171-2211 (1712211) is a 90A manual-reset circuit breaker designed for Caterpillar (CAT) equipment; within the industry, it is frequently referred to as a "Starter Relay" or "Overload Protector."

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I. Core Specifications

Part Number: 171-2211 (1712211)

Type: Manual-Reset Circuit Breaker / Overload Protector

Rated Current: 90 Amps (90A)

Voltage: 24V DC (Standard voltage for heavy machinery)

Function: Protects the starter motor, battery, and main electrical circuit against overloads and short circuits; requires manual depression of the top button to reset after a fault has been cleared. 

Mounting: Square black housing, bolt-mounted on both sides, featuring dual terminals (high-current posts).

II. Applicable Models (Primary)

Excavators: 312D, 313D, 315D, 318D, 319D, 320C, 320D, 323D, 336E, 349E, etc.

Engines: C7.1, C9, C9.3, etc.

III. Differences from Standard Relays

171-2211 (Circuit Breaker)

No Coil: Trips during overload via an internal thermal/magnetic bi-metal strip; operation is not electromagnetically controlled.

High Current: Rated at 90A; connected directly in series within the main power supply / starter motor circuit. 

Manual Reset: Must be manually reset by pressing the top button after tripping. 

Standard Electromagnetic Relay (e.g., 161-3128)

Features a Coil: Uses a low-current signal to control the switching of a high-current circuit.

Low Current: Typically rated at 10–30A; used to control components such as lights, fans, solenoid valves, etc.

Automatic Reset: Resets automatically upon power disconnection.

V. Common Faults and Diagnostics

Symptoms: Excavator fails to start, starter motor is unresponsive, or total power loss throughout the machine. Diagnosis: Use a multimeter to check for continuity across the two terminals. If there is no continuity, the device has tripped; press the reset button to re-test. If continuity is still not established, the internal components are damaged and the unit requires replacement.