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 or higher in order to correctly 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 4-cylinder, 4-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 each revolution. For every minute your engine idles, your points open and close 1,600 times (2 x 800 = 1,600). And that is just at idle. What about at 60 mph-
There are two ways to check breaker point gap: with a feel 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 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 swell of the engine, you also change the ignition timing. Therefore, if you adjust the points, you must also adjust the timing.
Certain 1978-79 and virtually all 1980 and later BMWs are equipped with electronic, breakerless ignition systems. See the following section for maintenance procedures.
See Figures 1, 2 and 3
- Disconnect the high tension wire from the top of the distributor and the coil.
- Remove the distributor cap by prying off the spring clips on the sides of the cap.
- Remove the rotor from the distributor shaft by pulling it straight up. Examine the condition of the rotor. If it is cracked or the metal tip is excessively worn or burned, it should be replaced. Clean the top with fine emery paper.
- Pry open the contacts of the points with a screwdriver and check the condition of the contacts. If they are excessively worn, burned or pitted, they should be replaced.
- If the points are in good condition, adjust them and replace the rotor and the distributor cap. If the points need to be replaced, follow the replacement procedure given below.
REMOVAL & INSTALLATION
See Figure 4
On 1970-74 cars, BMW recommends that the breaker points be inspected and adjusted every 6 months or 8,000 miles (13,000 km). If, upon inspection, the points prove to be faulty, they must be replaced with the condenser as a unit. On later models, replace the points every 12,500 miles (20,000 km), if the car is so equipped.
The usual procedure is to replace the condenser each time the point set is replaced. Although this is not always necessary, it is easy to do at this time and the cost is negligible. Every time you adjust or replace the breaker points, the ignition timing must be checked and, if necessary, adjusted. No special equipment other than a feeler gauge is required for point replacement or adjustment, but a dwell meter is strongly advised. A magnetic screwdriver is handy to prevent the small points and condenser screws from falling down into the distributor.
- Disconnect the coil high tension wire from the top of the distributor cap. Remove the distributor cap and place it out of the way. Remove the rotor from the distributor shaft by pulling straight up.
- Disconnect the electrical lead to the condenser, loosen the condenser bracket retaining screw and slide out the condenser.
- Disconnect the point set electrical lead.
- Remove the points assembly attaching screws and then remove the points. A magnetic screwdriver or one with a holding mechanism will come in handy here, so that you don't drop a screw into the distributor and have to remove the entire distributor to retrieve it.
After the points are removed, wipe off the cam and apply new cam lubricant. If you don't, the points will wear out in a few thousand miles.
- Slip the new set of points onto the locating dowel and install the screws that hold the assembly onto the plate. Don't tighten them all the way yet, since you'll only have to loosen them to set the point gap.
- Install the new condenser and attach the condenser lead to the points.
- Set the point gap and dwell, as described in the following procedures.
See Figures 5, 6 and 7
- If the contact points of the assembly are not parallel, bend the stationary contact so that they make contact across the entire surface of the contacts. Bend only the stationary bracket part of the point assembly; not the movable contact.
- Turn the engine until the rubbing block of the points is on one of the high points of the distributor cam. You can do this by either turning the ignition switch to the start position and releasing it quickly ("bumping" the engine) or by using a wrench on the bolt which holds the crankshaft pulley to the crankshaft.
- Place the correct size feeler gauge between the contacts (see the Tune-Up Specifications chart). Make sure that it is parallel with the contact surfaces.
- With your free hand, insert a screwdriver into the eccentric adjusting slot, then twist the screwdriver to either increase and decrease the gap to the proper setting.
- Tighten the adjustment lockscrew and recheck the contact gap to make sure that it didn't change when the lockscrew was tightened.
- Replace the rotor and distributor cap, and the high tension wire which connects the top of the distributor and the coil. Make sure that the rotor is firmly seated all the way onto the distributor shaft and that the of the rotor is aligned with the notch in the shaft. Align the tab in the base of the distributor cap with the notch in the distributor body. Make sure that the end of the high tension wire is firmly placed in the top of the distributor and the coil.
See Figure 8
The dwell angle or cam angle is the number of degrees that the distributor cam rotates while the points are closed. There is an inverse relationship between dwell angle and point gap. Increasing the point gap will decrease the swell angle and vice versa. Checking the swell angle with a meter is a far more accurate method of measuring point opening than the feeler gauge method.
After setting the point gap to specification with a feeler gauge as described above, check the dwell angle with a meter. Attach the dwell meter according to the manufacturer's instruction sheet. The negative lead is grounded and the positive lead is connected to the primary wire terminal which runs from the coil to the distributor. Start the engine, let it idle and reach operating temperature, and observe the dwell on the meter. The reading should fall within the allowable range. If it does not, the gap will have to be reset or the breaker points will have to be replaced.
Dwell can be checked with the engine running or cranking. Decrease dwell by increasing the point gap; increase by decreasing the gap. Dwell angle is simply the number of degrees of distributor shaft rotation during which the points stay closed. Theoretically, if the point gap is correct, the dwell should also be correct or nearly so. Adjustment with a dwell meter produces more exact, consistent results since it is a dynamic adjustment. If dwell varies more than 3° from idle speed to 1,750 engine rpm, the distributor is worn and will probably require adjustment.
- Adjust the points with a feeler gauge as previously described.
- Connect the dwell meter to the ignition circuit as per the manufacturer's instructions. One lead of the meter is connected to a ground and the other lead is connected to the distributor post on the coil. An adapter is usually provided for this purpose.
- If the dwell meter has a set line on it. adjust the meter to zero the indicator.
- Start the engine.
Be careful when working on any vehicle while the engine is running. Make sure that the transmission is in PARK or NEUTRAL and that the parking brake is applied. Keep hands, clothing, tools and the wires of the test instruments clear of the rotating fan blades.
- Observe the reading on the dwell meter. If the reading is within the specified range, turn off the engine and remove the dwell meter.
If the meter does not have a scale for 4-cylinder engines, multiply the 8-cylinder reading by 2.
- If the reading is above the specified range, the breaker point gap is too small. If the reading is below the specified range, the gap is too large. In either case, the engine must be stopped and the gap adjusted in the manner previously covered.
After making the adjustment, start the engine and check the reading on the dwell meter. When the correct reading is obtained, disconnect the dwell meter.
- Check the adjustment of the ignition timing.