Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsStarting System Diagnosis and RepairEasy
A technician is performing a resistance test on a starter motor's field windings. The ohmmeter reads 0.1 ohms. Given that typical field winding resistance is very low, what does this reading most likely indicate?
- ANormal resistance for the field windings.
- BA short circuit in the field windings.
- CAn open circuit in the field windings.
- DExcessive resistance due to corrosion.
Show answer & explanationAnswer & explanation
Correct answer: A. Normal resistance for the field windings.
Field windings are designed to have very low resistance to allow high current flow and create a strong magnetic field. A reading of 0.1 ohms is typical for a healthy field winding, indicating good continuity without a short or open circuit.
Why the other options are wrong
- B. A short circuit within the windings would show a resistance much lower than specified, potentially near zero, leading to excessive current draw but 0.1 ohms is still a measurable, expected value.
- C. An open circuit would show infinite resistance (OL or 'open loop') on an ohmmeter.
- D. Excessive resistance would show a higher reading than 0.1 ohms, potentially leading to reduced current and weak cranking.
Starter Field Winding Resistance
The electrical resistance of the starter motor's field windings, which create the magnetic field necessary for motor rotation; typically very low to allow high current flow.
- Measured with an ohmmeter after disconnecting power.
- Normal resistance is usually less than 1 ohm (e.g., 0.05 to 0.5 ohms).
- Infinite resistance indicates an open circuit; near-zero resistance could indicate a short.
Memory trick: Low ohms, good flow, that's how a strong starter will go.