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Pelican Technical Article:

Alternator Replacement

Tom Morr

Time:

1 hour1 hr

Tab:

$291 to $519

Talent:

**

Tools:

E10 Torx socket, pry bar, ratchet, sockets: 8mm, 13mm, 16mm

Applicable Models:

Mercedes-Benz R170 (1998-04)

Hot Tip:

Remove the flexible lower air duct for alternator bolt clearance

Performance Gain:

Restores proper battery charge and electrical system functions.
Like all moving parts, alternators eventually fail. Constant spinning wears out the bearings over time. Also, the voltage regulator can go bad, which will prevent the alternator from properly charging the battery.

To determine if the alternator is the root of charging-system problems, perform a few simple tests. If the battery warning light is illuminated on the cluster, the alternator might be bad--the light is wired to the alternator's circuit. If the light isn't on, make sure that its bulb is good. Turn the key on and see if the battery light comes on momentarily along with the rest of the warning lights. If not, replace the bulb.

A brittle serpentine belt or worn belt tensioner can also cause the alternator to malfunction. Bad grounds are another source of electrical-system gremlins. Verify that the engine's ground strap is securely anchored.

Next, check the battery's voltage. Its normal charge with the engine off should be slightly more than 12 volts. If the battery shows signs of acid leaks, the alternator's regulator could be faulty and allowing the battery to overcharge, causing acid to boil over. Clean up acid residue with a solution of baking soda and water. Be sure to wear eye and skin protection.

Start the car and check the battery voltage after a couple minutes. With the engine at 3,000 rpm (have a friend rev up the car, secured in park, if necessary), battery voltage should be between 13 and 14.5 volts. Any reading over 15.5 volts indicates a likely regulator problem.

If the alternator doesn't pass the tests above, it might need to be replaced or rebuilt. Removal isn't difficult: basically two bolts and two nuts once the belt is removed from the pulley. 

Always disconnect the battery's negative cable before beginning any electrical work. Raising and securing the car on jack stands or driving the front tires onto ramps creates beneficial work space underneath.

The belt tensioner is too close to the fan housing to use a normal ratchet for belt removal. Some owners report success with a swivel joint on the required E10 female Torx. We used locking pliers on the Torx socket.

Take a photo of the serpentine belt-routing or draw a schematic before removing the belt. This will come in handy later. The alternator's upper mounting bolt contacts the flexible air ducting during removal. We undid the duct's lower clamp and gently pushed the duct outboard to create bolt clearance.

The pulley will likely have to be swapped onto the new alternator. An impact wrench is helpful for removing the pulley's retaining nut.

The torque specs are as follows (111 Model Engine):

B+ terminal electrical nut - 15 Nm
D+ terminal electrical nut - 4 Nm
Alternator mounting bolts - 42 Nm
Always disconnect the negative battery cable before doing any electrical work.
Figure 1

Always disconnect the negative battery cable before doing any electrical work.

The belt must be removed from the alternator's pulley.
Figure 2

The belt must be removed from the alternator's pulley. Our 2000 SLK 230 has dual serpentines. The right-side belt's tensioner is close to the fan shroud. An E10 internal Torx socket (green arrow) fits on the adjuster's stud, but there isn't space for a ratchet. We clamped locking pliers on the Torx socket and pulled the makeshift tool counter-clockwise until the belt came loose.

From underneath, remove the lower engine cover (see that article for details).
Figure 3

From underneath, remove the lower engine cover (see that article for details). Then pop the protective boot off the larger electrical connection, labeled B+ on the alternator's plastic cover.

The lower, larger nut (the one with the â€Ã...
Figure 4

The lower, larger nut (the one with the "protection") can be removed with a 13mm socket.

Viewed from the front, the upper nut (D+ terminal) comes off with an 8mm socket.
Figure 5

Viewed from the front, the upper nut (D+ terminal) comes off with an 8mm socket.

Looking straight up, the alternator's two mounting bolts are at about 11:00 and 4:00 in relation to the pulley (green arrows).
Figure 6

Looking straight up, the alternator's two mounting bolts are at about 11:00 and 4:00 in relation to the pulley (green arrows). Their heads take a 16mm socket or wrench. We unclamped the lower air duct and moved it outboard to get the upper bolt out. The lower bolt can be shimmied around the fan shroud.

Theoretically, the alternator should drop down once the bolts are removed.
Figure 7

Theoretically, the alternator should drop down once the bolts are removed. In reality, we used a large screwdriver as a pry bar to slowly push the alternator out of its mounting bracket. The components are aluminum, so whacking them with a hammer isn't recommended. Reverse the removal steps to install the replacement alternator.

R170 SLK alternator replacement is basically a two bolt/two nut job.
Figure 8

R170 SLK alternator replacement is basically a two bolt/two nut job. An E10 Torx socket (arrow) is required for loosening the serpentine belt. Otherwise, basic hand tools and possibly some kind of pry bar are all that's required.

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Page last updated: Fri 8/18/2017 02:31:11 AM