Have you ever been in a situation where you need to engage your 4-wheel drive or disengage 4H, but nothing happens? If your Jeep’s 4-wheel drive is not engaging then you might want to read this article. There are a few possible reasons why this could happen and we’ll cover all the likely scenarios.
Why is your Jeep 4WD not engaging? The Jeeps 4WD system could fail to engage due to a few possible reasons. They could be any of the following but not limited to:
- Failed actuator
- Vacuum Pipe damage
- Faulty switch
- Missing or loose linkage
These are some of the most common reasons why your Jeep could fail to engage the 4WD system. The symptoms will differ somewhat between them, for example, the 4WD light will flash on the dash but never actually engage, or the 4WD light will shine solid, but you have no active 4WD engaged. The next part of this article will discuss the scenarios in more detail.
Later we look at the 4x4Posi-lock upgrade system. Stick around!
Table of Contents
Jeep 4 Wheel Drive not engaging
Below is an extract from an article titled: 4WD LIGHT BLINKING AND NOT ENGAGING
This part explains how the actuator functions
The actuator is a component found mostly on modern four-wheel drives and is designed to electronically engage 4 wheel drive. It accomplishes this by means of a small electric motor bolted onto the transfer case that shifts a pin in place to lock the center transfer differential.
The actuator allows you to shift from 2WD to 4WD on the fly and back to 2WD without stopping and can be engaged and disengaged by means of a switch inside the cabin, usually on the dashboard. The actuator can be activated electrically or with the use of an air vacuum system. The beauty of this device is that negates the need for the driver to stop the vehicle, having to get out, to manually engage the vehicle into four-wheel mode by locking wheel hubs. Hooray for technology!!!!
So now that we know the function of the actuator, it makes sense that when you press the 4WD or 4H button on your dashboard, and the 4WD light just flashes, that the possible cause could be the actuator motor, internal gears inside the motor or the actuator electronic module.Extract taken from article titled: 4WD LIGHT BLINKING AND NOT ENGAGING
Next, let’s look at the damaged or missing vacuum hose scenario
Vacuum Hose Damage
A common issue with the YJ Jeeps is a failure in the automatic vacuum system that locks and unlocks the hubs when going in and out of the 4-wheel drive. The vacuum hoses can also cause the 4WD system not to engage and this can occur when 4H is selected and everything in the vacuum system is working but the vacuum pressure does not travel all the way to the front axle shift motor.
You will need to trace all the hoses to ensure they are in good working order. The only way to know 100% if the hoses are reaching the shift motor is if there was an air pressure gauge right before the front axle input. That will be able to determine if there is an air leak in the pipelines due to low pressure once it reaches the axle shift motor. One way to test is to use a flexible pipe to your ear and run it all the way along the vacuum hoses to listen for air leaks when you attempt to engage 4WD.
Once Jeep owners experience vacuum leaks they simply upgrade to the 4x4Posi-Lock system. The instructions are simple and it’s a DIY job with some basic tools. With the 4x4Posi-Lock system, you never have to worry about your 4WD not engaging ever again.
Vacuum Hoses Unhooked or Ripped Off
If you have a failure in the vacuum system that actuates the shift motor on the front axle your 4WD will not engage or disengage. The vacuum lines for the 4wd axle disconnect system always seem to get unhooked, melted, or ripped totally off. You will need to lift the Jeep and examine all the vacuum lines from the shift motor and trace them all the way back to the transfer case. Remove the plastic housing around the vacuum harness to see the colored lines. Take special care not to break it.
Follow the below instructions to reconnect the vacuum hoses in the correct order.
- Locate the four nozzles on the vacuum switch that control your front disconnect system for your 4-wheel drive. (Located on top of your transfer case)
- Locate the red line (constant source) coming from the transfer case and connect it to the original red hose running behind your air cleaner.
- (Red Hose) Hooks up to a constant vacuum source off of the carb or intake manifold.
- (Yellow Hose) Connects directly to the shift motor. This is one of the lines that go into a two-line plug vertical on the shift motor.
- (Blue Hose) Ignore this hose. This hose has a check valve on it which does not get hooked to anything.
- (Hard black Hose) Connects with one of the yellow hoses to the shift motor. Plug located vertically on top.
- (Soft Black Hose) Connects to the plug on your firewall located at the battery. This is for the 4wd indicator on the dashboard.
- (Green Hose) Goes to the shift motor on the front axle. Horizontal on the shift motor.
If the remains of the original line that is hooked to the constant vacuum source are missing, find a port on your intake manifold or Carburetor that has a constant vacuum. Do your best to connect up what you can as a temporary fix, but I would suggest you buy a new vacuum harness here from 4wheelparts and save yourself the hassle.
Faulty Vacuum Switch
The vacuum switch referred to here is the vacuum switch on the transfer case. There are basically four vacuum lines that feed to the switch which is fairly easy to locate. There are four nipples on the switch where the vacuum hoses connect directly onto. The four vacuum hoses merge together into a 1 to 1-1/2-inch diameter connector that plugs directly into the nipples. The vacuum switch will fit all 87-96 Jeep Wrangler YJ & Jeep Cherokee XJ models with Transfer case model numbers NP207 or NP231.
Trouble-Shooting the Vacuum Switch
- Firstly, retrace all the vacuum hoses from the switch and confirm that all of them are connected securely as they should be.
- Remove the switch connection which has a vacuum harness with 4 ports that plug directly into the top of it. (The switch is keyed so it can only fit one way.)
- Next, unscrew the switch from the TC and inspect.
- Inspect the ball-bearing plunger at the bottom of the switch. This gets jammed inside the switch.
- Try and give it a few bangs to release the jammed ball-bearing inside the switch.
- If that is unsuccessful, you can purchase a new vacuum switch here.
- Screw in the new switch and reconnected the vacuum harness to the top of it.
Start the Jeep to test the 4WD functionality then take it for a drive on a low-traction dirt road and test between engaging 2H and 4H and back to 2H again.
Video – Jeep Wrangler 4-Wheel Drive Won’t Engage
Watch the video below to see how the linkage can be replaced.
The 4×4 Posi-Lock upgrade is a permanent cable replacement for the vacuum and electric front axle 4WD systems. This allows the drive to be in full control when activating 4WD from inside the cabin. This is made possible by removing the failure prove vacuum hose system and replacing it with a manual cable. A simple one-inch pull of the cable will engage 4WD on the front axle, provided the transfer case is engaged.
How to operate the Posi-Lock system
- Ensure no acceleration on the throttle
- Select 4H or 4Lo to engage the transfer case
- Press the button and give the cable a one-inch pull into a fully engaged position
- Release the button once 4WD is engaged.
You can buy this amazing posi-lock system here from
Jeep Wrangler 4-Wheel Drive Linkage Adjustment
If you are experiencing difficulty with engaging 4WD on your Jeep Wrangler, then this part of the article is for you. This section involves a linkage that has either failed or a junction that has completely worked itself loose causing the linkage to fall off.
The linkage we are referring to is the one that connects the transfer case shifter lever to the actual selector arm on the transfer case. This allows the driver to engage 4WD mode as well as select 4-Lo. If the lever feels more loose than normal or when you shift the lever back the Jeep fails to engage 4H or 4Lo.
If this happens then you need to investigate immediately by crawling underneath the Jeep and have a look at this transfer case linkage. It’s a very inexpensive part which can easily be purchased online. It is labeled as a linkage adjuster. The linkage has an adjuster which allows you to set the tightness.extract taken from article titled: My 4Wheel Drive shifter is loose
Watch the below video which explains the missing shifter linkage and how to fix it.
So we’ve learned there are at least 4 reasons why the 4WD won’t engage on your Jeep and how it can be rectified. The Jeep is an excellent 4WD and very basic to work on in most cases. Follow the instructions outlined above and you should be able to successfully troubleshoot and repair your Jeeps 4WD system.