How to Test Starter Motor Windings: A Comprehensive Guide

When troubleshooting a vehicle's electrical system, one common issue is a malfunctioning starter. A faulty starter motor can be caused by a variety of problems, including issues with the windings. In this article, we will explore the structure and function of starter motor windings, and provide a step-by-step guide on how to test them using an ohmmeter.

Understanding Starter Motor Windings

Starter motors typically consist of a main run winding and a secondary start winding. These windings are electrically isolated from each other, with the start winding only engaged during the initial start-up phase of the motor.

The start winding is connected in series with the centrifugal switch, which is usually located in one of the motor end brackets. When the motor is starting, the centrifugal switch remains closed, allowing current to flow through the start winding. As the motor speeds up, the centrifugal switch operates, disconnecting the start winding from the circuit once the motor reaches a certain speed.

The Role of the Centrifugal Switch

The centrifugal switch is an important component in the starter motor's operation. It consists of two parts: a stationary contact held in the motor housing and a rotating element attached to the rotor. As the motor accelerates, the centrifugal force causes the rotating element to spread apart, eventually opening the contacts in the stationary part of the switch. Once the contacts are opened, the start winding is no longer part of the circuit, ensuring that the motor runs efficiently without the excessive current draw of the start winding.

Over time, the contacts in the centrifugal switch can become fused together, which means the switch remains closed and the start winding remains in the circuit even when the motor is running. This can lead to overheating and potential failure of the start winding, as it is not designed to handle the full rated current of the motor.

Detecting a Failed Start Winding

To test the condition of the start winding, you will need an ohmmeter. The procedure for testing the start winding involves measuring the resistance between the appropriate leads on the motor. In North America, the leads are typically labeled in a specific pattern, but the exact lead numbering may vary depending on the manufacturer and the specific model of the starter motor.

Here is a step-by-step guide for testing the start winding using an ohmmeter:

Locate the start winding connectors on the motor. These are usually marked as terminals or leads 5 and 8 (numbers may vary). Set your ohmmeter to the lowest resistance setting (usually 200 ohms). Place one probe of the ohmmeter on terminal number 5 and the other probe on terminal number 8. Record the resistance reading on the ohmmeter. If the resistance is near zero, it indicates that the start winding is likely burnt out or short-circuited. If the resistance reading is in the higher range, it suggests that the start winding is functioning properly.

It is important to note that the exact lead numbering and testing procedure might differ between different models of starter motors. Always refer to the manufacturer's documentation for specific instructions.

Replacing a Failed Start Winding

Unfortunately, if the start winding is burnt out, it cannot be repaired and must be replaced with a new one. This is a relatively straightforward process, but it requires proper tools and knowledge of electrical connections. It is advisable to consult a professional mechanic or electrician if you are not confident in your ability to perform the work.

Conclusion

Checking and testing the starter motor windings is a crucial part of maintaining the reliability of a vehicle's electrical system. By understanding the function of these components and following the proper testing procedures, you can quickly identify and address any issues that may arise, ensuring that your starter motor operates efficiently and without fail.

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