RAM 1500 Service 4WD Light – Meaning And How To Fix

The first Dodge RAM truck was introduced in 1981, and since then has become an American institution, with the 2023 model already being announced. The model designated 1500 was introduced in 1993, and in 2023 it is powered by a Supercharged 6.2L HEMI® V8 producing 702hp.

When the RAM 1500 Service 4WD Light activates, it means that there is a problem with the 2WD/4WD system. The most common cause of the Service 4WD Light is a blown fuse or damaged wiring; however, it can be more serious and may involve a damaged component in the transfer case.

If the Service 4WD Light on your RAM lights up, first check the fuses, and if those are in order, conduct a visual inspection of the wiring harnesses and connector plugs. You need to dig a little deeper if the problem is not identified with these components.

Meaning of the RAM 1500 Service 4WD Light

The RAM 1500 has a full-time 4WD system installed.

The control for the 4WD system is located on the right-hand side behind the steering wheel. The control has three or four positions depending on the RAM model.

1. 2WD high range.
2. 4WD high range.
3. 4WD low range.
4. 4WD auto (on certain models.)

The system uses a central differential which distributes power to each of the four wheels depending on the road conditions and the available traction.

The 4WD auto system remains in 2WD drive mode, with power being supplied to the back wheels for most road conditions except when conditions require, or the driver manually overrides the 4WD auto system. This sends a signal to the transfer case shift motor.

The transfer case shift motor moves the cogs between 2WD and the two different 4WD modes.

If the Service 4WD Light activates, an error has occurred in the system.

What Causes the RAM 1500 Service 4WD Light?

The possible problems in the RAM 1500 4WD system can be traced to two main areas.

1. The electrical components in the 4WD system.
2. The mechanical components in the 4WD system.

To quickly find out why the Service 4WD Light is active, I recommend you plug in a diagnostic computer that will reveal the diagnostic code. This will show the specific problem and will prevent you from having to manually diagnose the issue.

If you don’t have the device, keep reading to learn how to check the system in a uniform sequence and hopefully track down the problem as quickly as possible.

The 4WD Electrical System

Early four-wheel drive systems were purely mechanical and required a second “gear shift” to physically manipulate the cogs and move them between 2WD and 4WD.

Some older models even required the driver to leave the vehicle and manually adjust the external locking hubs on the front wheels.

Modern 4WD electrical systems used in vehicles such as the RAM 1500 are activated electrically.

The following sequence is activated when the driver selects a different driving mode.

1. The driver presses the 2WD/4WD switch.
2. A signal is sent to the 2WD/4WD transfer case control module.
3. The 2WD/4WD transfer case control module sends a request to the transfer case actuator.

If the 2WD/4WD transfer case control module cannot send the message, it will activate the RAM 1500 Service 4WD Light “4WD SERV.”

The 4WD Mechanical System

The RAM 1500 4WD Mechanical System consists of a central transfer case with a series of gears with the following positions.

1. The transfer case disconnects the front wheels in 2WD mode.
2. The transfer case selects different cogs in 4WD low-range mode.

The following happens when the actuator receives an electronic request from the 2WD/4WD transfer case control module.

1. The shift motor in the transfer case activates and rotates the cogs to the required position.
2. If the operation is successful, the actuator sends a “completed” message to the 2WD/4WD transfer case control module.

If the transfer case control module does not receive a “completed” message from the transfer case message, it will activate the RAM 1500 Service 4WD Light with the description “4WD SERV.”

RAM 1500 Service 4WD Light – How To Fix

When the Service 4WD Light activates, the action to take is to diagnose the problem.

The Electrical System

The most common and easy-to-fix problems happen in the electrical side of the RAM 1500 4WD system. The electrical components to check include:

1. The 4WD wiring system
2. The fuses
3. The 4WD switch
4. The transfer case control module

The 4WD Wiring System

The wiring is relatively easy to check by sliding under the RAM 1500.

Check for loose, damaged, or broken wires at the front end. Concentrating particularly on the wiring loom connector at the transfer case shift motor.

There are parts of the wiring loom that are attached to the body with cable ties. If one of these has broken, the cable loom may chafe against the truck’s chassis.

If the wiring is damaged, splice new sections to replace the damaged ones.

The Fuses

Check the fuses (F42A / F42B) in the dashboard fuse box.

If these are damaged, replace them with the correct amperage fuses.

The 4WD Switch

If the switch is faulty, it will return the “4WD SERV” on the dashboard.

If you are comfortable removing the switch without damaging the trim, it may be worthwhile testing it with a replacement unit.

The Transfer Case Control Module

The only way to check if the fault lies with the transfer case control module is with a diagnostic computer.

The Mechanical 2WD/4WD Components

There are several mechanical components that may be damaged or broken, which could cause the “4WD SERV” warning light.

1. Shift motor
2. The transfer case shift motor and actuator

The Transfer Case Shift Motor And Actuator

A common failure point that causes the RAM 1500 Service 4WD Light to activate is the transfer case shift motor.

These are modular and are relatively easy to replace with replacement components.

The Gear System Inside The Transfer Case

Over a long period of time, particularly if the system has overheated, the transfer case fluid will break down and become dirty. This will restrict the moving parts and may ultimately cause failure.

The most effective way to test the condition of the transfer case is to empty the oil and examine the fluid for metal shavings or fragments.

A healthy transfer case will have clean oil with no contamination.

Conclusion

When the RAM 1500 service 4WD light is lit, first check the low cost, easy-to-fix parts of the system. These include the fuses and wiring harnesses underneath the vehicle, which are easy to check.

If the problem is not found with this check, it is time to plug a diagnostic computer in and check the error codes.

References

https://fuse-box.info/dodge/ram-1500-dodge-ram-2019-2021-fuses-and-relays