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The private blog of Andy Prevost. My experiences, my words. On any topic ...

When is a 4x4 NOT a 4x4?

My Ford F150 has 4X4 stickers all over it and is documented as a 4X4 in specifications and the user manual. Is is a 4X4?

EDIT: On a flat surface a vehicle in 4WD is driving all four wheels. 25% of the engines torque is going to each wheel. In 4WD, you have 50/50 torque split front to rear. With open differentials you can lose traction with one front or one rear wheel and still keep moving. If you had front and rear locked differentials you would only need one wheel with traction to move the truck (but steering would be next to impossible). With open differentials and the transfer case in 4WD, it only takes one front and one rear in the air, or on ice to stop the progress of the truck.

Here is some clarification on terminology:

  • 4x2 (four by two) means a vehicle with 4 wheels powered by 2 wheels, in effect a 2WD (two wheel drive)
  • 4x4 (four by four) means a vehicle with 4 wheels powered by 4 wheels, in effect a 4WD (four wheel drive)

Mine is a 4X3 at the very best and might possibly be a 4x2 – that is despite having an axle and drive train for the front wheels and an axle and drive train for rear wheels.

How can this be? Well, Ford uses a number of different rear axles on their F150 trucks. The common ones are known as 19, H9 and L9. You can see those in the door sticker of your own truck. Here's a picture of mine on the right.

The line near the bottom has the title "AXLE" and below that is the axle code (19). That means an open differential with 3.55 gear ratio. It has 31 splines, and is a 9.75 inch axle. Although it is an incredibly strong axle, only one wheel drives at any point in time. 

A differential has three tasks in the drive train:

  1. It transfers power from the engine/transmission to the wheels
  2. It varies this power so that the wheels can turn independently (differential)
  3. It is the final gear reduction in the drive system

In terms of the second point, there are three types. All are designed to allow some flexibility in one wheel turning a bit faster or slower than the other wheel. Without this flexibility, turning would be impossible. Here's the three types:

  • Open differential (open diff) has open spider gears that allows the wheels to turn freely. This the most common for rear-wheel drive passenger cars. The differential will transfer power to the wheel with the least resistance. If you have ever seen a vehicle stuck in snow or ice on the road with one wheel on the road and one wheel in snow/ice ... and it can't seem to get enough traction to pull itself out (despite one wheel on the road) ... it's the open differential that is the problem. It is transferring power to the wheel with the least resistance – the one spinning in the snow/ice.
  • Limited slip operates on a clutch mechanism. When it senses the two wheels are not rotating at the same speed, it will "lock up" its gears mechanically so that both wheels get the same amount of power. There is some slippage to allow for turning, but essentially it is a lock up system.
  • Locking axle. This is an electronic traction control system. When the computer senses slippage in one wheel, it will lock up the gears mechanically so that both wheels get the same amount of power. When not locked up, the locking axle functions like an open differential axle.

If you look up my axle code, I have an open differential axle. That means that despite flipping the switch to 4x4 (hi or lo), all I can possibly get is 4x3. There are four wheels, and only three are getting power. 

This F150 is my sixth 4X4 Ford truck. Four of them were Rangers, and two have been F150s. And this is the first that features this issue. I am actually quite disappointed in Ford. Calling this truck a 4x4 and selling it as such is misleading – at the very least, all trucks sold as 4X4's should have a limited-slip rear axle. 

This came to the fore front because I got stuck in snow. Not a lot of snow, just a bit of a snow pile. One side of the back of my truck was a bit raised and not really firm on the ground – and it ended up spinning because the open differential transferred power to that wheel. Had it functioned like a true 4x4, the other rear wheel should have picked up the load. Why didn't the front wheels help? I was on ice and the front wheels had no traction. I should have been able to just pull out of this easily ... but ...

I now have picked up a used H9 axle from a 2005 F150. This is a limited slip and a straight swap out. The only real issue I expect to run into is having to bleed the brake lines. Other than that, it is a straight mechanical swap with the same spline count and same dimensions. Just need a bit of a warm spell to get this done.

And, shame on you Ford. 

EDIT: In the first few paragraphs I pointed out that at best the final drive was 4X3 and quite possibly 4X2 even though promoted as a 4X4. Andrew Johnston at the Ontario F150 facebook group points out that the front axle is an open differential as well. That means that the front can only have one wheel driving. If the rear axle is open differential, you can put the truck in 4x4 Hi or 4X4 Lo, all you will end up getting is two wheels actually under power. A true 4X4 would require a locking or limited slip rear axle and a locking or limited slip front axle.

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