General 4WD

[davem]

I have some general questions about the behavior of my 2001 XLT, 4.0 V6, 4WD, 5 spd automatic. Hope this isn't too long of a post.

With the 4WD in 'auto' mode, I can sometimes feel a 'lurching' as the control trac decided to send more power to the front wheels - I assume this is 'normal'. Right ?

The other day, during a big snowstorm, the roads were basically 100 % snow/ice so I switched to 4High. While driving, I sometimes felt the same 'lurching sensation'. I really don't think there was any 'snow-free pavement' at all - was this due to some wheels getting better traction that others, ie 'normal' ? Or is it possible that, although the dial said 4High and the 4High light was on, I was somehow still in 4Auto ?

When I switch to 4Low (in neutral, stopped) I can hear it 'clunk' into gear. But when I switch from 4Auto to 4High there is no (perceptible) clunk - how can I easily verify its really working ?

Thanks,
Dave

 

[Blee1099]

Originally posted by davem
I have some general questions about the behavior of my 2001 XLT, 4.0 V6, 4WD, 5 spd automatic. Hope this isn't too long of a post.

With the 4WD in 'auto' mode, I can sometimes feel a 'lurching' as the control trac decided to send more power to the front wheels - I assume this is 'normal'. Right ?

The other day, during a big snowstorm, the roads were basically 100 % snow/ice so I switched to 4High. While driving, I sometimes felt the same 'lurching sensation'. I really don't think there was any 'snow-free pavement' at all - was this due to some wheels getting better traction that others, ie 'normal' ? Or is it possible that, although the dial said 4High and the 4High light was on, I was somehow still in 4Auto ?

When I switch to 4Low (in neutral, stopped) I can hear it 'clunk' into gear. But when I switch from 4Auto to 4High there is no (perceptible) clunk - how can I easily verify its really working ?

Thanks,
Dave

Welcome to the site.. You won't hear a clunk when you change from 4auto to 4High because there is no gear reduction compared to when your shift into 4LO. The lurching effect your feeling is probably just your 4WD engaging the front wheels. You can do a modification to the control trac to give you the option of 2WD.

 

[davem]

Is it normal to experience the lurching a few miles AFTER switching to 4High ?


Note - I also found that I have a 'limited slip' rear axle. Could this contribute to the lurch ?


(Forgive my ignorance - this is my 1st 4WD.)


Thanks,
Dave

 

[davem]

I have read about the option of converting to 2WD, but I seem to recall reading that there was little or no improvement in gas mileage - is that true ?

Are there other side effects to worry about when making this change ?


Thanks,
Dave

 

[jetmaker]

davem,

I read somewhere that in 4x4 high, you are not in "true" 4x4 mode. In the "auto" position, the power is split 4/96% between front and rear axles. In "4x4 Hi", there is a higher percentage transfered, how much I do not know. Also, it is not a manual lock, so it seems that power is shifted during driving. Only in "4x4 Lo" is it an equal split, and constant.

I have driven many 4x4's over the past 15 years. The one's I liked the best were fully manual systems. This way I knew what I had, when it was engaged, and I could fully trust my system. I can't say that I am thrilled with my stock X's 4x4 performance.

Regards,
jetmaker

 

[JDraper]

In 4x4 high, you are in true 4x4 with equal power distribution front and rear, as you are in 4x4 low. In 4x4 auto mode, the computer decides how much power to apply to each axle based on tire slippage measured by the sensors. In auto mode you will notice some rather hard thumps as the axles transition from good traction to bad traction.

What you may have been feeling is power shifting side to side on each axle as the traction conditions changed. The front axle has an open differential, so the power will shift to the wheel with the least traction. If one wheel is on an icy spot and the other on a snowy spot with good traction, you may feel a "lurch" or a "thump" as the wheel on the icy spot transitions back to better traction. Since you have a limited slip rear, I would suspect that you notice it more in the front than the rear.

 

[davem]

JDraper (and others),

Thanks for the explanation. I'm going to assume that what I experienced was normal.


Now another question about 4Auto - when tire slippage is detected and power is directed as needed, how quickly does it revert back when its no longer needed ? For example: driving along on pavement, hit an icey spot, directs power, then I'm back onto pavement in seconds - is it constantly checking/converting ? Seems like it could do a lot of swapping back and forth, right ? Or once it starts directing additional power to the front, does it remain that way for a certain amount of time ? (next stop, tranny shift, etc)

Thanks,
Dave

 

[JDraper]

It reverts back as soon as the slippage is gone, so it can jump back and forth quite rapidly. If I remember correctly, the original Control Trac system was developed by Bosch Automotive, and was very smooth in it's power transfer. Ford in their infinite wisdom decided that they could save money by watering down the system, and we have that result. Instead of the power being smoothly ramped up and down, it's done in a few discrete steps, leading to the "jerkiness" of the system.

Just a note. I rarely ever use the 4Auto mode. I have the 2wd modification in my '01, and I run in 2WD mode most of the time. Once the roads get snowy, I use 4high. I do this for two reasons. First, the Control Trac Xfer case has a clutch system in it that can wear out. The less it shuttles power back and forth, the less wear. By doing the "brown wire mod" the clutches don't engage and disengage at all, saving wear and tear on the Xfer case (I know several people with slipping xfer cases). Second, I'm not fond of the way that the auto mode works on changing traction conditions. I don't like not knowing when it's gonna grab or let go, because it does change how the vehicle handles. In 4high, there is no change in how the power is applied, so it's more predictable.

 

[Nick26]

Originally posted by davem
JDraper (and others),

Thanks for the explanation. I'm going to assume that what I experienced was normal.


Now another question about 4Auto - when tire slippage is detected and power is directed as needed, how quickly does it revert back when its no longer needed ? For example: driving along on pavement, hit an icey spot, directs power, then I'm back onto pavement in seconds - is it constantly checking/converting ? Seems like it could do a lot of swapping back and forth, right ? Or once it starts directing additional power to the front, does it remain that way for a certain amount of time ? (next stop, tranny shift, etc)

Thanks,
Dave

I may have read this wrong but the way you worded it makes it sound like the power is being transfered to the front wheel with the most traction, which is not the case at all. With an open differential the power absolutely always goes to the wheel with the least traction, because naturally thats the easiest one for it to spin. Ofcourse this is not ideal but it is just a side effect of the reason open differentials were designed which is to allow wheels to turn at different speeds when cornering. Check out how differentials work on www.howstuffworks.com for a good explanation and diagram of this... http://auto.howstuffworks.com/differential.htm

 

[JDraper]

Originally posted by JDraper
....The front axle has an open differential, so the power will shift to the wheel with the least traction. .......

Read the whole thread...........

 

[Nick26]

Originally posted by JDraper
Read the whole thread...........

no need to be rude about it, the thread was only 7 posts long its not like i didn't read the whole thing... I wasn't trying to show you up or anything I was mearly clarifying things.

when he says this after what you said it most likely means he didn't understand it because 9 times out of 10, wording something like the following would mean that they think the power is going to the wheel with more traction. The keyword here being needed.

Originally posted by davem
when tire slippage is detected and power is directed as needed

and dave just to clear one more thing up the axles are purely mechanical and there is nothing there to redirect power. hope what i said helped...

 

[Billy177]

drain your transfercase fluid and see what comes out there is a very small possibility that the 4wd clutch is burnt up