All modern cars are fitted with alternators, normally driven by a belt which powers the generator producing power for all the electrically powered components and for charging the battery, thereby replacing the power drawn from the battery when for instance starting the car or where the amount of power demanded by the electrical components exceeds the amperage output of the alternator. This can be the case where the car is for instance waiting at traffic lights junctions or any instance where the engine/alternator is not turning sufficiently to provide the demand required the battery will provide the short fall, the power being replaced as soon as the engine is running normally and where excess power to the demand is available from the alternator. This of course can only be done efficiently if the battery is in good condition and contains water/acid covering the plates of the 6 cells of the 12volt battery.
It is well known that where a car is used in the winter months for only short runs, the battery power used will not be replaced fully thus the battery will require a trickle charge from a battery charger. from time to time (see my page13, battery charging.) this is due to the fact that the alternator cannot power all the various electrical components which includes such items as, lights, audio equipment, power steering, heated seats, etc etc as well of course the extras that the owner has fitted him/herself, There is no doubt that the alternator does a fantastic job and yet from a service point of view is often forgotten until of course it goes defective and packs up working, having done so the car eventually fails to start due to loss of stored battery power. An indication that the alternator has stopped producing power will be detected in the power steering on this car, as this provision requires a signal from the alternator circuit for the PSP to run. This is only the case is when the alternator is rotating and generating power.
In the case of the 'A class the alternator is located on the back right hand side of the engine and is driven by the poly 'v' belt , were the car is fitted with air conditioning the alternator is sandwiched between the compressor and the rear of the engine casing, albeit is possible to replace the alternator without discharging the air conditioning system. driving the alternator with the poly 'V' Belt is fine providing the belt is inspected and replaced as necessary, loose your belt and the car will not run for very long as all electrical power is lost, due to the generator having lost it is power souse. i.e. the poly 'V' belt drive.
The red arrow is pointing to the alternator located forward of in this case the air Conditioning compressor.
An indication that the alternator has ceased to generate power will be given by the ignition light not going out or flickering while the engine is running, and or the power steering pump(PSP) failing to work, for that reason always check the alternator before considering a repair/replacement of the PSP.
Owners should also bear in mind that both the power steering pump electronics and the alternator are situated at not more that 16 inches (400mm) from the road surface, therefore if you encounter flooding which appears to be coming more common consider the situation before entering the flooded road surface or even ford, as severe damage may be caused to these components. although also applies to the air conditioning compressor it is less likely to suffer any long term damage.
Replacement of the alternator
When faced with having to replace your alternator always quote your Vin number to the provider the information on this site is in favour of the W168/A160 other models are know to have different alternators. some models for instance have water cooled alternators which cost considerably more money, please do not me why this is because I cannot work out how hot water in the cooling system can possibly cool an alternator. which is possibly why not all models have them!
It should noted that when testing the alternator output at the power steering contacts nearside junction box 13+volts will be recorded if the alternator is working correctly.
As can be see by the label on this alternator the amperage max output is 90amps , if all the the bulbs and electrical components on the amperage by each were added up then it would be seen how many amps were left for recharging battery As can be see by the label on this alternator the amperage max output is 90amps , if all the the bulbs and electrical components on the amperage by each were added up then it would be seen how many amps were left for recharging battery
Servicing
Only limited servicing can be done on the alternator while in situ, In the case of failure check the contacts for security and cleanliness, ensure the ignition is off when working on electrics or fault codes will be possibly stored on your ECU.
However should the unit still not work, having removed it from the car the alternator can be stripped and possibly repaired, brushes replaced etc, I'm unsure if these can be obtained from Mercedes Parts however a good auto electrician should be able to provide or indicate where theses can be purchased. As is the case with most items fitted to the engine and car Mercedes-Benz do not make them themselves, it is therefore worth shopping around for either a reconditioned or new unit which may be cheaper than Mercedes parts but check their price first , some times they are as, if not cheaper than other outlets/suppliers.
Another tip from a fellow owner Roy, he had what he thought was a noisy alternator, albeit working. When he investigated further it was the bearing on the poly 'V' Belt tensioner pulley wheels that were at fault the alternator was OK, so it is worth checking before spending out for a replacement unit and if not doing it yourself, ask you garage to do the same.
Roy has also highlighted the fact that on the unit fitted to his car, the alternator is fitted with an inertia clutch system, i.e.. when the car engine is switched off the armature of the generator (alternator) continues to spin (turn) independent of the pulley wheel for a few moments after the engine is switched off. This is obviously designed to take the shock off the main shaft of the alternator which is connected to the pulley wheel of the alternator by a ratchet clutch system.
As a point of interest my car 2002/A160 is fitted with a 'Valeo' as was the other owners but do make sure you get the right replacement unit or you could involve your self in unnecessary cost. I am aware for instance that at least some of the earlier A170CDI were fitted with water cooled alternators 'Yes water cooled', connected into the cooling system !!!
How hot water in the cooling system of the car cools an alternator beats me, Well of course it doesn't In fact it's quite the reverse , The alternator generates heat and well as electricity (DC) and as a result heats the water in the cooling system of the car , this in the case of diesels is useful; because heaters in diesel cars are not so efficient as petrol cars and so any heat that can be gleaned and used to heat the car in the colder months is in this case well used. This was the first time I had heard that such a component existed, we have often as 'A' class owners heard that the 'A' class was built as a test bed. Well this could well be one such case?? So if you are looking at a replacement take care to get the right unit your Vin number should identify the correct unit when ordering.
This is an alternator suitable for Mercedes-Benz vehicles fitted with fluid units.( because of the coolant fluid added to the water of the cars cooling system we choose to use the word fluid i.e Water coolant mix) This particular unit is apparently suitable for=
MERC - E-CLASS (W210) - E 200 CDI (210.007) 09/2000-03/2002
MERC - E-CLASS (W210) - E 200 CDI (210.007) 09/2000-03/2002
MERC - E-CLASS (W210) - E 220 CDI (210.006) 09/2000-03/2002
MERC - E-CLASS (W210) - E 270 CDI (210.016) 07/1999-03/2002
MERC - E-CLASS Estate (S210) - E 220 T CDI (210.206) 09/2000-03/2003
MERC - E-CLASS Estate (S210) - E 270 T CDI (210.216) 07/1999-03/2003
MERC - E-CLASS Estate (S210) - E 320 T CDI (210.226) 07/1999-03/2003
MERC - S-CLASS (W220) - S 320 CDI (220.026, 220.126) 081999-09/2002
MERC - M-CLASS (W163) - ML 270 CDI (163.113) 12/1999-06/2005
My comment about the 'A' Class W168/A170CDI being a test bed for the fluid cooled alternators appears to be out of order as these models are already fitted with fluid cooled units as standard..
However it does substantiate my comment about using you vehicles Vin number to get the correct unit for your car, go to the car breakers and I feel that on this component at least you will be wasting your money, as the chances of getting the correct unit look pretty slim.
When Roy a fellow owner had noise coming from under the car in the alternator, poly 'V' Belt area, he assumed the worst and removed the alternator however further inspection revealed that the poly 'V' Belt tensioner pulley bearings had dried out, which could only be rectified by replacing with new pulley wheels complete with bearings, He could have replaced the bearing alone but opted to change the pulley wheels as the working surfaces were also worn and pitted. Cost approx £20.00 each (10/04/08) Always use your Vin Number to obtain the correct spares for your car.
One example of a water / coolant cooled alternator
Remember to quote your VIN the W168 A170CDI has a water cooled alternator get the wrong type for your car and you could get stuck with it so check using your Vin before purchase and paying out for postage or delivery. As can be seen they vary not just in appearance but cost as well they are not interchangeable. SO USE YOUR VIN
I would like to thank Roy a fellow owner, for some of the photographs on this page.
Hello. We spoke a week ago and I was asking about the whining noise that came out of my engine.I finally found a service that agreed to work with my parts. So now I have to buy a new alternator (I could not find a service that could fix my old one). The problem is that when i wanted to buy the alternator thay asked me "How many "nervures" does my old alternator have?". Now I ask you kindly to help me.
These are the specifications for the 2 model of alternator - 5 and 6 "nervures"
- Alternateur VALEO 436732
[436732]
Engine code 166.940
Voltage [V] 14 V
Charge curent (Amps) 90 Amps
Position/degré 0 °
Direction of rotation clockwise
Poulies with poulie
Poulie for belt- Ø [mm] 51 mm
Number of nervures 5
................................................... Référence constructeur
MERCEDES-BENZ : 0101543802 - 0111545602 - 011154560280 - A0101543802 - A0111545602
- Alternateur VALEO 437806
[437806]
Engine code 166.940
Voltage [V] 14 V
Charge curent (Amps) 143 Amps
Position/degré L 60 °
Poulie for belt - Ø [mm] 44 mm
Number of nervures
6
Référence constructeur
MERCEDES-BENZ : 0091545602 - A0091545602
as can be seen these units are very differnt make sure you use your VIn to obtain the corrrect unit.
All modern cars are fitted with alternators, normally driven by a belt which powers the generator producing power for all the electrically powered components and for charging the battery, thereby replacing the power drawn from the battery when for instance starting the car or where the amount of power demanded by the electrical components exceeds the amperage output of the alternator. 
The red arrow is pointing to the alternator located forward of in this case the air Conditioning compressor.
However should the unit still not work, having removed it from the car the alternator can be stripped and possibly repaired, brushes replaced etc, I'm unsure if these can be obtained from Mercedes Parts however a good auto electrician should be able to provide or indicate where theses can be purchased. 
This is an alternator suitable for Mercedes-Benz vehicles fitted with fluid units.( because of the coolant fluid added to the water of the cars cooling system we choose to use the word fluid i.e Water coolant mix) 
