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Spark Plugs Coils and Wire Replacement
 
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Pelican Technical Article:

Spark Plugs Coils and Wire Replacement

Steve Vernon

Time:

2 hours2 hrs

Tab:

$20 to $600

Talent:

**

Tools:

5/8th thin walled spark plug socket, extensions, 5mm Allen

Applicable Models:

Mercedes-Benz 260E (1987-89)
Mercedes-Benz 300CE (1988-93)
Mercedes-Benz 300E (1986-93)
Mercedes-Benz 300TE (1988-93)
Mercedes-Benz 400E (1992-93)
Mercedes-Benz 500E (1992-93)
Mercedes-Benz E320 (1994-95)
Mercedes-Benz E420 (1994-95)
Mercedes-Benz E500 (1994)

Parts Required:

New plugs, wires and or coils

Hot Tip:

Remove the air intake pipe

Performance Gain:

Proper idle

Complementary Modification:

Change air filter

Replacing the spark plugs on your W124 is recommended routine maintenance. Mercedes-Benz recommends replacing the spark plugs on W124 models every 30,000 miles and I suggest replacing your spark plugs every three years regardless of mileage. If the last time they were serviced is unknown, replace them right away.

Changing spark plugs is one of those time-honored maintenance tasks that any self-respecting car enthusiast should do themselves. It's usually easy to do and it gives you a chance to feel like a real gear head without having to get all that dirty or spend hours writhing around on the floor of your garage.'

In the case of the M104 engine, however, it's quite easy to change the spark plugs. On the M104 engine, Mercedes has eliminated the use of a distributor by integrating a 'wasted spark' ignition setup with three coils. In a wasted spark system, the spark plugs fire in pairs even though one plug is on the compression stroke and the other plug is on its exhaust stroke. The extra spark on the exhaust stroke has no effect and is considered to be "wasted." The main advantage of using this setup is that there are half as many components needed for the engine and thus it is less expensive to produce.'The single coils work the number 1 and 6, number 2 and 5 and number 3 and 4 spark plugs. Half of the plugs are a coil-on-plug ignition system. All this means for the home mechanic is that the coil must be removed in order to access the spark plugs, rather than a cable running between the coil and the plugs.

One other thing you may want to consider is putting a small amount of dielectric grease on the ends of the connectors where it attaches to the spark plug. This will help with the conductivity and seal out moisture. I do NOT recommend putting anti seize on the plugs.

It is a very good idea to change the plugs when the engine is cold. If the engine is hot or warm the threads in the head are much more susceptible to damage from stripping or cross threading.

Begin by disconnecting the ground on the battery. Please see our article on battery maintenance and replacement.

The six spark plugs on the M104 engine are located under a cover on the top of the valve cover (red arrow).
Figure 1

The six spark plugs on the M104 engine are located under a cover on the top of the valve cover (red arrow). You will need to remove the air intake tube (green arrow) to get access to the plugs, coils and wires.

Start by disconnecting the MAF sensor.
Figure 2

Start by disconnecting the MAF sensor. The MAF is held in place by two clips to the air filter box (red arrow, one shown, one below), a single clamp to the intake tube (yellow arrow) and the wiring harness (green arrow).

Turn the locking ring on the harness (red arrow) counter clockwise; this will separate it from the connection on the MAF (yellow arrow).
Figure 3

Turn the locking ring on the harness (red arrow) counter clockwise; this will separate it from the connection on the MAF (yellow arrow).

Separate it from the air filter lid by pulling back on the two clips (red arrows).
Figure 4

Separate it from the air filter lid by pulling back on the two clips (red arrows). You can leave the MAF attached to the tube and remove the MAF and tube together.

Remove the two 10mm nuts where the intake tube attaches to the valve cover (red arrows).
Figure 5

Remove the two 10mm nuts where the intake tube attaches to the valve cover (red arrows).

Disconnect the air intake temperature sensor (red arrow).
Figure 6

Disconnect the air intake temperature sensor (red arrow).

There is a single hose clamp that holds the intake tube to the throttle body.
Figure 7

There is a single hose clamp that holds the intake tube to the throttle body. You will need to reach between the water pump and the intake runners with a long flathead screwdriver (red arrow) to loosen the clamp. With the clamp loose, remove the intake tube and MAF from the top of the engine.

There are six 5 mm Allen head bolts that hold the access panel to the top of the valve cover (red arrows).
Figure 8

There are six 5 mm Allen head bolts that hold the access panel to the top of the valve cover (red arrows).

Use a 5mm Allen and remove the bolts and washers (red arrow).
Figure 9

Use a 5mm Allen and remove the bolts and washers (red arrow).

There is a diagram on the top of the cover that illustrates the coil and plug distribution (red arrow).
Figure 10

There is a diagram on the top of the cover that illustrates the coil and plug distribution (red arrow).

Remove the cover from the top of the engine (red arrow).
Figure 11

Remove the cover from the top of the engine (red arrow).

This photo illustrates how the number 6 coil also fires the number 1 plug (red arrows); the number 2 coil also fires the number 5 plug (yellow arrows); and the number 4 coil also fires the number 3 plug (green arrows).
Figure 12

This photo illustrates how the number 6 coil also fires the number 1 plug (red arrows); the number 2 coil also fires the number 5 plug (yellow arrows); and the number 4 coil also fires the number 3 plug (green arrows). If you are going to be removing the coils and wire connectors completely from the head, it is a good idea to number them first for ease of reinstallation.

Pull the spark plug connectors up by the hard plastic part (red arrow).
Figure 13

Pull the spark plug connectors up by the hard plastic part (red arrow). Never pull the connector up from the wires.

You can pull the connector for the coil over plugs up by the coil itself (red arrow).
Figure 14

You can pull the connector for the coil over plugs up by the coil itself (red arrow). The wires, coils and harness all disconnect from each other for replacement.

Use a 5/8th-inch thin walled spark plug socket and remove the plugs.
Figure 15

Use a 5/8th-inch thin walled spark plug socket and remove the plugs. When installing plugs always thread them gently by hand until you are sure they are properly threading. You do not want to strip or cross thread the plug holes.

Examine the plugs for any issues with the motor (red arrow).
Figure 16

Examine the plugs for any issues with the motor (red arrow). Installation is the reverse of removal.

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