Diagnosing ignition issues in a classic car like a 1972 Mercedes 280se can seem daunting, but with systematic testing, you can pinpoint the problem. Often, the culprit lies within the points system. This guide will walk you through a series of tests, based on factory training manual procedures, to help you identify whether your 1972 Mercedes 280SE ignition problems stem from faulty points, coil, resistors, or the switching unit.
First, a basic check to see if the points are functioning. Connect your ohm meter to the green wire leading to the distributor. Check for continuity both when the car is sitting still and while cranking the engine. You should observe a ground connection when the points are closed and on/off pulses as the engine rotates. If there’s no continuity or pulsing, it indicates the points might not be opening, or they could be dirty and failing to conduct electricity. Replacing the points is an inexpensive starting point, usually around $8, and worth considering.
From this point, we’ll delve into more detailed factory training manual tests to further diagnose the 1972 Mercedes 280SE ignition system.
Factory Voltage Tests for Your 1972 Mercedes 280SE
These tests require a voltmeter and a non-conductive tool to manually open the points.
Points Open Voltage Test
Begin by using a non-conductive object to block the points in the open position. Then, using your voltmeter, check the voltage to ground at both coil connections and at both resistors. You should be reading battery voltage at all these points. If you find voltage on one side of a component but not the other, that component is likely faulty and needs replacement. If you are getting a voltage reading that is not consistent with battery voltage, disconnect the transistor unit and re-check. If battery voltage returns after unplugging the transistor unit, it indicates a faulty switching unit. In such cases, upgrading to a more modern and reliable system like a Crane or Pertronix electronic ignition system might be a better and more cost-effective long-term solution than repairing the original factory unit.
Points Closed Voltage Test
For this test, ensure the points are closed. Attach the red lead of your voltmeter to terminal 7 of the junction block (this is the green wire). Connect the black lead to the distributor housing. Turn the ignition to the ‘on’ position. The voltmeter reading should be less than 0.1V if the points are in good working order. If the reading is higher than 0.1V, the points may be dirty or worn and require cleaning or replacement.
Next, keep the black lead grounded and check the voltage at terminals 15 (resistor side) and 1 (switching unit side) on the coil. Terminal 15 should read between 8.2 to 9.0V, and terminal 1 should read between 4.8 to 6.3V. If these voltage readings are outside of the specified ranges, you will need to check the resistance of the coil and both resistors. A healthy coil should measure between 0.38 to 0.43 ohms. The 0.4-ohm resistor should measure 0.35 to 0.43 ohms, and the 0.6-ohm resistor should measure 0.55 to 0.65 ohms. Out-of-spec resistance readings indicate a faulty component.
Switching Unit Voltage Drop Test
To assess the switching unit itself, connect the black lead of your voltmeter to ground and the red lead to the 0.6-ohm resistor connection that goes to the switching unit. The voltage reading must be less than 2.5V. A reading higher than 2.5V indicates a problem with the switching unit, suggesting it may be failing.
An engine bay detail of a 1972 Mercedes 280SE showcasing the distributor and ignition components, relevant to troubleshooting procedures.
Direct Points Wiring for Troubleshooting
In some cases, to isolate the problem, it’s beneficial to bypass the switching unit and wire the points directly to the coil for troubleshooting purposes. Disconnect the switching unit entirely. Then, using a wire, connect terminal 7 (green wire) directly to the side of the 0.6-ohm resistor that leads to the coil, but insert a 1-ohm resistor in series within this wire connection. This setup effectively replaces both original resistors with a single 1-ohm resistor and allows the points to directly switch the coil.
If the engine starts and runs with the points wired directly, it confirms that the issue lies within the switching unit or its associated circuitry. In this scenario, you will need to consider replacing the switching unit (if you can source one), attempt to repair it – often the large switching transistor is the failed component – or, as previously mentioned, upgrade to a more modern breakerless ignition system from brands like Crane or Pertronix. These aftermarket systems are often easier to maintain and provide improved reliability for your classic 1972 Mercedes 280SE.
