Ignition Points and Condenser Replacement

Replace breaker points and condenser in the Bosch distributor of the M127 engine, set proper dwell angle and base ignition timing to ensure reliable spark generation and optimal engine performance.

Warnings

Tools required

Parts

• Breaker point set × 1 — Bosch 1 237 013 015 or equivalent for 6-cylinder Bosch distributor
• Condenser × 1 — Bosch 1 237 330 080 or equivalent (0.22-0.25 microfarad)
• Distributor rotor × 1 — Recommended replacement while distributor is open
• Distributor cap × 1 — Inspect and replace if cracked or carbon-tracked

Preparation

1. Disconnect the negative battery cable and secure it away from the terminal.
2. Allow the engine to cool completely if it has been recently run.
3. Clean the exterior of the distributor and surrounding engine area to prevent debris from entering the distributor during service.
4. Gather all tools and parts before beginning, as the distributor should not remain open for extended periods to prevent contamination.
5. If available, use a piece of chalk or paint pen to mark the position of the distributor body relative to the engine block and the rotor position relative to the distributor housing for reference during reassembly.

Procedure

1. 1

   Access the distributor assembly
   The distributor on the M127 engine is located on the right side (passenger side) of the engine block, driven by the camshaft through a vertical shaft. It sits roughly mid-engine height, between the cylinder head and the mechanical fuel injection pump. Remove the spark plug wires from the distributor cap by pulling firmly on the boots, not the wires themselves. Label each wire or take a photograph showing wire positions corresponding to cylinder numbers. The firing order 1-5-3-6-2-4 must be preserved. Once wires are clear, remove the two spring clips securing the distributor cap by prying them away from the cap body with a flat-blade screwdriver. Some distributors may use screws instead of clips; if so, use a screwdriver to remove them. Lift the distributor cap straight up and set it aside with the wires still attached if wire length permits, otherwise carefully disconnect all wires and label them clearly.
2. 2

   Remove the distributor rotor
   With the distributor cap removed, the rotor is now visible on top of the distributor shaft. The rotor on the Bosch distributor used in the M127 is typically held by a single screw at the center or may be a press-fit design with a small retaining screw on the side. If a center screw is present, use a Phillips screwdriver to remove it, then lift the rotor straight up off the shaft. Note the orientation of the rotor before removal—the square drive tang on the bottom of the rotor fits into the distributor shaft in only one position, but marking its orientation relative to the distributor body helps during reassembly. Inspect the rotor contact for burning or erosion; if the brass contact is heavily pitted or the plastic body shows cracks or carbon tracking, replace it. Set the rotor aside in a clean location.
3. 3

   Inspect the distributor interior and cam
   With the rotor removed, you now have a clear view of the breaker points assembly, condenser, and the distributor cam (the lobed shaft that operates the points). The M127 uses a 6-lobe cam, meaning there are six high points that open the points once per cylinder firing. Carefully inspect the cam lobes for wear, grooving, or flat spots. The cam surface should be smooth and uniform; excessive wear on the lobes will cause erratic point gap and dwell variation across cylinders, leading to misfiring and timing inconsistencies. If the cam shows significant wear (grooves deeper than 0.020 inch or visible flat spots), the entire distributor shaft assembly should be replaced, which is a more involved procedure. Also check the felt wick or pad at the center of the cam; this small felt piece should be lightly lubricated to reduce friction on the breaker point rubbing block. If the felt is dry, hard, or missing, it must be replaced or lubricated during reassembly.
4. 4

   Remove the old condenser
   The condenser is a cylindrical component mounted on the interior wall of the distributor body or on the breaker plate. It has a wire lead that connects to the primary terminal of the breaker points. Before removing anything, note the wire routing and connection point—typically the condenser wire and the wire from the breaker points both connect to the same terminal screw. Use a small screwdriver or wrench to loosen the terminal screw that holds the condenser wire. Slide the condenser wire terminal off the screw. Next, locate the mounting screw that secures the condenser to the distributor body; this is usually a single screw with a slotted or Phillips head. Remove this screw and carefully extract the condenser from the distributor. The condenser can be tested with a multimeter or condenser tester to confirm failure, but replacement is recommended whenever points are changed as condensers are inexpensive and critical to ignition system function. A condenser's purpose is to absorb the voltage spike that occurs when the points open, preventing arcing across the point contacts which causes burning and erosion. A failed condenser shows either infinite resistance (open circuit) or zero resistance (short circuit), or may have intermittent behavior. A properly functioning condenser should show high resistance on an ohmmeter and should charge/discharge when tested with appropriate equipment.
5. 5

   Remove the old breaker points
   The breaker point assembly consists of a fixed contact mounted on a spring arm and an adjustable contact on a plate with a rubbing block that rides on the distributor cam. Begin by locating the primary wire connection—this is typically a terminal screw on the movable portion of the point assembly where the spring arm connects to the ignition coil circuit. Loosen this screw and disconnect the wire, noting its routing for reinstallation. Next, identify the two screws that secure the point assembly to the breaker plate: one is the pivot screw that the point arm rotates on, and the other is the locking screw for the adjustable fixed contact. The adjustable contact will have an eccentric adjustment screw or slot that allows gap adjustment. Remove the locking screw completely. Then remove the pivot screw, holding the point assembly to prevent it from falling. Carefully lift the entire breaker point assembly out of the distributor. Inspect the removed points for the pattern of wear: if the contacts show a transferred metal buildup on one side and a crater on the other, this indicates arcing due to condenser failure or improper gap. If points show even gray pitting with no transfer, they were functioning correctly but reached end of life. Clean the mounting area on the breaker plate with a clean rag to remove any oil, corrosion, or debris that could affect the new points.
6. 6

   Install the new condenser
   Take the new condenser and position it in the same mounting location as the old unit. The body of the condenser should sit flush against the distributor housing or breaker plate. Insert the mounting screw and tighten it securely, but do not overtighten as the distributor housing may be made of softer metal or the condenser body could crack. Route the condenser wire lead toward the primary terminal where it will connect along with the breaker point wire. Do not connect it yet—this will be done after the points are installed and properly positioned. Ensure the condenser wire does not interfere with the movement of the breaker point arm or contact the rotating distributor shaft or cam. The wire should have a smooth path without sharp bends that could cause work-hardening and eventual breakage from engine vibration.
7. 7

   Install the new breaker points
   Unpack the new breaker point set and verify that all components are present: the point assembly with the movable contact arm and rubbing block, the fixed contact mounting bracket, pivot screw, lock screw, and any washers or insulators. Position the breaker point assembly on the breaker plate with the pivot hole aligned with the corresponding hole in the plate. Insert the pivot screw with any washers or insulators in the correct order—there is typically an insulating washer that prevents the point assembly from grounding to the plate except through the intended wire connection. Thread the pivot screw finger-tight but do not fully tighten it yet; the assembly needs to move slightly for adjustment. Next, position the adjustable fixed contact portion of the assembly. Some point sets have the adjustment mechanism integrated; others have a separate eccentric screw or slot for adjustment. Ensure that the rubbing block on the movable arm is properly positioned to ride on the distributor cam. The rubbing block is the fiber or plastic pad that contacts the cam lobe—it must be centered on the cam surface and aligned so that it rides smoothly on all lobes without side-loading. Verify that the point contacts are aligned face-to-face when closed; they should meet squarely without offset. If alignment is off, gently bend the fixed contact bracket (not the spring arm) until alignment is achieved. Once aligned, insert the locking screw for the adjustable contact and thread it finger-tight. The points are now mounted but not yet adjusted.
8. 8

   Set initial point gap with feeler gauge
   Before setting the gap, rotate the engine crankshaft or distributor shaft until the rubbing block of the breaker points is resting on the highest point of one of the six cam lobes—this is when the points are at maximum opening. You can rotate the engine by turning the crankshaft pulley bolt with a wrench, or if the ignition system is safe and battery disconnected, you can carefully rotate the distributor cam by hand. The specification for point gap on the M127 engine distributor is 0.016 inch (0.40mm), which is typical for Bosch distributors of this era, though you should verify this specification in the original Mercedes-Benz service documentation as specifications can vary slightly by production year. Select a feeler gauge blade of 0.016 inch. Loosen the locking screw on the fixed contact slightly to allow adjustment. Insert the feeler gauge between the open point contacts and adjust the position of the fixed contact using the eccentric adjusting screw or slot until the feeler gauge slides between the contacts with a very slight drag—not so loose it falls through, not so tight you cannot insert it. The feel should be consistent and smooth. Once the correct gap is achieved, carefully hold the adjustment in place and tighten the locking screw securely. After tightening, recheck the gap to ensure it did not shift; if it did, loosen and readjust. Check the gap at all six cam lobes by rotating the distributor shaft through a complete revolution. The gap should be consistent at each lobe within 0.001-0.002 inch. If variation is greater, the distributor cam is worn and should be replaced. Feeler gauge setting is adequate for initial setup, but dwell angle measurement is the preferred final adjustment method.
9. 9

   Connect the primary wire and condenser wire
   With the point gap initially set, connect the electrical components. The primary wire from the ignition coil negative terminal and the condenser wire must both connect to the primary terminal on the breaker point assembly. This terminal is typically a small screw on the movable arm or on an insulated post on the breaker plate. Route the primary wire from the coil to this terminal, ensuring the wire does not contact any grounded metal or moving parts. Place the terminal eyelet of the primary wire onto the screw post, then place the condenser wire terminal eyelet on top of it. Install any washers provided, then thread the terminal screw and tighten it securely. A loose connection here will cause intermittent ignition failure or weak spark. Give the wires a gentle tug to verify they are securely fastened. Ensure that all wiring inside the distributor is routed neatly and will not interfere with the rotor when it is reinstalled.
10. 10

    Lubricate the distributor cam and wick
    Proper lubrication of the distributor cam is essential for long point life and consistent dwell angle. The rubbing block on the breaker points rides on the cam lobes millions of times during normal operation, and without lubrication, both the cam and the rubbing block will wear rapidly, causing changing dwell and timing. Locate the felt wick or pad at the center of the distributor cam. Apply a small amount of distributor cam lubricant or high-temperature grease specifically designed for this purpose—do not use ordinary grease or oil, which can be thrown onto the point contacts by centrifugal force and cause misfiring. If the wick is hardened or missing, replace it with a new one. Apply only 2-3 drops of lubricant to the wick if using liquid cam lube, or a very small dab if using grease. The wick will distribute the lubricant to the cam lobes as they rotate. Also apply a very thin film of lubricant directly to each of the six cam lobes, using your finger or a clean rag to spread it evenly. Wipe away any excess—too much lubricant is as bad as too little, as it can contaminate the points. Do not lubricate the point contacts themselves or the breaker plate pivot; these must remain clean and dry.
11. 11

    Reinstall the rotor and distributor cap
    Place the distributor rotor back onto the distributor shaft, aligning the square or keyed drive tang with the slot in the shaft. The rotor will only fit in one orientation. If the rotor has a center mounting screw, insert it and tighten securely but not excessively—overtightening can crack the rotor body. Push the rotor down firmly to ensure it is fully seated; a loose rotor will cause erratic timing and possible arcing. Once the rotor is secure, inspect the inside of the distributor cap for cracks, carbon tracking, or corrosion on the terminals. Carbon tracking appears as thin black lines running from the center terminal to the outer terminals or to ground—this indicates high-voltage leakage and requires cap replacement. If the cap is acceptable, position it over the distributor body, aligning the tabs or notches so it seats fully. Secure the cap with the spring clips or screws, ensuring they are fully engaged. Reconnect the spark plug wires to the correct terminals on the cap according to your earlier marking or the firing order 1-5-3-6-2-4. The cylinder numbering on the M127 runs from front to rear, with cylinder 1 at the front of the engine. Ensure each wire boot is pushed fully onto the cap terminal to ensure good contact.
12. 12

    Set dwell angle with dwell meter (preferred method)
    While feeler gauge adjustment is sufficient for initial setup, setting the dwell angle with a dwell meter provides a more accurate and consistent result, especially for the M127 engine which requires precise ignition timing for optimal performance with the mechanical fuel injection system. Dwell angle is the number of degrees of distributor shaft rotation during which the points are closed and the ignition coil is energizing. For a 6-cylinder engine, dwell is measured on a 60-degree scale (360 degrees divided by 6 cylinders). The specified dwell angle for the M127 distributor is typically 33-37 degrees on the 6-cylinder scale, with 35 degrees being ideal. Check your specific service manual for exact specifications. Connect a dwell meter to the engine according to the meter's instructions—typically the positive lead connects to the distributor side of the ignition coil (terminal 1 or the negative coil terminal) and the negative lead connects to ground. Reconnect the battery negative cable. Set the dwell meter to the 6-cylinder scale. Start the engine and allow it to idle. Observe the dwell reading on the meter. If dwell is too low (below specification), the point gap is too wide and must be decreased; if dwell is too high (above specification), the point gap is too narrow and must be increased. The relationship is inverse: wider gap equals lower dwell; narrower gap equals higher dwell. To adjust, stop the engine, remove the distributor cap and rotor, and adjust the point gap as described in step 8, making small changes. Reinstall the rotor and cap, restart the engine, and recheck dwell. Repeat until dwell is within specification. Proper dwell is critical: insufficient dwell means the coil does not have enough time to fully energize, resulting in weak spark and misfiring, especially at high RPM; excessive dwell means the points remain closed too long, causing excessive current flow through the points which leads to overheating, burning, and premature point failure.
13. 13

    Set base ignition timing
    After setting dwell angle, base ignition timing must be checked and adjusted, as changing the point gap affects timing. The breaker points control the moment at which the spark is generated, so any change in when they open (due to gap adjustment) changes ignition timing. Base timing for the M127 engine is typically specified at idle speed and is usually in the range of 5-10 degrees BTDC (before top dead center), but you must consult the emissions label on the vehicle or the factory service manual for the exact specification, as this varies by year and market. Connect a timing light to cylinder number 1 spark plug wire according to the timing light instructions, and connect the power leads to the battery. Locate the timing marks on the crankshaft pulley and the timing pointer or scale on the front engine cover. Start the engine and allow it to reach normal operating temperature. Disconnect and plug any vacuum advance line if present on the distributor. Aim the timing light at the timing marks while the engine idles. The timing light will strobe each time cylinder 1 fires, making the timing mark on the pulley appear stationary. Verify that the mark aligns with the correct specification on the timing scale. If adjustment is needed, loosen the distributor hold-down clamp bolt (located at the base of the distributor where it enters the engine block) just enough to allow the distributor body to rotate. Rotate the distributor body slightly—rotating it counterclockwise advances the timing (moves the mark toward more BTDC), rotating it clockwise retards the timing. Make small adjustments and recheck with the timing light until the timing is correct. Once correct, hold the distributor firmly in place and tighten the hold-down clamp bolt securely. Recheck timing after tightening to ensure it did not shift. Reconnect any vacuum lines that were disconnected.
14. 14

    Perform final ignition system checks
    With dwell and timing set, perform final checks of the ignition system. With the engine running at idle, observe the dwell reading for stability—it should be steady, not fluctuating more than 1-2 degrees. Fluctuating dwell indicates worn distributor components, loose point mounting, or cam wear. Rev the engine to 2500-3000 RPM and observe dwell again; it should remain within specification. Check the timing at higher RPM if a vacuum advance is present; timing should advance smoothly as RPM increases or vacuum is applied. Listen for any misfiring or hesitation during acceleration. Turn off the engine and inspect all spark plug wire connections, ensuring they are fully seated and routed away from hot or moving engine components. Check that the distributor cap is fully secured and that no tools or parts were left in the engine compartment.

Reassembly

1. Ensure all distributor cap clips or screws are fully secured and the cap is properly seated.
2. Verify that all spark plug wires are connected to the correct distributor cap terminals according to the firing order 1-5-3-6-2-4.
3. Double-check that the distributor hold-down clamp is tightened securely after timing adjustment to prevent distributor rotation during operation.
4. Reconnect the battery negative cable and ensure it is tight.
5. Clean any tools or rags from the engine compartment and ensure no parts or fasteners were left loose.

Verification

• Start the engine and verify smooth idle with no misfiring or hesitation.
• Confirm that dwell angle remains stable at idle and under light acceleration, reading 33-37 degrees on a 6-cylinder dwell meter.
• Verify ignition timing with a timing light matches the specification for the M127 engine (typically 5-10 degrees BTDC at idle, verify exact specification).
• Road test the vehicle under various load conditions to ensure smooth acceleration with no misfiring, hesitation, or loss of power.
• After 100-200 miles of operation, recheck dwell angle and ignition timing, as new points may seat slightly and cause minor changes in gap and dwell.
• Inspect the engine compartment after the first drive for any fuel leaks, loose wires, or abnormal conditions related to the work performed.

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