it is not under the hood at all: it sits beneath the rear passenger-side seat cushion. First, we need to remove the cabin air filter housing. Push up the 3 clips in the picture below and pull out the housing. accessing the battery on a mercedes benz c class_mercedes_Benz
    3. Remove the bolt on the NEGATIVE battery terminal clamp and disconnect the cable. You will need to loosen the 10 mm bolt; no need to remove the bolt altogether. DSCN0584_mercedes_Benz
    4. Remove the engine cover. It comes right up. It has clips that hold it in place and no screws.remove engine cover
    5. Remove the serpentine belt. Read this article and watch the video on replacing the serpentine belt.
    6. Remove the protective cap from the alternator B+ (Terminal 30) connection. Remove the retaining nut and disconnect the B+ cable. This is the terminal the negative cable was disconnected for. B+ is wired straight to the battery positive and is live whenever the battery is connected, with no fuse and no switch in between, so a wrench bridging that stud to anything earthed will arc, weld itself in place and can set fire to the wiring. With the battery negative off, the same slip does nothing at all. Then disconnect the smaller Terminal 61/D+ electrical connector.
      mercedes bosch valeo alteranor_mercedes_Benz
    7. You should see a nut. Remove the 13 mm nut on the back of the alternator. See the close-up picture below. positive cable to alternaor on mercedes benz_mercedes_Benz
    8. Unplug the connector that goes into the back of the alternator. Press on the tab and pull it away from the alternator. unplug the connector alternator_mercedes_Benz
    9. Remove the two bolts that hold the alternator in place using the female E-Torx sockets we listed in the required tools section above.serpentine belt on alterantor pulley_mercedes_Benz
    10. Remove the alternator. This is the part that takes effort. The alternator can be tight in its mounting bracket. If necessary, carefully work it free without damaging the mounting bracket, electrical connections, or surrounding components. pulling out the old alternaor_mercedes_Benz
    11. Install the new alternator. Here are some pictures of our new alternator, which we bought for $115 on Amazon. Buying the unit yourself is where the money is saved: online prices run well below dealer prices and are often below the local parts store too. Note the position of the spacer sleeve/bushing at the alternator mounting bolt and make sure it is correctly positioned before tightening the bolts.order now on amazonback of mercedes alteranor_mercedes_Benz cost of new mercedes benz alternator_mercedes_Benz
    12. Install the new alternator and tighten the two bolts to 42 Nm.
    13. Connect the electrical cable to the alternator. Tighten the nut for the electrical cable (positive +) between 15 Nm and 18 Nm.
    14. Plug the electrical cable on the back of the alternator and make sure it clicks when you push it in. That way, you know it is locked in place.
    15. Route the serpentine belt according to the vehicle’s belt-routing diagram and verify that the belt is fully seated in every pulley groove before starting the engine. Refit the engine cover, then reconnect the battery negative cable.

Torque Specifications

  • 2 Bolts that hold the alternator in place. Max 45 Nm.
  • 1 Bolt on the back of the alternator. Between 15 Nm and 18 Nm.

How much does a Mercedes-Benz alternator replacement cost?

The answer depends on where you live. With that in mind, we will give you a general idea. First is the price of the alternator. If you buy it at the dealer, you can spend around $1000 to replace the alternator. If you do it yourself, you can save hundreds and replace the alternator for 1/10 of that. How?

First, buy the alternator itself online from your favorite online retailer. You can find a new alternator online for $100 to $200. Find one for your Mercedes-Benz on Amazon; see the link here. We, the ones who replaced it, bought it on Amazon for $115, and it worked out great.
Replacing the alternator on a Mercedes-Benz should not take more than 2 hours. For an experienced technician, the job may take around 1 to 2 hours on the vehicle shown, while a first-time DIY repair may take considerably longer. Even Valeo or Bosch alternators, with the Original Equipment installed on the Mercedes-Benz cars, can be bought for a fraction of what the dealer charges. For example, check out the links below.check-price-at-amazon


Applicable Models

Years: 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, and 2007
E Class Models: E320 E430 E55
C Class Models: C240 C320 C32 C280
CLK Class: CLK320 CLK430 CLK55
SLK Class: SLK320 SLK32 AMG R170
CROSSFIRE

Bosch Alternator 12 120 Pulley Rotation Polarity
Bosch 0124515046, 0986042550, 0124515132
Delco DRB2550
Chrysler 5097755AA, 5097756AA
Mercedes 0111546802, 011-154-72-02
Mercedes A011-154-68-02, A011-154-72-02
Valeo 2542317, 437222, 542317, SG12B021
Bosch 0-124-515-056, 0-124-515-132, 0-986-047-540

Alternator part numbers vary by model, engine, production date, electrical equipment, and alternator manufacturer (Bosch or Valeo). Always verify the replacement alternator by VIN or by matching the part number and amperage of the original unit.

What Scanner Can I Use to Troubleshoot Mercedes-Benz Alternator Problems?

If you’re experiencing problems with your Mercedes-Benz, a YOUCANIC full system diagnostic scanner can help you troubleshoot the issues. This advanced diagnostic tool can read and interpret the fault codes stored in your car’s computer system, providing detailed information about any problems with the engine, transmission, brakes, or other critical systems.

YOUCANIC UCAN II full system diagnostic scanner

Please use the comments section below to let us know how your job went. Feel free to post any recommendations or suggestions to help others.

Next step

Run a full system scan on your Mercedes-Benz

Before you buy parts, scan every control module, not just the engine. It shows the root fault behind the warning lights and tells you whether the problem is still active. See how to do a full health check.

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