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Pelican Technical Article:

Alternator Replacement

Steve Vernon

Time:

2 hours2 hrs

Tab:

$140 to $400

Talent:

**

Tools:

17mm, 15mm, 8mm wrench, 17mm socket

Applicable Models:

Mercedes-Benz W123 (1977-85)

Parts Required:

Alternator, bracket, adjustment bolt (if needed)

Hot Tip:

Remove the air filter housing and intake pipe

Performance Gain:

Working electrical system

Complementary Modification:

Change air filter

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 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.

Inspect the belts that drive the alternator. Are they tight and amply turning the alternator? If not, then adjust the tension on the belt. Modern belts seldom break, but they get brittle and glazed with age, and can slip on their pulleys. Replace the belts with new ones 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 voltage regulator is probably bad and you should see our article on voltage regulator replacement. 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.

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 alternators and support brackets designs were changed over the years, some mounting brackets have welded nuts on the front and some have them on the back, some alternators have the electrical connections secured by nuts and others have plugs. Depending on the vehicle you have and the type of equipment in it, this article may not be an exact procedure for replacing your alternator but the basic procedures included here should help you get through it.

Vehicle models change and evolve as they grow older, so the vehicle shown in our illustrations may vary slightly from yours. If something seems different, let us know and share your info to help other users. Questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.

Before you start working on the alternator make sure to disconnect the ground strap on the battery (red arrow) and place it somewhere where it cannot accidentally come in contact with the post while you are working.
Figure 1

Before you start working on the alternator make sure to disconnect the ground strap on the battery (red arrow) and place it somewhere where it cannot accidentally come in contact with the post while you are working.

On our project 1985 300TD I found it much easier to remove the air filter housing and air intake pipe (red arrow), this gives you all kinds of room to see what you are doing and get your hands and tools in there.
Figure 2

On our project 1985 300TD I found it much easier to remove the air filter housing and air intake pipe (red arrow), this gives you all kinds of room to see what you are doing and get your hands and tools in there. Please see our article on air filter housing and intake pipe removal for additional instruction.

The rear of our alternator was covered with a plate (red arrow) that needed four 8mm bolts removed to access to the wiring on the rear of the alternator.
Figure 3

The rear of our alternator was covered with a plate (red arrow) that needed four 8mm bolts removed to access to the wiring on the rear of the alternator.

Disconnect the wiring from the rear of the alternator; on our project car it was a three pronged electrical plug (red arrow).
Figure 4

Disconnect the wiring from the rear of the alternator; on our project car it was a three pronged electrical plug (red arrow).

Next, working from below, loosen but do not remove the 17mm bolt (red arrow).
Figure 5

Next, working from below, loosen but do not remove the 17mm bolt (red arrow).

Next, loosen but do not remove the 17mm pass through tensioning bolt (red arrows).
Figure 6

Next, loosen but do not remove the 17mm pass through tensioning bolt (red arrows).

There will be an adjustment bolt with a nut on the end of it that is used for tensioning the belts that drive the alternator; this threaded rod is welded to the 17mm pass through bolt that you just loosened.
Figure 7

There will be an adjustment bolt with a nut on the end of it that is used for tensioning the belts that drive the alternator; this threaded rod is welded to the 17mm pass through bolt that you just loosened. The rod adjustment bolt on our project car was broken off and the tension was set by tensioning the alternator belts with a pry bar on the alternator and then just really tightening everything down, which is not a good way to guarantee that your alternator is going to work. If your adjustment rod or bolt is broken make sure to replace it. If your vehicle has this, and hopefully it does, loosen the adjustment nut.

Push the alternator towards the motor and slip the belts off.
Figure 8

Push the alternator towards the motor and slip the belts off.

Remove the two 17mm pass through bolts and remove the alternator out through the bottom.
Figure 9

Remove the two 17mm pass through bolts and remove the alternator out through the bottom. Check your mounting bracket for bends or damage. If the alternator bearing failed because the belts where not lined up correctly because the bracket is bent then there is no point installing a new alternator onto a bent bracket as it will just fail again. You will need to replace the bent bracket before you proceed. Installation is the reverse of removal.

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