AutoCodes – Generic – P1840

P1840 Code - Internal Mode Switch 'S' Circuit Low Voltage

Quick Answer

The internal mode switch indicates the Transmission Control Module (TCM) which gear position the vehicle operator has selected. Common causes include Faulty Transmission Internal Mode Switch, Transmission Internal Mode Switch harness is open or shorted, Transmission Internal Mode Switch 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 indicates Internal Mode Switch 'S' Circuit Low Voltage.
Common causes: Common causes include Faulty Transmission Internal Mode Switch, Transmission Internal Mode Switch harness is open or shorted, Transmission Internal Mode Switch 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 19,026

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 Mar 23, 2024

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code P1840 Description

The internal mode switch indicates the Transmission Control Module (TCM) which gear position the vehicle operator has selected. The internal mode switch consists of 5 separate hall effect switches. Each hall effect switch is supplied with a 9 V reference circuit and a signal circuit from the TCM. Each signal circuit for each gear selector position will have either a voltage reading of 0.70–0.96 V indicating On or 1.68–2.38 V indicating Off. The voltage values on each internal mode switch circuit will change and are dependent on the position of the gear selector. The state of each internal mode switch A/B/C/P/S signal circuit is displayed on the scan tool. The Diagnostic Trouble Code (DTC) will be set when the TCM has detected the internal mode switch S signal circuit voltage is less than 0.7 V, for 1.75 s.

OBDII Code P1840 - Internal Mode Switch 'S' Circuit Low Voltage - AutoCodes.com — P1840 Code - Internal Mode Switch 'S' Circuit Low Voltage

What does this mean?

What are the Possible Causes of the DTC P1840?

Faulty Transmission Internal Mode Switch
Transmission Internal Mode Switch harness is open or shorted
Transmission Internal Mode Switch circuit poor electrical connection
Faulty Transmission Control Module (TCM)

How to Fix the DTC P1840?

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 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.

Need more information about the P1840 code?

Need more information on how to fix the P1840 code? Get Access to Factory Service Manuals

Need Help with the Code?

Get the P1840 code diagnosed by a professional: Find a repair shop in your area

Information for Specific Makes

P1840 Buick — Internal Mode Switch 'S' Circuit Low Voltage
P1840 Cadillac — Internal Mode Switch 'S' Circuit Low Voltage
P1840 Chevrolet — Internal Mode Switch 'S' Circuit Low Voltage
P1840 Ford — Transfer Case Shift Motor Short Circuit To Battery
P1840 GMC — Internal Mode Switch 'S' Circuit Low Voltage
P1840 Lincoln — Transfer Case Shift Motor Short Circuit To Battery
P1840 Mercury — Transfer Case Shift Motor Short Circuit To Battery

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.

Frequently Asked Questions about P1840 Code

1. What are common symptoms of OBDII code P1840?

Common symptoms include issues with shifting gears, difficulty engaging the transmission, and potentially a "Check Engine" light illuminated on the dashboard.

2. What causes the Internal Mode Switch 'S' Circuit Low Voltage code (P1840) to trigger?

The code P1840 is typically triggered by a malfunction in the internal mode switch (IMS) circuit, often due to wiring issues, a faulty IMS, or a problem with the transmission control module.

3. How can I diagnose the cause of OBDII code P1840 at home?

You can start by checking the wiring connected to the internal mode switch for any damage or loose connections. Using a multimeter to test the voltage at the switch can also help pinpoint the issue.

4. Can a DIY enthusiast repair OBDII code P1840, or is it best left to professionals?

Depending on your skill level and comfort working with automotive electronics, repairing P1840 may be feasible for some DIY enthusiasts. However, if you are unsure or lack experience, it's recommended to seek help from a professional mechanic.

5. Is it safe to continue driving with OBDII code P1840 present?

It is not recommended to continue driving with the P1840 code active, as it can lead to further transmission issues and potential safety hazards. It's best to address the problem promptly.

6. What are some potential repair options for resolving OBDII code P1840?

Repair options may include fixing or replacing the internal mode switch, repairing any damaged wiring, resetting the code after making necessary repairs, and ensuring the transmission control module is functioning correctly.

7. How much can I expect to pay for repairs related to OBDII code P1840?

The cost of repairs for code P1840 can vary depending on the underlying issue, parts needed, and labor costs. It's advisable to get estimates from different mechanics or repair shops to compare prices.

8. Can a low vehicle battery voltage cause OBDII code P1840 to trigger?

Yes, a low vehicle battery voltage can potentially cause the internal mode switch 'S' circuit to have low voltage, triggering code P1840. Ensuring the battery is in good condition is essential for proper electrical system operation.

9. Are there any preventive maintenance steps to avoid OBDII code P1840 in the future?

Regularly inspecting the internal mode switch, checking for wiring issues, and maintaining the transmission system can help prevent the recurrence of code P1840. Keeping up with scheduled maintenance can also help catch potential problems early.

10. If I clear the OBDII code P1840, how long should I wait to see if it reoccurs?

After clearing the P1840 code, it's recommended to drive the vehicle for a few days under normal conditions to see if the code reappears. If the issue persists, further diagnostics and repairs may be necessary.