Diagnosing ignition problems in a classic car like a 1972 Mercedes 280se can seem daunting, but with a systematic approach, you can pinpoint the issue and get your vintage vehicle running smoothly again. This guide, tailored for your 1972 Mercedes 280SE, will walk you through essential troubleshooting steps, focusing on the key components of its ignition system.
Initial Checks: Points Inspection and Continuity Test
The first step in diagnosing ignition issues is to inspect the points and check for continuity. This is crucial because the points act as a mechanical switch, controlling the flow of electricity to the coil.
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Visual Inspection of Points: Begin by visually inspecting the points inside the distributor. Ensure they are opening and closing correctly as the engine is cranked. Look for signs of excessive wear, burning, or pitting on the contact surfaces. Dirty or corroded points can prevent proper electrical contact.
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Continuity Test: Use an ohm meter to test the continuity of the points circuit.
- Connect the Ohm Meter: Connect one lead of your ohm meter to the green wire leading to the distributor (terminal 7 of the junction block). Connect the other lead to a good ground point on the engine or chassis.
- Static Test (Points Closed): With the ignition system static (engine off), the ohm meter should show continuity (close to zero ohms) when the points are closed. This indicates a proper connection to ground.
- Dynamic Test (Cranking Engine – Points Opening/Closing): Crank the engine. The ohm meter should now display on/off pulses, indicating that the points are opening and closing correctly as the distributor shaft rotates. If you don’t see these pulses, it suggests either the points are not opening or they are too dirty or corroded to conduct electricity effectively.
- Points Cleaning/Replacement: If the points are dirty, try cleaning them with a points file. If they are worn, burnt, or cleaning doesn’t restore proper function, replacement is necessary. Points are relatively inexpensive, typically around $8, making replacement a cost-effective troubleshooting step.
Voltage Testing the Ignition System Components
If the points appear to be functioning correctly but ignition issues persist, voltage testing is the next crucial step. These tests help identify whether each component in the ignition circuit is receiving the correct voltage.
Safety First: Before proceeding with voltage tests, ensure the ignition is switched OFF unless explicitly instructed to turn it ON for a specific test.
Voltage Test Procedure:
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Block Points Open: To isolate the circuit for testing, manually block the points in the open position using a non-conductive material like a piece of cardboard or plastic.
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Battery Voltage Check at Coil and Resistors:
- Coil Connections: Check the voltage to ground at both terminals of the ignition coil. You should measure battery voltage at both terminals.
- Resistors: The 1972 Mercedes 280SE ignition system uses resistors to reduce voltage to the points and coil. Check the voltage to ground at both sides of each resistor (typically a 0.4 ohm and a 0.6 ohm resistor). You should also measure battery voltage at all test points on the resistors.
- Interpreting Results: If you find battery voltage on one side of a component (coil or resistor) but not the other, it indicates a break or fault within that component. Replace the faulty component. If you find no voltage at all, trace back the wiring to identify the break in the circuit.
- Voltage Deviation and Transistor Unit Test: If you measure a voltage value significantly lower than battery voltage at any of these points, unplug the transistor switching unit. Re-check the voltage at the same points. If you now measure battery voltage after unplugging the transistor unit, this strongly suggests that the transistor switching unit is faulty and needs replacement. In this scenario, consider upgrading to a more modern and reliable system like Crane or Pertronix, which are often better and more cost-effective than repairing the original factory transistor unit.
Points Closed Testing: Voltage Drop Across Points
This test assesses the voltage drop across the points when they are closed, indicating the effectiveness of their contact.
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Voltmeter Setup:
- Connect the red lead of your voltmeter to terminal 7 of the junction block (green wire leading to the distributor).
- Connect the black lead of your voltmeter to the distributor housing (a good ground).
- Turn the ignition ON.
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Voltage Reading and Interpretation:
- Acceptable Voltage Drop: A reading of less than 0.1V is considered good, indicating the points are making good contact and providing a low resistance path to ground.
- Excessive Voltage Drop: If you measure more than 0.1V, it indicates excessive resistance across the points. This is often due to dirty or corroded points. In this case, try cleaning the points or replace them if cleaning doesn’t resolve the issue.
Coil and Resistor Voltage and Resistance Checks
These tests verify the voltage reaching the coil and the resistance values of the coil and resistors are within the specified ranges.
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Voltage Checks with Points Closed:
- Terminal 15 (Resistor Side) on Coil: With the black lead of your voltmeter grounded, measure the voltage at terminal 15 of the coil (the side connected to the resistors). The voltage should be between 8.2V and 9.0V.
- Terminal 1 (Switching Unit Side) on Coil: Measure the voltage at terminal 1 of the coil (the side connected to the switching unit). The voltage should be between 4.8V and 6.3V.
- Out-of-Specification Voltages: If the measured voltages are outside these specified ranges, proceed to resistance testing of the coil and resistors.
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Resistance Testing of Coil and Resistors (Ignition OFF):
- Coil Resistance: Disconnect the coil wires. Measure the resistance across the coil terminals. The specified resistance should be between 0.38 ohms and 0.43 ohms.
- 0.4 Ohm Resistor Resistance: Disconnect the 0.4 ohm resistor and measure its resistance. The specified range is 0.35 ohms to 0.43 ohms.
- 0.6 Ohm Resistor Resistance: Disconnect the 0.6 ohm resistor and measure its resistance. The specified range is 0.55 ohms to 0.65 ohms.
- Resistance Deviations: If any of these components’ resistance values are outside their specified ranges, replace the faulty component.
Voltage Drop Test of the Switching Unit
This test assesses the voltage drop across the transistor switching unit itself, helping to identify internal faults.
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Voltmeter Setup:
- Connect the black lead of your voltmeter to ground.
- Connect the red lead to the connection point of the 0.6 ohm resistor that goes to the switching unit.
- Turn the ignition ON.
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Voltage Drop Reading and Interpretation:
- Acceptable Voltage Drop: The voltage reading must be less than 2.5V.
- Excessive Voltage Drop: If the reading is higher than 2.5V, it indicates an excessive voltage drop across the switching unit, suggesting an internal fault within the unit. In this case, the switching unit is likely faulty and needs replacement.
Direct Points Connection for Troubleshooting (Bypass Test)
As a troubleshooting step, you can temporarily bypass the transistor switching unit and resistors to isolate the issue.
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Bypass Wiring:
- Unplug the transistor switching unit.
- Connect a wire from terminal 7 of the junction block (green wire) to the side of the 0.6 ohm resistor that normally goes to the coil.
- Insert a 1 ohm resistor in series within this newly connected wire. This 1 ohm resistor replaces both the original 0.4 ohm and 0.6 ohm resistors for this test.
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Test Run: Attempt to start the engine with this direct points connection.
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Interpreting Results:
- Engine Runs: If the engine runs with the points connected directly, it confirms that the issue lies within the transistor switching unit or the original resistors. Since you’ve bypassed both, the most probable cause is a faulty switching unit. You would then need to replace the switching unit (if you can find a factory replacement), repair it (often the large switching transistor inside fails), or consider upgrading to a breakerless ignition system like Crane or Pertronix.
- Engine Still Doesn’t Run: If the engine still doesn’t run with the direct points connection, the problem is likely elsewhere, such as in the coil, points themselves (despite initial checks), or other parts of the ignition system or engine. Revisit your initial checks and tests, or expand your troubleshooting to other potential causes of engine starting issues.
By following these detailed steps, you can systematically diagnose ignition problems in your 1972 Mercedes 280SE. Remember to be methodical, double-check your connections, and prioritize safety throughout the process.
