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Voltage Regulator Replacement
 
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Pelican Technical Article:

Voltage Regulator Replacement

Steve Vernon

Time:

1 hour1 hr

Tab:

$15 to $50

Talent:

**

Tools:

Philips head screwdriver

Applicable Models:

Mercedes-Benz W123 (1977-85)

Parts Required:

Voltage Regulator

Hot Tip:

ALWAYS disconnect the battery before beginning

Performance Gain:

Car charges again

Complementary Modification:

Change drive 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 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 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.

A fair amount of the time 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 and because it is easier, fixes the problem and most people do not understand how an electrical system works on their car. If your electrical issues are caused by a faulty voltage regulator you can save your self a lot of money by just replacing the defective part.

Over the years your car may have been serviced by multiple people including previous owners and this can lead to parts replaced with different size fasteners and hardware. The sizes of the nuts and bolts we give may be different from what you have so be prepared with different size sockets and wrenches.

Our vehicle may vary slightly from yours as Mercedes redesigned systems and sub-system as the models developed over the years. If something is different on your vehicle please let us know and share your info to help other users. If you have any questions or comments or even a different procedure you would like to share please leave it below and please leave your vehicle information.

Whether you are going to attempt to change the voltage regulator with the alternator in the engine bay or remove the alternator first make sure you disconnect the ground cable (red arrow) from the battery and place it where it cannot accidentally come in contact with the battery post while you are working.
Figure 1

Whether you are going to attempt to change the voltage regulator with the alternator in the engine bay or remove the alternator first make sure you disconnect the ground cable (red arrow) from the battery and place it where it cannot accidentally come in contact with the battery post while you are working.

While I have heard of people on the internet claiming to change out the voltage regulator on their W123 while it is in the car, I would not even attempt to do it with the alternator in our 300TD.
Figure 2

While I have heard of people on the internet claiming to change out the voltage regulator on their W123 while it is in the car, I would not even attempt to do it with the alternator in our 300TD. The voltage regulator is located on the rear of the alternator (red arrow) and there is no way I could access it while mounted in the vehicle. Please see our article on alternator replacement for step by step instructions on how to remove your alternator.

Turn your alternator pulley side down on your bench and you can see the voltage regulator (red arrow) along with a resistor (yellow arrow) that reduces 60 second hum on your radio.
Figure 3

Turn your alternator pulley side down on your bench and you can see the voltage regulator (red arrow) along with a resistor (yellow arrow) that reduces 60 second hum on your radio.

Use a Philips head screwdriver and remove the two screws (red arrows) holding the regulator in place.
Figure 4

Use a Philips head screwdriver and remove the two screws (red arrows) holding the regulator in place.

Pull the regulator out form the alternator.
Figure 5

Pull the regulator out form the alternator.

You can now see the two brushes (red arrow) that make contact with the poles on the alternator.
Figure 6

You can now see the two "brushes" (red arrow) that make contact with the poles on the alternator. Check the condition of the brushes for excessive wear or pitting. The brushes are held in place against the polls by springs that can wear out; push them down with your finger and they should pop right back up. As you can see, the brushes on our voltage regulator were at the end of their wear life.

While the regulator is out make sure you check the condition of the poles (red arrow) on the alternator: they can be discolored but should not be excessively worn or corroded and should not have grooves worn in them.
Figure 7

While the regulator is out make sure you check the condition of the poles (red arrow) on the alternator: they can be discolored but should not be excessively worn or corroded and should not have grooves worn in them.

Before installing the regulator inspect and clean the ground clip that the regulator presses against (red arrow).
Figure 8

Before installing the regulator inspect and clean the ground clip that the regulator presses against (red arrow).

Also make sure to clean up the alternator connectors with some Emery cloth and apply a little di-electric grease to them (red arrow).
Figure 9

Also make sure to clean up the alternator connectors with some Emery cloth and apply a little di-electric grease to them (red arrow). Installation is the reverse of removal.

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