AutoCodes – Mercury – P1840 Mercury

P1840 Mercury Code - Transfer Case Shift Motor Short Circuit To Battery

Quick Answer

The Mode Select Switch (MSS) communicates the operator's choice to the Transfer Case Control Module (TCCM). Common causes include Faulty Transfer Case Control Module (TCCM), Transfer Case Control Module harness is open or shorted, Transfer Case Control Module circuit poor electrical connection. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means: P1840 Mercury indicates Transfer Case Shift Motor Short Circuit To Battery.
Common causes: Common causes include Faulty Transfer Case Control Module (TCCM), Transfer Case Control Module harness is open or shorted, Transfer Case Control Module circuit poor electrical connection.
Severity: High - diagnose this code as soon as possible.
Can I drive?: Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

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Code Popularity 1,321

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Advanced (Pro) (3 / 3)

What is this?

Table of contents Created and updated by William Orellana on Aug 15, 2020

Description
Possible Causes
How To Fix
Cost to Diagnose
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Code P1840 Mercury Description

The Mode Select Switch (MSS) communicates the operator's choice to the Transfer Case Control Module (TCCM). As the TCCM commands the gearmotor encoder assembly to 4H mode, the transfer case shift fork moves to engage 4H mode which also engages the dual-cone synchronizers to bring the front driveline to the same speed as the rear driveline. The TCCM also activates the hublock vacuum solenoid, providing a vacuum and engaging the Constant Vacuum Hublock (CVH) at each front wheel. If the vehicle is not responding to the operator's intentions, systematically check the necessary inputs and outputs of the TCCM, components of the transfer case, CVH components, and axle shafts. Check all circuits for opens and shorts to power or ground.

OBDII Code P1840 Mercury - Transfer Case Shift Motor Short Circuit To Battery - AutoCodes.com — P1840 Mercury Code - Transfer Case Shift Motor Short Circuit To Battery

What does this mean?

What are the Possible Causes of the DTC P1840 Mercury?

Faulty Transfer Case Control Module (TCCM)
Transfer Case Control Module harness is open or shorted
Transfer Case Control Module circuit poor electrical connection
Faulty Transfer case
Faulty Front axle assembly

How to Fix the DTC P1840 Mercury?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P1840 Mercury code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

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About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.