Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide

Breaker Points and Condenser

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The diesel engine has no distributor so there are no breaker points or condensers to replace. Also, 1976 and later V-8's, 1977 and later 6-cylinder engines and 1977 and later 4-cylinder engines use a breakerless, electronic ignition which has no breaker points.

The points function as a circuit breaker for the primary circuit of the ignition system. The ignition coil must boost the 12 volts of electrical pressure supplied by the battery to as much as 25,000 volts in order to fire the plugs. To do this, the coil depends on the points and the condenser to make a clean break in the primary circuit.

The coil has both primary and secondary circuits. When the ignition is turned on, the battery supplies voltage through the coil and onto the points. The points are connected to ground, completing the primary circuit. As the current passes through the coil, a magnetic field is created in the iron center core of the coil. When the cam in the distributor turns, the points open, breaking the primary circuit. The magnetic field in the primary circuit of the coil then collapses and cuts through the secondary circuit windings around the iron core. Because of the physical principle called "electromagnetic induction," the battery voltage is increased to a level sufficient to fire the spark plugs.

When the points open, the electrical charge in the primary circuit tries to jump the gap created between the two open contacts of the points. If this electrical charge were not transferred elsewhere, the metal contacts of the points would start to change rapidly.

The function of the condenser is to absorb excessive voltage from the points when they open and thus prevent the points from becoming pitted or burned.

If you have ever wondered why it is necessary to tune-up your engine occasionally, consider the fact that the ignition system must complete the above cycle each time a spark plug fires. On a four-cylinder, four-cycle engine, two of the four plugs must fire once for every engine revolution. If the idle speed of your engine is 800 revolutions per minute (800 rpm), the breaker points open and close two times for each revolution. For every minute your engine idles, your points open and close 1600 times (800 - 1600). And that is just at idle. What about at 60 mph-

There are two ways to check breaker point gap: with a feeler gauge or with a dwell meter. Either way you set the points, you are adjusting the amount of time (in degrees of distributor rotation) that the points will remain open. If you adjust the points with a feeler gauge, you are setting the maximum amount the points will open when the rubbing block on the points is on a high point of the distributor cam. When you adjust the points with a dwell meter, you are measuring the number of degrees (of distributor cam rotation) that the points will remain closed before they start to open as a high point of the distributor cam approaches the rubbing block of the points.

If you still do not understand how the points function, take a friend, go outside, and remove the distributor cap from your engine. Have your friend operate the starter (make sure that the transmission is not in gear) as you look at the exposed parts of the distributor.

There are two rules that should always be followed when adjusting or replacing points. The points and condenser are a matched set; never replace one without replacing the other. If you change the point gap or dwell of the engine, you also change the ignition timing. Therefore, if you adjust the points, you must also adjust the timing.

IGNITION SYSTEM PRECAUTIONS



Mercedes-Benz has determined that some transistorized switching units have been damaged due to improper handling during service and maintenance work. The following precautions should be observed when working with transistorized switching units.

  1. Do not shut off a running engine by shorting terminal 15 of the ignition coil to ground or the transistorized switching unit will be destroyed.
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  3. Do not steam clean or apply water pressure to transistorized switching units, fuel injection control units, or ignition components, since water may enter these and short them.
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  5. Do not assume that transistor switching units are defective without checking the plug terminals. The plug terminals are frequently corroded because the rubber boot was not properly seated. In addition, the terminals can become corroded even if the rubber boot is properly seated. Mercedes-Benz recommends that all contacts be cleaned before assuming that a transistorized switching unit is defective.
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INSPECTION & CLEANING



The breaker points should be inspected and cleaned at 6000 mile intervals. To do so, perform the following steps:

  1. Disconnect the high-tension lead from the coil.
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  3. Unsnap the two distributor cap retaining clips and lift the cap straight up. Leave the leads connected to the cap and position it out of the way.
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  5. Remove the rotor and dust cover by pulling them straight up.
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  7. Place a screwdriver against the breaker points and pry them open. Examine their condition. If they are excessively worn, burned, or pitted, they should be replaced.
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  9. Clean the distributor cap and rotor with alcohol. Inspect the cap terminals for looseness and corrosion. Check the rotor tip for excessive burning. Inspect both cap and rotor for cracks. Replace either if they show any of the above signs of wear or damage.
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  11. Check the operation of the centrifugal advance mechanism by turning the rotor clockwise. Release the rotor; it should return to its original position. If it doesn't, check for binding parts.
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  13. If the points do not require replacement, proceed with the adjustment section below. Otherwise perform the point and condenser replacement procedures.
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REMOVAL & INSTALLATION



See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Lubricate the point set rubbing surface with high temperature multi-purpose grease before installation

  1. Raise the hood and locate the distributor. Remove the rubber or plastic cover (if equipped).
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  3. Release the clips on the side of the distributor cap and remove the cap. Lay it aside.
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  5. Remove the rotor and dust shield from the distributor shaft. The rotor only fits one way.
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  7. Some distributors have a protective cover installed over the points, which must be removed.
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  9. Remove the distributor contact holder by removing the screw or screws. Some models also have a snapring on the bearing contact lever, which must also be removed. Pry the wire from the connecting terminal or loosen the screw at the terminal and remove the wire from the connecting terminal.
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  11. Disconnect the condenser wire and remove the condenser from its bracket. The condenser screw is located on the outside of the distributor.
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To install:
  1. Before installing new points, clean the contact surfaces by squeezing them against a clean matchbook cover. This will remove any film or grease. Also be sure the point contact faces are aligned. If not, bend the fixed contact arm so that the points align and close squarely.
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  3. Lightly coat the rubbing arm of the contact breaker with high temperature multipurpose grease. It is no longer necessary to lubricate the felt pad.
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  5. Install a new condenser and connect the wire.
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  7. Install a new contact set or sets into the distributor.
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  9. Install the hold-down screw(s) or the snaprings on the bearing pins of the contact plate.
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  11. Connect the wire to the terminal and tighten the nut, if necessary.
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  13. If equipped, install the cover over the breaker points. Be sure it does not interfere with the distributor cam.
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  15. Install the plate and rotor on the shaft.
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  17. Install the cap and secure it in place with the clips.
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  19. Check the dwell angle and ignition timing. Adjust if necessary.
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ADJUSTMENT Feeler Gauge Method



See Figures 2 and 3

Click image to see an enlarged view

Fig. Fig. 2: Insert a feeler gauge between the points and check the gap



Click image to see an enlarged view

Fig. Fig. 3: With the feeler gauge inserted, adjust the points until a slight drag is felt

Perform the gap adjustment procedure whenever new points are installed, or as part of routine maintenance. If you are adjusting an old set of points, you must check the dwell as well, since the feeler gauge is really only accurate with a new point set.

  1. Rotate the engine by hand or by using a remote starter switch, so that the rubbing block is on the high point of the cam lobe.
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  3. Insert a feeler gauge between the points; a slight drag should be felt.
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  5. If no drag is felt or if the feeler gauge cannot be inserted at all, loosen, but do not remove, the point hold-down screw.
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  7. Insert a screwdriver into the adjustment slot. Rotate the screwdriver until the proper point gap is attained. The point gap is increased by rotating the screwdriver counterclockwise and decreased by rotating it clockwise. On some models it is possible to adjust the point gap by means of an eccentric adjustment screw provided for this purpose in the breaker plate.
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  9. Tighten the point hold-down screw.
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Lubricate the cam lobes, breaker arm, rubbing block, arm pivot, and distributor shaft with special high-temperature distributor grease. Check the dwell.

Dwell Meter Method See Figure 4

Click image to see an enlarged view

Fig. Fig. 4: Dwell is the amount of time the points are open (shaded areas)

A dwell meter virtually eliminates errors in point gap caused by the distributor cam lobes being unequally worn, or human error. In any case, point dwell should be checked as soon as possible after setting with a feeler gauge because it is a far more accurate check of point operation under normal operating conditions.

The dwell meter, actually a modified voltmeter, depends on the nature of contact point operation for its usefulness. In this electro-mechanical system, a fiber block slides under tension, over a cam (see illustrations). The angle (in black) that the block traverses on the cam, during which time current is made available to the coil primary winding, is an inverse function of point gap. In other words, the wider the gap, the smaller the "dwell" (expressed in degrees); the closer the gap, the greater the "dwell."

Because the fiber block wears down gradually in service, it is a good practice to set the dwell on the low side of any dwell range (smaller number of degrees) given in specifications. As the block wears, the dwell becomes greater (toward the center of the range) and point life is increased between adjustments.

To connect the dwell meter, switch the meter to the six-, four- or eight-cylinder range, as the case may be, and connect one lead to ground. The other lead should be connected to the coil distributor terminal (the one having the wire going to contact points). Follow the manufacturer's instructions if they differ from those listed. Zero the meter, start the engine and gradually allow it to assume normal idle speed. (See "Tune-Up Specifications.") The meter should agree with the specifications. Any excessive variation in dwell indicates a worn distributor shaft or bushings, or perhaps a worn distributor cam or breaker plate.

Up until 1976 (V8), 1977 (6-cylinder) or 1977 (4-cylinder), Mercedes-Benz engines use transistorized ignitions. These can be identified by a "blue" ignition coil. Occasionally, a dwell meter or tachometer will not work on these ignitions because of internal design.

It is obvious from the above procedure that some means of measuring engine rpm must also be employed when checking dwell. An external tachometer should be employed. Hook-up is the same as for the dwell meter and both can be used in conjunction. Most commercial dwell meters have a tachometer scale built in and switching between them is possible.

Diesel engines, 1976 and later V8's, 1977 and later 6-cylinder engines and 1977 and later 4-cylinder engines have no provision for adjusting dwell.

  1. The dwell angle should be measured at idle speed.
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  3. Raise the hood and connect a dwell meter/tachometer.
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  5. Start the engine and allow it to reach normal idle speed. Read the dwell angle from the meter on the appropriate scale.
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  7. If the dwell angle is not according to specifications, remove the distributor cap and adjust the dwell angle. Reduce the point gap if the dwell angle is too small, or increase the contact point gap if the dwell angle is too large.
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  9. To actually adjust the point gap, stop the engine and loosen the hold-down screw and insert a screwdriver between the lugs on the breaker plate. Move the plate to the desired location. Tighten the hold-down screw. On some models it is possible to adjust the point gap by means of the eccentric screw provided for this purpose in the breaker plate.
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  11. Recheck the dwell angle and adjust the gap again if it is still not satisfactory. Repeat the process until the dwell angle is as specified.
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