If your W203 is at or over 100,000 miles (160,000 kM) you should attend to these items to prevent future costly repair bills:
- Check/replace the voltage regulator brushes. The Bosch alternator in your W203 has high output (the V6 models produced 150A) but the regulator brushes wear and need replacement after 100,000 miles. Unfortunately due to the location of the alternator, it’s not possible to remove the brushes without first removing the alternator from the car. It’s kind of a bear of a job, but if you don’t do it, you risk being stuck somewhere.
- On V6 models (C240, C320) replace the belt tensioner. The factory tensioners had a high failure rate, you may want to proactively replace the tensioner, otherwise you may find yourself stuck with a shredded belt. While you are doing the tensioner, install a new serpentine belt.
- Replace the plugs and plug wires. Your W203 came with Platinum spark plugs good for 100,000 miles. The V6 models used 2 per cylinder. A special tool is needed to remove the plug wires. Be warned the plugs at the two rearmost cylinders are hard to reach,
- Replace the antifreeze. The antifreeze is good for 7-10 years or 100k miles. Make sure to use only Mercedes Benz antifreeze from the dealer.
- Replace the differential fluid (front and rear for 4matic models). MB says you can use a standard 90W gear oil, consult your owners manual for the fluid spec.
- For 4matic models, replace the Transfer case fluid. Be warned, this is not a small job. It requires either a lift, or 2 sets of jack stands so that you can get all 4 wheels in the air. You will need to remove the transfer case cross member, a mount, a heat shield and disconnect the driveshaft and remove the flex disc. If you’re not up to the task, have your mechanic do the Transfer case fluid at the same time you do the transmission service.
- Believe it or not MB did not specify a service interval for your 5-speed automatic transmission. Rather, the transmission was advertised as “sealed for life” and not requiring fluid changes. This concept later proved to be BS, and a wise owner will do a trans. service consisting of dropping the pan, new filter and fluid change, every 40-50k miles. The service costs about $200 and is well worth it.