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Replacing Your Alternator on your Mercedes-Benz
 
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Pelican Technical Article:

Replacing Your Alternator on your Mercedes-Benz

Steve Vernon

Time:

2 hour2 hr

Tab:

$6 to $240

Talent:

**

Tools:

8mm, 13mm, 17mm wrench and sockets, Philips head screw driver

Applicable Models:

Mercedes-Benz W126 (1981-91)

Parts Required:

New or rebuilt alternator

Hot Tip:

ALWAYS disconnect the battery before beginning

Performance Gain:

Car charges again

Complementary Modification:

Change belt

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. It's 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, you'll 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 the condition of the belt, they 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. This should read a little more than 12 volts with the engine off. Wait two minutes after starting the vehicle for (accessory cycling) voltage to stabilize. When the car is running, the voltage must read in the range of 13 to 14.5 volts with the engine at 3000 rpm. 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 15.5 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 to neutralize the acid considerably, and prevent it from eating 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 you've checked all of these things and you still have charging problems, it's 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 and give yourself a heck of a shock. Be smart here and disconnect it.

If you are replacing the alternator it is a good idea to install a new belt "while you are in there," unfortunately that also means removing the air pump belt and air conditioner belt.

With the other two belts removed, you need to loosen the 17mm bolt attaching the alternator to the solid mount. There is a nut and bolt that attaches the alternator to the arm bracket. This bolt has the tensioner teeth built into it like the other accessories. You need to use a 17mm socket and loosen the nut on the back of the alternator. With this loose, you can use your 17mm wrench on the front bolt to take the tension off the belt. Remove the belt and use a 13 wrench to remove the upper bracket from the solid mount. This will give you more room to remove the alternator and get access to the electrical connections at the rear.

If you are just replacing the voltage regulator you can simply turn the alternator over, use a Philips-head screwdriver to remove the old one and install a new one.

If you are replacing the alternator remove the two electrical connectors using an 8mm and 13mm wrench.

Installation is the reverse of removal.

Begin by removing the ground cable from the battery and making sure it can not accidentally come in contact with it while working.
Figure 1

Begin by removing the ground cable from the battery and making sure it can not accidentally come in contact with it while working.

Loosen the 17mm bolt (red arrow) attaching the alternator to the solid mount.
Figure 2

Loosen the 17mm bolt (red arrow) attaching the alternator to the solid mount. There is a nut and bolt that attaches the alternator to the arm bracket. This bolt has the tensioner teeth built into it like the other accessories (green arrow).

You need to use a 17mm socket and loosen the nut on the back of the alternator (red arrow).
Figure 3

You need to use a 17mm socket and loosen the nut on the back of the alternator (red arrow).

With this loose you can use your 17mm wrench on the front bolt to take the tension off the belt.
Figure 4

With this loose you can use your 17mm wrench on the front bolt to take the tension off the belt.

Use a 13 wrench to remove the upper bracket from the solid mount.
Figure 5

Use a 13 wrench to remove the upper bracket from the solid mount. This will give you more room to remove the alternator and get access to the electronic at the rear.

If you are just replacing the voltage regulator you can simply turn the alternator over, use a Philips head screw driver to remove the old one (red arrows) and install a new one.
Figure 6

If you are just replacing the voltage regulator you can simply turn the alternator over, use a Philips head screw driver to remove the old one (red arrows) and install a new one.

If you are replacing the alternator remove the two electrical connectors using an 8mm and 13mm wrench.
Figure 7

If you are replacing the alternator remove the two electrical connectors using an 8mm and 13mm wrench. Installation is the reverse of removal

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Comments and Suggestions:
Joe Comments: Where is the engine ground strap on 91 mercedes benz 420sel,because car was fine at night driving home except lots of snow,the next morning i started it an lights on dash came on,srs abs,traction control,an bright battery light.. an now alt isnt charging,could the ground strap have been broke ,tore,or something from the snow would it cause this issue ?
March 9, 2017
  Followup from the Pelican Staff: See this tech article for ground strap location:

http://www.pelicanparts.com/techarticles/Mercedes-W126/08-ELEC-Ground_Strap_Problems/08-ELEC-Ground_Strap_Problems.htm">http://www.pelicanparts.com/techarticles/Mercedes-W126/08-ELEC-Ground_Strap_Problems/08-ELEC-Ground_Strap_Problems.htm">http://www.pelicanparts.com/techarticles/Mercedes-W126/08-ELEC-Ground_Strap_Problems/08-ELEC-Ground_Strap_Problems.htm - Nick at Pelican Parts
 
Joey Comments: How do you remove alt,on 91 mercedes benz 420 sel.i got off the 2 bolts an removed bracket but the 3rd 17mm will not budge with heat,vibration etc.Is there a hidden nut that needs loosening to get last bolt out
March 9, 2017
  Followup from the Pelican Staff: As far as I recall, only two fasteners. The article you commented on should show the steps. Possible it needs to be removed out of the mounting bracket using a gentle levering. - Nick at Pelican Parts  
Joey Comments: I replaced everything on my charging system on 91 420 sel. I dont know where my grounding strip is? That goes from motor to frame i think.?? There was alot of snow it ran fine until i got home an the next morning all lights came on...HELP ME PLEASE IM ON DISABILITY
March 8, 2017
  Followup from the Pelican Staff: See this tech article for ground strap location:

http://www.pelicanparts.com/techarticles/Mercedes-W126/08-ELEC-Ground_Strap_Problems/08-ELEC-Ground_Strap_Problems.htm

- Nick at Pelican Parts
 
joey Comments: i replaced my volt reg on my alt,with it not running battery says 12.5 when started it says 11.2 it has 100k 91 420sel benz what cld it be any suggestions
March 7, 2017
  Followup from the Pelican Staff: You probably have bad alternator. - Nick at Pelican Parts  
350sdlvr Comments: Is there an easy way to replace the serpentine belt on the 350sd. Looks like the fan has to come off in order to properly route the belt. Thanks.
February 23, 2017
  Followup from the Pelican Staff: Taking the fan off is the easy way, but I have had luck turning the fan and popping the belt over the blades while turning the fan, one by one...it's slow and you're guaranteed a few new scrapes on your hands and arms but it is possible! - Casey at Pelican Parts  
joey Comments: all my lights came on the battery lights the brightest on dash even my fuel an traction control lights are on..i disconectd negative terminal an car dies .Is it volt regulator?the car was fine when i got home the next morning after snow an coldness all lights came on..1991 mercedes benz 420sel
February 17, 2017
  Followup from the Pelican Staff: Check the voltage at the battery with the car running, should be 13-14.5 volts. If it is less than that it could be the voltage regulator. - Casey at Pelican Parts  
Tobias Comments: Do you know if anyone sells the bolt that has the tensioner teeth? Mine has a broken teeth and can no longer get the required tension. Thanks!
July 25, 2016
  Followup from the Pelican Staff: Yes. Give The Pelican Parts parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need. - Nick at Pelican Parts  
Milidon Comments: My battery light won't come on in the cluster either when I turn the key for the first time while the other lights come on. Does this mean that my alternator is not charging because if that, always referring to the explanation above. Thank you.
May 14, 2016
  Followup from the Pelican Staff: I would assume the circuit for the light is faulty. Grab a wiring diagram and check the fuse and wiring for the bulb. - Nick at Pelican Parts  
Jerry Comments: I checked the battery with the engine off after recaharging the battery. It was 12.6 .I started the car and the voltage was 13.93. Indication that alternator might be good.I guess checking battery is in order
June 18, 2015
  Followup from the Pelican Staff: You will want to check charging current as well. You could have voltage with low current. - Nick at Pelican Parts  
malbuddy Comments: Thank you for the huge help. Starting at ground zero and first checking the batter light on the cluster, I noticed not only does my battery light come on, but the only lights that temporarily come on are the ABS, SRS and Check Engine light. Those three lights turn on and off, but no other lights come on. Could this be bulbs or a deeper concern?
July 30, 2014
  Followup from the Pelican Staff: That sounds like bulbs. If the other lights are coming on, it shouldn't be something major. - Nick at Pelican Parts  
Chris Comments: Do you have Info on how to reinstall the pulley assembly from old alternator to the new....I took the old pulley off and did not note the correct order to reinstall....lots of washers etc....also does the pulley itself have a washer inside where the belt goes threw....any picture would be great
December 30, 2013
  Followup from the Pelican Staff: The attached photo show the washer orientation on the front of the pulley. This may be able to help you sort out what is supposed to be at the pulley rear..- Nick at Pelican Parts

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Page last updated: Fri 11/17/2017 02:32:01 AM