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The maintenance manual recommends inspecting ignition components at least once a year. This article will help you replace distributor cap, wires, rotor, and spark plugs, in addition to performing a compression test to track the engine's internal condition.

(1) Is this really $200 worth of parts? Welcome to Mercedes Benz maintenance. Actually, when you compare the quality of these components to the typical Pep Boys stuff, the price seems well worth it. Today we'll replace the distributor cap and rotor, spark plug wires, and spark plugs. In addition, since we'll be pulling the plugs anyway, we'll do a compression check to keep an eye on engine wear. (NOTE: Bosch brand wires are shown here. These were found to be of low quality and failed quickly. Use BERU brand wires if possible. They're much higher quality, and nearly identical in price. - ed.)

(2) Step one is to remove the old distributor cap. That means inserting a flat bladed screwdriver into each of the two hold down screws, pushing down, and giving a quarter turn counter-clockwise. When pressure on the screwdriver is released, the screw should pop up indicating it's free of the cap's hold down retainer. When both screws have been released, the cap can be lifted free exposing the rotor. Pull straight up on the rotor to remove it from the center shaft of the distributor.

(3) Here's the new rotor (left) I got from my local MB mechanic, compared to the old one. They both have a potted supression resistor in the rotor arm. The main difference in the two is that the new rotor has this ingenious little rev limiter built into the back side of the rotor (green arrow). At high RPM, a spring loaded weight travels outward, contacting a metal tab, grounding out the spark from the coil. Pretty neat!
(4) Before replacing the rotor, we'll put a drop of light machine oil on the fiber pad under the rotor to keep the top end of the distributor properly lubricated.
(5) And here's the new rotor all in place, ready for replacement of the distributor cap. Replacement of the distributor cap is the reverse of removal. Put the cap in place on the distributor and rotate the cap until you feel the locator tab click into place. At that point you won't be able to rotate the cap any more. Hold the cap down with your hand while you push your flat bladed screwdriver down lightly on one of the cap's clamping screws and rotate the screw clockwise until you feel it stop. Then push the screw down until you're able to turn it farther, and turn it an additional quarter turn. This is tough to explain in words, but it should be obvious when the angled legs on the bottom of the screws are holding the cap securely.

(6) Next remove all six spark plugs. About this time you should also remove your fuel pump relay, attach the remote starter switch to the starter, and remove the ignition sense line from the ignition control box. Instructions for these operations can be found in the article on Valve Clearance Adjustment. And as always, when using the remote starter, have the wheels securely chocked, parking brake applied, and transmission in neutral.
(7) Here's one of the old spark plugs. It shows some deposits from fuel additives, but its a perfect light tan color, indicating that the fuel mixture is just right, and there is no oil or coolant leaking into the cylinder.
(8) We've threaded the compression tester's hose into the #1 spark plug hole, being very careful not to cross thread it. There are four important steps to getting a good compression test reading. One is to have a good quality gage that's easy to read. The second is to perform the test on a warm engine so all parts are at their normal operating clearances. Third is to have all the spark plugs removed so the engine spins as freely as possible. And last, but not least, the throttle plate must be held in the wide-open position during the test. Here a screwdriver inserted into the throttle linkage (green arrow) holds the butterfly wide open.

(9) Now the compression tester gage is attached to the hose and the engine cranked by pressing the remote starter button. Alternatively, you may have a friend turn the ignition key to turn the engine over. In either case, crank the engine until a steady reading is achieved on the gage. In this case the reading is 100 psi. Not bad for this engine, considering I'm performing the test at 9000 ft elevation (thinner air means lower compression). Release the compressed air in the gage and repeat the test to verify you have the correct reading. Write the reading down and repeat the test for the remaining cylinders. In this case my 6 cylinders read, 100, 100, 100, 95, 95, 100. This is well within specification. At least as important as the actual reading is the fact that all readings are within 10% of the highest reading. In this case, all are within 5%, so the engine should run smoothly. E-mail me if you'd like more information about how to diagnose erratic compression readings.

(10) Once the compression test is complete, log all of your results in a place where you can refer to them next time you do a compression test, either routinely, or in an attempt to diagnose some problem. If you have a problem later, and need to do a compressino test, it will be of limited value unless you have baseline compression data from when the engine was running well. Next we'll place all of the new spark plug wires onto the distributor cap. The wire with the odd end is for the coil and should be attached to the center post of the distributor. Since Mercedes is kind enough to mold the correct cylinder number into the distributor cap adjacent to each wire tower, all we have to do is start with the shortest wire in the bunch, attach it to the #1 tower, attach the next shortest to #2, and so on through #6. I like to spread a little silicone grease inside each rubber boot (green arrow) before attaching it to the cap, just to keep moisture out and make the wires easier to remove later.

(11) The last step is to insert the new spark plugs. I verified the gap of all the new plugs to be the correct 0.90 mm, and spread a little anti-seize grease on the threads to keep them from sticking in the aluminum head. The orange things in the picture are a couple of old spark plug boots. A piece of thick-walled tubing would also work. What I'm doing is giving myself a handle I can use to start the plugs into the head by hand. Using the rubber tubing means I can't apply enough torque to the plugs to cross-thread them in the head. Get all six plugs started at least a few turns this way before putting a socket wrench on to tighten them down. Once you feel the resistance of the plug meeting the head, it should only require anout another 1/6 of a turn to have the plug plenty tight enough to do it's job. Any more force and you risk stripping the threads. It's difficult and expensive to repair stripped spark plug threads in this engine.

Once the plugs are in place, connect the proper wires to them, remove the remote starter switch and screwdriver proping the throttle open. Replace any parts you removed to gain access (the intake pipe, in this case), reconnect the ignition coil and fuel pump relay, and start the engine. It should be purring like a kitten!

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