Q: 4WD will not engage

First, if there are any other lights on the dash, (such as the VSC, ABS etc.,) the 4WD module may not allow the 4WD to engage. As for the symptoms that could be causing this condition, it could be:

• faulty switch on the dash
• faulty 4WD module
• problem with the communication between the related computer modules

The most common failures are:

• transfer case actuator failure
• wheel speed sensor failure

There are other switches involved that indicate to the engine computer which state the 4WD is in, whether it be four low, four high or two wheel drive. A failure with any of these parts will cause problems for the four wheel drive.

As for what is wrong with your particular vehicle, without direct testing, I would only be guessing here. It could be any of the above components. I can, however, relay to you some common failures with the Toyota 4WD drive system.

I personally own a 2006 Sequoia, which is the same as the Tundra. I hear many reports that faulty wheel speed sensors can cause issues like this. Personally, I have experienced the lights flashing and the 4WD drive wouldn’t engage. In my case, I was on a hill in a driveway and some wheels were slipping while I was engaging the 4WD. I remedied the situation by turning the key off and restarting the motor. For me, this reset the ABS system that engaged because a wheel was slipping. This sort of situation can be avoided by doing the following.

Toyota high speed 4WD drive systems are designed to engage while the vehicle is moving. Refer to your owner’s manual for the top safe speed to engage the 4WD. Engaging the 4WD at speeds of less than 20 mph works seamlessly. Four high doesn’t like to engage when the car is not moving. This has to do with the physical engagement of the gears in your transfer case. Like a bike gear system, it won’t shift if it isn’t moving. So the slower you are moving the more time it will take the 4WD drive to engage.

Other conditions that confuse the Toytota 4WD system are when there are wheels slipping when attempting to engage it. It utilizes information from the wheel speed sensors and it becomes confused when it is attempting to engage while slipping. The same thing happens when a wheel speed sensor fails. There are quite a few reports of broken wheel speed sensor wires. So if you are inclined, crawl under your rig and take a look.

Engaging four low is another story. It should be done with the vehicle stopped.

Aside from these few ideas, this can be a difficult problem to diagnose. Definitely find someone who has diagnosed Toyota 4WD drive systems before. Their experience will be what saves you money and time. Most likely a scanner will reveal some trouble codes that point to the actual failure. Keep in mind, a typical code reader from an auto parts store will not do the trick here. The technician you choose should have equipment that can see ABS, VSC and 4WD PID (parameter ID) data. This could mean that you will need a dealer, but that doesn’t necessarily mean they will have much experience with it. The best place will be a shop that specializes in off road vehicles.

Good luck.