Delivering the Goods - May - 2021

Engaging Moments! Ford’s Vacuum-Actuated Locking Hub System, Diagnosis and Issues

Convenience and ease of operation are things we have come to expect out of our daily driving experiences. Pushing a button should give us exactly what we want immediately. This is what we have grown to expect from our 4-wheel drive systems. Since the introduction of shift-on-the-fly 4-wheel drive, automakers have taken a multi-step process and reduced it to one button! We can transform our vehicle from a road polisher to a sure-footed driving machine in an instant. That is, provided everything works correctly. An important part of this process is the front axle locking system.

On truck applications, many manufacturers use different types of systems to engage and disengage the front axle. Ford has utilized everything from manual actuated wheel-end locks to fully automatic hub assemblies. While all of these systems offer their pros, they also have their cons. In their latest venture, Ford has struck their version of the best of all worlds as they see it with the introduction of the Integrated Wheel End (IWE) actuator system. The IWE actuators are a part of the Electronic Shift-On-The-Fly (ESOF) and Torque-On-Demand (TOD) systems which control the entire 4-wheel drive operation. We will look at the IWE system for controlling on-demand power flow to the front axle on Ford medium to heavy-duty trucks.


Ford’s IWE system works off of engine vacuum. Diesel applications are equipped with a vacuum pump for the brake booster reservoir and also serves the IWE system. It consists of IWE actuators (one in each front hub), a vacuum storage reservoir, vacuum lines, one-way check valves, and an actuator control solenoid (figure 1). The IWE actuators are also referred to as Constant Vacuum Hub lock (CVH) actuators. This design was first introduced in 2004 applications and worked opposite of the earlier Ford design CVH actuators. For this design, the vacuum is placed on the system unlocking the hubs while the truck runs in 2-wheel drive mode. When shifting to 4-wheel drive, vacuum is released, which locks the hubs. This leaves the front axle locked for fail-safe operation. Note that if the front axle remains locked, it will not cause driveability or durability issues. However, it’ll decrease fuel mileage due to the additional rotating parts.

The system’s check valves are designed to maintain the highest vacuum level in the reservoir and control system to maintain commanded operation. The IWE solenoid controls the vacuum signal based on the selection of 2WD or 4WD. In 2WD mode, the solenoid is energized, providing vacuum to the IWE actuators. When 4WD is selected, the IWE solenoid is de-energized, which causes vacuum to be evacuated, allowing the IWE actuators to lock.

Ford uses two different kinds of 4WD shift systems on late model applications: Electronic Shift-On-the- Fly (ESOF) and Torque-On-Demand (TOD). Both systems use the same IWE actuator setup, however, the selection strategies differ. The ESOF system relies on the driver to select between modes of operation, where the TOD system uses computer inputs and sensors to determine when 4WD is necessary.

The ESOF 4WD system consists of the following:

  • Transfer case
  • Transfer Case Control Module (TCCM)
  • High-Speed Controller Area Network (HS-CAN)
  • Mode Select Switch (MSS)
  • Transfer case shift motor
  • 4WD indicators and 4WD system messages in the IPC message center
  • Integral Wheel End (IWE) actuator
  • IWE solenoid and vacuum lines
  • Vacuum pump (diesel engine only)
  • On heavy-duty applications, manual select hubs are available in conjunction with the IWE actuators. This gives the driver additional control to engage, release or allow the IWE actuator to select based on the Mode Select Switch position. When manual hubs are present, verify their mode and operation before attempting to diagnose the IWE system.

The TOD 4WD system consists of the following:

  • Mode Select Switch (MSS)
  • Transfer Case Control Module (TCCM)
  • IWE solenoid
  • Integral Wheel End (IWE) actuators
  • 4WD mode indicators and messages in the IC message center
  • Transfer case assembly
  • Transfer case shift motor
  • Steering wheel angle signal from ABS module via HS-CAN
  • APP output from the PCM via HS-CAN
  • Wheel speed signals from the ABS via HS-CAN

The TOD systems are found on the Lincoln Navigator (1-speed) and the Ford Expedition (2-speed) models.


On late model applications, there are a few codes that help diagnose electrical and command-related issues, however, they do not offer mechanical or vacuum evaluation (figure 2). Once you determine the transfer case is functioning correctly, testing the IWE system is very straightforward. Most scan tools offer bidirectional control of the IWE solenoid. Even if you don’t have a scanner handy, all you need is a good vacuum pump (with a gauge), jumper wires, and some test vacuum hose.

Note, you can use a vacuum gauge teed into the system to monitor the real-time response. I recommend starting at the IWE solenoid (figure 3). Remove the switched vacuum line that goes to the IWE actuators and install your vacuum pump. Pump and hold the vacuum and wait for 60 seconds (1 minute). Verify that the IWE actuators release (unlocked). Observe your vacuum gauge reading from start to finish. If your reading drops by 1 inch or less after 60 seconds, the system checks good through all of the hoses and both actuators. Note if either IWE actuator does not move, there could be a blockage in the vacuum line(s) or a frozen or damaged IWE actuator.

You can also test each IWE actuator at the assembly. The large inlet is where the vacuum is applied. The smaller inlet is a vent. It is important to make sure that there are no obstructions to the vent and vent hose. The vent hose is routed into the underhood area and is positioned so that moisture cannot enter. If moisture enters the IWE actuator, it will damage the actuator and cause it to fail.

Check the IWE solenoid the same way you test shift solenoids. With no voltage applied, the vacuum switch is closed, which blocks vacuum from the control port. You can use jumper wires from the vehicle’s battery or an external power source. Applying a 12-volt source opens the switch, allowing vacuum to the control port. No voltage applied means no vacuum at the control port. Use a DVOM to verify the vehicle is giving the proper signal by back probing the control circuit in 2WD mode with the engine running.

If you hear grinding or whining noises from one or both front hubs at certain speeds or loads in 2WD mode, you probably have a leak in your IWE vacuum system. On vehicles equipped with the TOD system, grinding or whining may occur after heavy acceleration or after an ABS-induced 4WD command. A quick check is to remove and plug the vacuum feed into the IWE actuators and retest. If the grinding/whining noises go away, then unplug and reconnect the hoses and continue diagnosing the system.

If vacuum drops to or below 5 inches, the hubs may be partially released or applied. An intermittent issue is best diagnosed with a vacuum gauge teed into the IWE control system. Drive the vehicle while observing the vacuum gauge versus the grinding/whining issue. Also, verify that the vacuum coming to the IWE reservoir is correct and consistent. On gas engines, it is supplied by the engine manifold. It is typical to see anywhere from 16 to 22 inches of vacuum at idle. Diesel applications should record a similar range from the vacuum pump.

There may be additional steps on later model applications based on the operating characteristics that differ between models using the IWE system. For example, while working on a 2013- 2018 Ford Expedition, F150, or Lincoln Navigator equipped with the ESOF or TOD (AWD) systems, a TCCM module programming update is necessary in conjunction with any necessary repairs. The reflash allows for a delayed activation of the IWE solenoid after initial startup in 2WD mode. The updated programming requires the vehicle to be operated for ó mile before the IWE solenoid commands vacuum to release the hubs.


If the vehicle you are working on came in for a grinding noise related to the hub partially engaging, the hub teeth will most likely be damaged. It may be necessary to replace the axles as well. Upon installing each drive axle into the hub, you must line up the IWE hub teeth with the axle teeth before pulling the axle into the hub and tightening the axle bolt (figure 4). Remember, with no vacuum applied to the hub, it is in the LOCKED state, which means the teeth must be meshed to seat the axle properly; otherwise, you will destroy the new IWE and/or the axle!

Ensure that all hoses are free of debris and moisture before installing on the IWE ports. The larger diameter hose is the vacuum control source, and the smaller is the vent. Check to make sure the vent is free and clear of obstructions.

Ford offers an extreme-duty knuckle shield kit for off-road use and vehicles exposed to extremely dirty conditions (Ford part number GC3Z-54024A38-A). It helps to keep contaminants away from the IWE’s.

As a final note, several aftermarket fixes for the IWE actuators involve permanently locking the front hubs. It is important to realize that there may be a noticeable difference in fuel mileage if this repair path is taken. While it does not affect the durability of the 4-wheel drive components, it is something to notify the customer about before proceeding.

Vehicles capable of 4-wheel drive are becoming more and more popular. This opens the door for engineers to create new ways to give consumers what they want to keep ease of operation and reduce fuel consumption in the forefront. Needless to say, we will see more versions of systems that will give us the 4-wheel drive experience we want, whenever we want it. For us technicians, that ensures we will have many ‘engaging’ experiences to look forward to!