Mercedes Coupes/Sedans/Wagons 1974-1984 Repair Guide



Ignition timing is the measurement in degrees of crankshaft rotation of the instant the spark plugs in the cylinders fire, in relation to the location of the piston, while the piston is on its compression stroke.

Diesel engines use no distributor, so they require no ignition timing adjustment.

Ignition timing is adjusted by loosening the distributor locking device and turning the distributor in the engine.

Ideally, the air/fuel mixture in the cylinder will be ignited (by the spark plug) and just beginning its rapid expansion as the piston passes top dead center (TDC) of the compression stroke. If this happens, the piston will be beginning the power stroke just as the compressed (by the movement of the piston) and ignited (by the spark plug) air/fuel mixture starts to expand. The expansion of the air/fuel mixture will then force the piston down on the power stroke and turn the crankshaft.

It takes a fraction of a second for the spark from the plug to completely ignite the mixture in the cylinder. Because of this, the spark plug must fire before the piston reaches TDC, if the mixture is to be completely ignited as the piston passes TDC. This measurement is given in degrees (of crankshaft rotation) before the piston reaches top dead center (BTDC). If the ignition timing setting for your engine is seven degrees (7-) BTDC, this means that the spark plug must fire at a time when the piston for that cylinder is 7- before top dead center of its compression stroke. However, this only holds true while your engine is at idle speed.

As you accelerate from idle, the speed of your engine (rpm) increases. The increase in rpm means that the pistons are now traveling up and down much faster. Because of this, the spark plugs will have to fire even sooner if the mixture is to be completely ignited as the piston passes TDC. To accomplish this, the distributor incorporates means to advance the timing of the spark as engine speed increases.

The distributor in your Mercedes-Benz has two means of advancing the ignition timing. One is called centrifugal advance and is actuated by weights in the distributor. The other is called vacuum advance and is controlled by that larger circular housing on the side of the distributor.

In addition, some distributors have a vacuum-retard mechanism which is contained in the same housing on the side of the distributor as the vacuum advance. The function of this mechanism is to retard the timing of the ignition spark under certain engine conditions. This causes more complete burning of the air/fuel mixture in the cylinder and consequently lowers exhaust emissions.

Because these mechanisms change ignition timing, it is necessary to disconnect and plug the one or two vacuum lines from the distributor when setting the basic ignition timing.

If ignition timing is set too far advanced (BTDC), the ignition and expansion of the air/fuel mixture in the cylinder will try to force the piston down the cylinder while it is still traveling upward. This causes engine "ping," a sound which resembles marbles being dropped into an empty tin can. If the ignition timing is too far retarded (after, or ATDC), the piston will have already started down on the power stroke when the air/fuel mixture ignites and expands. This will cause the piston to be forced down only a portion of its travel. This will result in poor engine performance and lack of power.

Ignition timing adjustment is checked with a timing light. This instrument is connected to the number one (No. 1) spark plug of the engine. The timing light flashes every time an electrical current is sent from the distributor, through the No. 1 spark plug wire, to the spark plug. The vibration damper or balancing plate are marked with a timing pointer and a timing scale. When the timing pointer is aligned with the mark (pin) on the timing scale, the piston in No. 1 cylinder is at TDC of its compression stroke. With the engine running, and the timing light aimed at the timing pointer and timing scale, the stroboscopic flashes from the timing light will allow you to check the ignition timing setting of the engine. The timing light flashes every time the spark plug in the No. 1 cylinder of the engine fires. Since the flash from the timing light makes the crankshaft pulley seem stationary for a moment, you will be able to read the exact position of the piston in the No. 1 cylinder on the timing scale on the front of the engine.

There are three basic types of timing light available. The first is a simple neon bulb with two wire connections (one for the spark plug and one for the plug wire, connecting the light in series). This type of light is quite dim, and must be held closely to the marks to be seen, but it is inexpensive. The second type of light operates from the car battery. Two alligator clips connect to the battery terminals, while a third wire connects to the spark plug with an adapter. This type of light is more expensive, but the zenon bulb provides a nice bright flash which can even be seen in sunlight. The third type replaces the battery source with 110 volt house current. Some timing lights have other functions built into them, such as dwell meters, tachometers, or remote starting switches. These are convenient, in that they reduce the tangle of wires under the hood, but may duplicate the functions of tools you already have.

If your Mercedes has electronic ignition, you should use a timing light with an inductive pickup. This pickup simply clamps onto the No. 1 plug wire, eliminating the adapter. It is not susceptible to crossfiring or false triggering, which may occur with a conventional light, due to the greater voltages produced by electronic ignition.

INSPECTION & ADJUSTMENT All Engines (Except Diesels)

See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Disconnect and plug both vacuum lines when adjusting the timing280, 280C and 280S engines

  1. Warm-up the engine. Connect a tachometer and check the engine idle speed to be sure that it is within the specification given in the "Tune-Up Specifications" chart at the beginning of the section.
  3. If the timing marks are difficult to see use a dab of paint or chalk to make them more visible.
  5. Connect a timing light according to the manufacturer's instructions.
  7. Disconnect the vacuum line(s) from the distributor vacuum unit. Plug it (them) with a pencil or golf tee(s).
  9. Be sure that the timing light wires are clear of the fan and start the engine.

Keep fingers, clothes, tools, hair, and leads clear of the spinning engine fan. Be sure that you are running the engine in a well-ventilated area.

  1. Allow the engine to run at the specified idle speed with the gearshift in Neutral with manual transmission and Drive (D) with automatic transmission.

Be sure that the parking brake is set and that the front wheels are blocked to prevent the car from rolling forward, especially when Drive is selected with an automatic.

  1. Point the timing light at the mark indicated previously. With the engine at idle, timing should be at the specification given on the "Tune-Up Specifications" chart at the beginning of the section.

The balancer on some engines has two timing scales. If in doubt as to which scale to use, rotate the crankshaft (in the direction of rotation only) until the distributor rotor is aligned with the notch on the distributor housing (No. 1 cylinder). In this position, the timing pointer should be at TDC on the proper timing scale.

  1. If the timing is not at the specification, loosen the pinch-bolt at the base of the distributor just enough so that the distributor can be turned. Turn the distributor to advance or retard the timing as required. Once the proper marks are seen to align with the timing light, timing is correct.
  3. Stop the engine and tighten the pinchbolt. Start the engine and recheck timing. Stop the engine; disconnect the tachometer and timing light. Connect the vacuum line(s) to the distributor vacuum unit.