P1812 – Transmission 4-Wheel Drive Mode Select Circuit Failure (Ford)

By Bojan Popic (Contact Me)
Last Updated 2023-01-25
Master Mechanical Engineer

Manufacturer Specific Definitions for P1812

Table of Contents

1. What Does Code P1812 Mean?
2. Where is the P1812 sensor located?
3. What are the common causes of code P1812?
4. Get Help with P1812

What Does Code P1812 Mean?

OBD II trouble code P1812 is a manufacturer-specific code, which Ford defines as Transmission 4-Wheel Drive Mode Select Circuit Failure. This code is caused by an electrical error during the engagement of the 4-wheel-drive system. In some cases, it can be accompanied by other transmission-related errors, but it can also appear on its own.

Be aware other carmakers also use this code, with different definitions between different makes. This guide applies primarily to Ford and some Lincoln vehicles.

Where is the P1812 sensor located?

This article will focus on Ford’s pickup trucks and full-size SUVs that use the same platform, as these are usually affected by this error. The vehicles in question are available either with a traditional rear-wheel drive or an optional four-wheel drive. But even with the latter one, only the rear axle is driven under normal circumstances. Here, the driver has to engage the four-wheel-drive system manually by turning the dash-mounted selector knob in an appropriate position.

In most Ford models, there are two or three available options. One of them is 4H, or four-high, which engages the transfer box that sends the power to the front wheels. The other is 4L, which is short for four-low. This mode engages the low-ratio box within the transmission, effectively ‘shortening’ the gears.

Several other components, apart from the selector knob, are a part of the four-wheel-drives operation. This includes two solenoids, which do the mechanical work of engaging and disengaging the transfer and low-ratio boxes. In addition, the Transfer Case Control Module, or TCMM, oversights the whole process to ensure smooth operation and prevent any damage. Lastly, several sensors monitor different transmission’s running parameters and feed the data to the TCMM.

What are the common causes of code P1812?

Malfunctioning Mode Select Switch
As explained, the 4-wheel-drive system is operated with a rotary-style control knob. Ford refers to this component as Mode Select Switch, or MSS, which the driver uses to select the desired drivetrain configuration. If used frequently, the knob’s internal components may wear out, resulting in a loss of electrical conductivity. Measuring the resistance at the switch’s pin 3 will help determine if this is the case. The expected values, depending on the selected mode are:

• 2H – 619 ohms
• 4A – 270 ohms
• 4H – 130 ohms
• 4L – 62 ohms

If any of these values are significantly off, the Mode Select Switch should be replaced.

Damaged wiring
If the MSS resistance test clears out, checking the wiring for damage is the next step. The Transfer Case Control Module (TCMM), which controls the 4-wheel-drive system’s operation, is connected to it with a wire harness. Being underneath the vehicle, the transfer case and its wiring are exposed to damage caused by moisture, road debris, or rodents. This may result in shorts or broken electrical circuits. Checking the voltages and resistances on wires C284-2 and C284-3 will reveal if any damage is present.

Loose pins inside the MSS switch
A batch of 2021-2022 Super Duty trucks (F-250 and bigger) came from the factory with a potentially faulty Mode Select Switch. In this particular case, one of its pins may be loose. As a result, the wiring connector may bend or push it out of its place. This issue is described in Ford’s service bulletin SSM50322.

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