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Pelican Technical Article:

Voltage Regulator Replacement

Steve Vernon

Time:

2 hours2 hrs

Tab:

$35 to $50

Talent:

**

Tools:

13mm, 8mm sockets, Philips head screwdriver

Applicable Models:

Mercedes-Benz 260E (1987-89)
Mercedes-Benz 300CE (1988-93)
Mercedes-Benz 300E (1986-93)
Mercedes-Benz 300TE (1988-92)
Mercedes-Benz E320 (1994-95)

Parts Required:

New voltage regulator

Hot Tip:

ALWAYS disconnect the battery before beginning

Performance Gain:

Car charges again

Complementary Modification:

Change serpentine belt

The alternator charges the battery and provides your car with a constant source of electricity while the engine is running. The voltage regulator does just what it says; it regulates the voltage from the alternator. Your battery 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 the engine running at idle. Turn on the headlights and HVAC blower motor. Never disconnect a battery cable while the engine is running to test the alternator. You may cause damage to the alternator or other electrical components from the surge in amperage. If your battery appears to be leaking, 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, the voltage 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.

Often when there is an electrical problem with the charging system it is the voltage regulator. Yet people just swap out the entire alternator because it comes with a new voltage regulator included. They do this because it is easier, fixes the problem and most people do not understand how the electrical system on their car works. If your electrical issues are caused by a faulty voltage regulator you can save yourself a lot of money by just replacing the defective part.

The alternator is located on the lower right side at the front of the motor. If your Mercedes still has the under engine tray, you will need to remove it before you begin as well as safely jack up and support the vehicle. Please see our articles on safely jacking up and supporting your vehicle as well as removing your under trays.

The voltage regulator is located on the rear of the alternator. You can replace the voltage regulator while the alternator is still on the car. Note - we removed the alternator for better pictures but you do not need to remove it to perform this work.

It is extremely dangerous to work on your alternator with the battery still connected.
Figure 1

It is extremely dangerous to work on your alternator with the battery still connected. Begin by removing the negative or ground cable. With the ground cable unattached no electricity will be flowing in the car. Use a 13mm wrench (red arrow). Verify you are working on the negative or ground cable (yellow arrow) and disconnect the cable. Make sure the cable cannot accidentally make contact with the battery ground post while working on the car.

The voltage regulator is located on the rear of the alternator.
Figure 2

The voltage regulator is located on the rear of the alternator. You can replace the voltage regulator while the alternator is still on the car. Begin by removing the cap (red arrow) and then the 13mm nut underneath. Next, remove the 8mm nut on the smaller wire (yellow arrow). Place both wires out of the way.

Note: we removed the alternator for better pictures but you do not need to remove it to perform this work.
Figure 3

Note: we removed the alternator for better pictures but you do not need to remove it to perform this work. Remove the three small Philips head screws that help hold the protective cover on the rear of the alternator (red arrows).

Gently release the three plastic clips holding the cover on (red arrow, one shown).
Figure 4

Gently release the three plastic clips holding the cover on (red arrow, one shown). Use care as these clips get fragile and can break.

Pull the cover off the back of the alternator (red arrow).
Figure 5

Pull the cover off the back of the alternator (red arrow).

Remove the two Philips head screws holding the voltage regulator to the alternator (red arrows).
Figure 6

Remove the two Philips head screws holding the voltage regulator to the alternator (red arrows).

You can now remove the voltage regulator (red arrow) from the alternator.
Figure 7

You can now remove the voltage regulator (red arrow) from the alternator.

With the voltage regulator removed you can now see the two brushes that make contact with the poles on the alternator (red arrows).
Figure 8

With the voltage regulator removed you can now see the two "brushes" that make contact with the poles on the alternator (red arrows). These are held in place against the poles by springs that can wear out; the brushes can also wear out or become excessively corroded over time.

Make sure you check the condition of the poles on the alternator (red arrows).
Figure 9

Make sure you check the condition of the poles on the alternator (red arrows). They can be discolored but should not be excessively worn, corroded and should not have excessive grooves worn in them. If there is heavy grooving in the poles or other damage, you will need to replace the alternator. Please see our article on alternator replacement for further assistance. Installation of the voltage regulator is the reverse of removal.



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Comments and Suggestions:
derek Comments: thanks for the article - great help. I had to loosen the alternator however to get the plastic cover off because of hard pipes in the way coming from the sump. 1994 E300D estate....
October 26, 2015
  Followup from the Pelican Staff: Thanks for the additional info and feedback. We appreciate it.
- Nick at Pelican Parts
 
Craymon Comments: Thanks for the reply Nick... My car is a 1994 e320 Mercedes. From everything I'm reading here and there it may be the voltage regulator. Your thoughts? Also, in changing out the voltage regulator would you recommend the Bosch OEM? Thanks again...
May 20, 2015
  Followup from the Pelican Staff: Once you confirm charging system voltage and current is low, check voltage at the alternator wiring engine off and running, large cable should be battery volts. Small will be close to battery volts engine off, then rise to charging system voltage when the engine is running. - Nick at Pelican Parts  
Craymon Comments: I've installed new wiring harness, computer and a/c blower motor and ovp relay. Engine still shifts erratically in the first couple of gears, surges on cold and some hot starts, A/C blower and cruise control cuts in and out. Works when they want to. Stereo also acts a little squirrelly at times. I did notice a little leakage at the battery and a build up around the terminal. Voltage readings at the battery while idling is up and down and not constant. Is the voltage regulator my next culprit to change? Thanks for your help...
May 16, 2015
  Followup from the Pelican Staff: What vehicle are you working on? I would check the charging system. Check voltage and charging current. You may have a faulty alternator.- Nick at Pelican Parts  
threejak Comments: Nick:
Upon closer review...Sorry Disabled and opportunity under a lift is a bit rare...but got there FINALLY Had purchased a bone-yard Alt. just as something to get me by "last surgery" previous summer. It turns out prob was arising from a "trashed" vot. reg. but also one that had been jerry-rigged into Alt. housing as a result of broken screw...Had to drill an equal hole into new voltage-reg to make it fit..Whatever they are using in Hungry metal wise is super strong t'il ya hit the brass or copper..Mechanic friend was amazed that Bosch product held up under all the drilling and heat...Oh and the ground plate too..Good readings on the meter post complete and all good on the first turn of the key this morning...Reserve alt. now scheduled for boxing and off for a proper Rebuild..Any suggestions or cost estimates? Since you folks have the rest of the answers thought I'd throw this one in too.. Tku.
March 7, 2015
  Followup from the Pelican Staff: No clue on cost, as that is regionally driven. I would look up a few local electric motor rebuilders and get quotes. That is your best bet. You'll likely find the smaller operations have the best service and pricing. - Nick at Pelican Parts  
threejak Comments: Nick:
TKU for the time and efforts spent here. What I didn't include was the fact that if the car isn't driven for even a couple of days battery goes kaput. Don't own any gear to check charge etc. so hopped down to a local Auto Zone and he said Battery Charge was fine but strongly guessed at the voltage regulator given given odd diode reading...Think I got that right...Will follow thru on your suggestion via a couple of Pro Electric guys here in town. Appreciate your input as I'm way outta my depth here...
February 23, 2015
  Followup from the Pelican Staff: Sounds like a parasitic draw, as I mentioned. Follow my previous instructions. - Nick at Pelican Parts  
threejak Comments: Wow! This was good! I've got Gremlins or something that is zapping my battery on '87 300E. New Interstate- July '14
All connections are good etc.super clean. Just periodically goes Kaput. Any help? TKU -j
February 21, 2015
  Followup from the Pelican Staff: You may have a parasitic draw. You'll want to monitor the draw on the battery when the vehicle is asleep.

Once you confirm there is a draw. You will have to isolate the circuit with the draw. Then determine which components on that circuit is causing the draw You can isolate by removing fuses. be sure to have all the latches on the doors and hood closed (pin switches) so the vehicle will sleep during testing. If it wakes up, you will have to wait until it goes to sleep again. - Nick at Pelican Parts
 
cwpalmer Comments: Do you have instructions for replacing the distributor cap on a 1989 300E? Many thanks for your excellent articles.
August 17, 2014
  Followup from the Pelican Staff: We don't currently have that tech article. I would grab a repair manual. It will have the procedure, special tools and torque specs.

Give our parts specialists a call at 1-888-280-7799. - Nick at Pelican Parts
 

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