Diagnosing problems with a classic car can be a rewarding challenge, especially when it comes to the ignition system of a vintage Mercedes. If you’re experiencing issues with your 1972 Mercedes starting or running smoothly, a systematic approach to checking the ignition points and related components is essential. This guide will walk you through the process of diagnosing potential ignition problems in your 1972 Mercedes, drawing on factory testing procedures to ensure accuracy and reliability.
Preliminary Checks: Inspecting the Points
Before diving into detailed voltage tests, a visual inspection of the points is a crucial first step. The points are responsible for interrupting the current to the ignition coil, creating the spark that ignites the fuel-air mixture in your 1972 Mercedes engine.
To begin, locate the distributor. Inside, you’ll find the points. Visually examine the points for:
- Cleanliness: Points need to be clean to conduct electricity effectively. Look for any signs of corrosion or carbon buildup.
- Contact: Ensure the points are opening and closing correctly as the engine turns. They should open and close with the rotation of the distributor cam.
- Condition: Check the points for excessive wear or pitting. Severely worn or pitted points may need replacement.
For a more precise check of point conductivity, use an ohm meter. Connect your ohm meter to the green wire leading to the distributor and ground. Observe the reading as you manually rotate the engine or crank it. You should see:
- Continuity (near zero ohms): When the points are closed.
- Open circuit (infinite ohms): When the points are open.
- Pulsing: A clear on/off pulse as the distributor rotates, indicating the points are opening and closing.
If you don’t see these readings, the points might be improperly adjusted, dirty, or faulty. Cleaning the points with a points file might resolve conductivity issues, but replacement might be necessary if they are worn or damaged. Points are relatively inexpensive, typically around $8, making replacement a straightforward option.
Factory Training Manual Tests: Voltage and Resistance Checks
For a more in-depth diagnosis, Mercedes-Benz factory training manuals provide specific voltage and resistance tests to pinpoint ignition problems. These tests help isolate issues within the coil, resistors, switching unit, and points themselves.
Voltage Test (Points Open)
This test checks for voltage supply to the ignition components when the points are held open, simulating an “ignition on” state but with no spark being generated.
- Block Points Open: Use a non-conductive object (like a piece of plastic or wood) to keep the points in the open position.
- Voltage Measurement: With the ignition switched ON, use a voltmeter to measure the voltage to ground at the following points:
- Both coil connections (+ and – terminals).
- Both sides of each resistor in the ignition circuit.
Expected Result: You should measure battery voltage at all these points.
Troubleshooting:
- No Voltage on One Side: If you have battery voltage on one side of a component (coil or resistor) but not the other, the component is likely broken and needs to be replaced.
- Voltage Other Than Battery Voltage: If you measure a voltage significantly lower than battery voltage at any point, unplug the transistor switching unit. Re-check the voltage.
- Battery Voltage Restored: If voltage returns to battery level after unplugging the switching unit, the switching unit is likely faulty and needs replacement. In this scenario, consider upgrading to a more modern and reliable system like Crane or Pertronix, which are often more cost-effective and perform better than repairing the original switching unit.
Points Closed Testing
This test evaluates the voltage drop across the points when they are closed and conducting current.
- Voltmeter Setup: Connect the red lead of your voltmeter to terminal 7 of the junction block (this is typically the green wire leading to the distributor). Connect the black lead to the distributor housing (ground).
- Ignition ON: Turn the ignition switch to the ON position.
- Voltage Reading: Observe the voltage reading on your voltmeter.
Expected Result:
- Less than 0.1V: Points are in good condition and making good contact.
- More than 0.1V: Indicates excessive resistance across the points. They may be dirty or require replacement. Try cleaning the points first; if the high voltage drop persists, replace them.
Voltage Drop of Switching Unit
This test assesses the voltage drop across the switching unit, which can indicate if the unit is functioning correctly or is failing.
- 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.
- Voltage Reading: Observe the voltage reading on your voltmeter.
Expected Result: The voltage reading should be less than 2.5V.
Troubleshooting:
- Voltage Higher Than 2.5V: Indicates an excessive voltage drop across the switching unit, suggesting the unit is likely faulty and needs replacement.
Coil and Resistor Resistance Checks
If voltage readings are outside the specified ranges in the previous tests, checking the resistance of the coil and resistors is necessary to identify faulty components. Use an ohm meter to measure the resistance of:
- Coil: Disconnect the coil wires and measure the resistance between the coil terminals. Expected range: 0.38 to 0.43 ohms.
- 0.4 Ohm Resistor: Expected range: 0.35 to 0.43 ohms.
- 0.6 Ohm Resistor: Expected range: 0.55 to 0.65 ohms.
Replace any component that falls outside these specified resistance ranges.
Direct Points Connection for Troubleshooting
As a troubleshooting step, you can temporarily bypass the switching unit and resistors to isolate the problem. This involves directly wiring the points to the coil.
- Disconnect Switching Unit: Unplug the transistor switching unit.
- Direct Wiring: 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. Include a 1-ohm resistor in series with this wire. This 1-ohm resistor replaces both the 0.4-ohm and 0.6-ohm resistors in the original circuit for this test.
Purpose: This direct connection simplifies the ignition circuit, using only the points and a single resistor to switch the coil.
Diagnosis:
- Engine Runs with Direct Connection: If the engine starts and runs with this direct points connection, it indicates that the issue lies within the switching unit or the original resistors.
- Engine Still Doesn’t Run: If the engine still fails to run, the problem is likely with the points themselves, the coil, or other fundamental ignition components before the switching unit in the circuit.
Solutions: Repair or Upgrade
If your troubleshooting points to a faulty switching unit, you have a few options:
- Replace the Switching Unit: Finding an original replacement switching unit for a 1972 Mercedes can be challenging and potentially expensive if sourced as a new old stock part. Used units may be available, but their reliability can be questionable.
- Repair the Switching Unit: In many cases, a faulty switching unit is due to a failed switching transistor. If you have electronics repair experience, replacing this transistor might be possible.
- Upgrade to a Breakerless System: Consider upgrading to a modern breakerless ignition system from brands like Crane or Pertronix. These systems offer improved reliability, performance, and are often more readily available and cost-effective in the long run compared to sourcing or repairing original components. These systems eliminate the points and often the original switching unit entirely, providing a more robust and maintenance-free ignition solution for your 1972 Mercedes.
By following these diagnostic steps and tests, you can systematically troubleshoot ignition problems in your 1972 Mercedes and determine the best course of action to get your classic car running smoothly again.
