The alternator charges the battery and provides your car with a constant source of electricity while the engine is running. Over time, the alternator will begin to fail and cause trouble with the various electrical devices in the car. If you suspect alternator trouble, you need to check to see that is operating correctly, and is indeed the cause of the problems with your charging system. Sometimes bizarre electrical problems can be caused by a number of faults other than the alternator. Its important to troubleshoot the system prior to replacing your alternator.
The first thing to check is the battery light on the instrument cluster. This light bulb is part of the alternator field circuit; if it burns out the alternator will not charge. Simply turn the key on and see if it illuminates briefly then goes off. If the light does not illuminate, youll need to replace it before continuing.
Inspect the belt that drives the alternator. Is it tight and amply turning the alternator? If not, then check that the belt tensioner is working correctly. Modern belts seldom break, but they get brittle and glazed with age, and can slip on their pulleys. Replace the belt with a new one as required.
The next item to check is the voltage at the battery. Before performing any tests of your alternator, charge and test your battery. If the battery is faulty, your alternator tests will not be accurate. This should read a little more than 12 volts with the engine off. Next check the voltage of your charging system under load, the voltage should be within 13.2: 14.5 volts d/c. To load, have engine running at idle, turn on headlights and HVAC blower motor. Never disconnect a battery cable while engine is running to test alternator, you may cause damage to alternator or other electrical components from the surge in amperage. If your battery appears to be leaking, then your voltage regulator has probably failed.
The battery will usually only leak acid if it has been overcharged at a much higher voltage. If the voltage measured at the battery is more than 14.5 volts when the engine is running, then the regulator is probably bad. If your battery has boiled over and has acid overflowing out the top, make sure that you clean up any spilled acid immediately. Dousing the area with a water and baking soda solution should help considerably to neutralize the acid, and prevent it from eating away at the metal.
An important item to check on your car is the engine ground strap. The engine is electrically isolated from the chassis by rubber motor mounts. If the engine ground strap is damaged or disconnected, you will have problems, including electrical system malfunctions, no and/or difficult starter cranking.
If youve checked all of these things and you still have charging problems, its likely the alternator will need to be replaced.
The first step in replacing any alternator is to disconnect the battery. You will be working around live electric wires here. If you happen to touch the lead going to the alternator against something, you can cause permanent damage to the electrical system if the battery is hooked up. Be smart here and disconnect it. Please refer to our article on battery replacement for more info.
You will need to remove the two air inlet ducts as well as the front engine cover to get access to the coolant temperature sensor on the front of the engine. Remove each duct by compressing them towards the engine and slipping them off the air inlet. With the ducts off remove the front engine cover. It pulls up and away from the engine. The cover is held on by five clips and will easily come off with hand pressure.
You will need to remove the fan next. Please see our article on removing the fan and then return here.
Next, use an E10 (reverse Torx socket) and driver. Locate the tensioner and using the Torx driver turn the nut on the wheel counter-clockwise 45 degrees. You can now slip the belt off the tensioner. There is a picture below that shows you the routing of the belt that you can use when you reinstall the belt.
You can also lock the tensioner into the open position if you need both hands to install the new belt. Turn the tensioner all the way counter-clockwise and insert a retaining pin between the rotating part and the tensioner base. If you do not have a retaining pin you can use a 5mm Allen key.
The alternator is held to the engine by two bolts. Use an E14 Torx and remove the two bolts. There is a spacer that is pressed in the mount that you may or may not have to press out a little to get the new alternator installed.
Lower the alternator down a little so you can get access to the connections on the back. Unplug the harness, pull the plastic cover off the cable connector and use a 15mm socket to remove the cable.
Now you can slide the alternator out of the engine. You will need to move the auxiliary coolant hose out of the way.
Make sure you disconnect the ground terminal (red arrow) from the battery before you begin this project.
The cover is held on by five clips (red arrows) and will easily come off with hand pressure.
Note: You will need to remove that fan from the car at this point. Please see our article on removing the fan and then return here.
You can also lock the tensioner into the open position if you need both hands to install the new belt. Turn the tensioner all the way counter-clockwise and insert a retaining pin between the rotating part and the tensioner base. If you do not have a retaining pin you can use a 5mm Allen.
Use an E14 Torx (red arrow) and remove the two bolts. There is a spacer that is pressed in the mount (green arrow) that you may or may not have to press out a little to get the new alternator installed. The rear nut (yellow arrow) is part of the block and will not fall into the engine compartment when you loosen the bolts.
Lower the alternator down a little so you can get access to the connections on the back. Unplug the harness (yellow arrow), pull the plastic cover off the cable connector (red arrow) and use a 15mm socket to remove the cable.
Comments: what is the max wats alternator that i can install in my 03 c320 mbz with out damage to the wireing
June 24, 2014
Followup from the Pelican Staff: Whatever the rating is on the alternator in your vehicle. Unless there is a higher rated alternator for your model, you may be able to use that one. Give our parts specialists a call at 1-888-280-7799. They can help you find the right part.
- Nick at Pelican Parts
Comments: Had alternater rebuilt and new belt went to start and nothing happened no clicks in starting no headlights nothing at all before low lights and clicking battery good could wires that run from alternater to battery be bad? How can i test? At max know how please help!! Thanks ken
June 9, 2014
Followup from the Pelican Staff: Yes, you can voltage drop the wires from battery to starter and alternator. I would start there. - Nick at Pelican Parts
Comments: I have a 2005 C320 with the Valeo alt. I've been told that I have to stay with Valeo instead of a Bosch. Is this true?
June 5, 2014
Followup from the Pelican Staff: No. Give our parts specialists a call at 1-888-280-7799. They can help you find the right part.
- Nick at Pelican Parts
Comments: Should I change anything else when I change my alternater in my slk 320 2003 belt ect.ect.
May 28, 2014
Followup from the Pelican Staff: Inspect the other pulleys and components. If all OK, just the belt and alternator. - Nick at Pelican Parts
Comments: The spacer mount is killing my mojo! I cannot get it to budge, either one. The replacement I bought does not seem to have room for the spacer to actually fit inside of it, is this going to be a problem? Also, could you provide more detail as to how you actually used your technique to back them out? Thanks!
May 1, 2014
Followup from the Pelican Staff: The spacer can be a pain! I suggest squeezing it to compress it. It does have a slit cut into the spacer. Once compressed, it should slide. Try using channel locks. Do not hit it with a hammer, you can break the bracket. - Nick at Pelican Parts
Comments: Does this information apply to the 2003 C230K model? I don't see it listed above as an applicable model is why I ask. Awaiting your reply. Thanks
April 4, 2014
Followup from the Pelican Staff: This tech article applies to:
SLK320 (1998-2003) - Nick at Pelican Parts
Comments: Just wanted to say thanks for posting this with specific model detail and follow up. Next time, I'll order my MB parts from Pelican!
January 6, 2014
Followup from the Pelican Staff: Thanks for the feedback. Glad we could help. - Nick at Pelican Parts
Comments: On my 2001 E320 Wagon, there were a couple of differences; specifically, a tensioner was bolt to the lower left that I used a socket instead of the Torx and b I needed a 25mm socket to transfer the pulley on the old alternator to the new one.
And ditto on the spacers - save yourself a lot of hassle and press them outward so that the new alternator slides in easily otherwise you'll have a hard time getting the bolts aligned.
November 12, 2013
Followup from the Pelican Staff: Some times aftermarket parts use different nuts and bolts and these may have been changed already at some point. - Kerry at Pelican Parts
Comments: Did you connect a memory setting saver prior to disconnecting the battery?
October 29, 2013
Followup from the Pelican Staff: No, it being a subject vehicle we weren't worried. Usually I write down the radio presets and record seat positions, etc. I prefer not to put power on the vehicle when major B+ connections are loose. There is a chance fa short, I want to avoid that at all costs. - Nick at Pelican Parts
Comments: Great article. Does it matter which replacement I use? If my current one is Valeo could I install a bosch? Thanks!
May 27, 2013
Followup from the Pelican Staff: Glad this diy helped you. If the Bosch is an option, yes.
Comments: Mercedes told me that my alternator pulley is causing some noise coming from my engine while at idle. Would you have a proper procedure to do this or would replacing the alternator with a pulley already installed on it be a better way to go? Also how can I tell if I have a Bosch or Valeo alternator? Thanks! if you could email me the pulley question that would be wonderful
May 25, 2013
Followup from the Pelican Staff: You can tell what alternator you have by looking at the label on the back side.
I would replace the entire alternator if the pulley is failing. - Nick at Pelican Parts
Comments: "There is a spacer that is pressed in the mount green arrow that you may or may not have to press out a little to get the new alternator installed."
This turned out to be the hardest part of the job. These bushings "spacers" required a lot of force to move. I would love to know if there is an easy way to do this.
I was able to use a C clamp and a large socket to move the top bushing; but the bottom one had a bracket blocking access. I had to go buy a threaded rod. I inserted the rod through the bushing and turned a nut on the backside of the bushing to push it out.
April 12, 2013
Followup from the Pelican Staff: Sorry you had corrosion issues, happy the DIY was able to help you through this job.
In high corrosion areas, what you describe is common.
A C-clamp and socket works for the first.
As you noted, the second is easiest with a threaded rod pulling it back.
Thankfully neither bushing needs to move very far.
There where several $150 - $1;300 USD, after market tool sets made for this job in the late 1980's, few where sold, and I have not seen one in years.
Check out some other sample projects
from the book: