Hi,
I have a 99 XLT and I wonder what happen when the truck is in AUTO mode. My dealer told me 95 % of the power is behind and 5% is in front wheels but that could change if rear wheels loses grip... is this true? also how come i can dammage powertrain by using 4x4 in rain or light snow? The manual seem to say to not use 4x4 high or low unless apocaliptics situation!

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Phil
'99 XLT SOHC 4x4

Hi Phil,

The 4WD system you have is what Ford calls Control Trac. The Control Trac system uses an electrically controlled clutch in the transfer case to engage the front wheels. Normally in a typical part time 4 wheel drive system in 2-High mode, only the rears would be mechanically connected to the engine/transmission, with the front drive train freewheeling.

Now, keep in mind that when you turn, the front wheels must travel farther than the rear ones, since the actual turning circle for the outside front wheel is larger than for the inside rear (actually, each wheel travels at a different speed through a turn, and the difference depends on the tightness of the turn). This is important on 4WD vehicles. Keep it in mind throughout the rest of this description.

In 4-High, the clutch in the transfer case is fully engaged, and the front drive train is locked "in step" with the rear, that is, the front axle turns at the same speed as the rear. If you try to turn on solid ground such as wet or dry road surfaces, there is binding as the front wheels try to turn faster than the rear but cannot. Something has to give, and it could be the tires sliding and jumping across the pavement or it could be a broken axle shaft, drive shaft, or other part. This is why you shouldn't use 4-High on the road unless it is quite slippery.

The same holds true for 4-Low. A different gear is used in 4-Low to allow for steep hill climbing and descending in low range, but like 4-High, the front and rear drive trains are locked together through the electric clutch in the transfer case.

Now, in the Auto mode, things get more complicated. While the rear axle is the primary drive axle, the fronts do have some power applied as well. This is controlled electronically. The GEM module (I think that stands for generic electronic module) actually continually pulses the electric clutch such that it is engaged for short bursts, long enough to allow some drive forces to go through the front axle, but short enough to relieve the binding that can occur when turning. These pulses can occur several times a second.

Now, when traction is good, these short periods of engagement occur such that the front axle is engaged approximately 5% of the time. That is where Ford gets the 95% rear/ 5% front power transfer levels.

When things get a little more slippery, the system reacts. The wheel speed sensors that are part of the ABS system (anti-lock brakes) are used to detect rear wheel spin. If the computer senses that the rear wheels are turning faster than the front wheels, the computer automatically tells the clutch to be engaged for a longer portion of the time. This is done in 10% increments, up to a duty cycle of 100%. (It will go to 15% front/ 85% rear and so on until traction is regained.) The change begins to occur within about 1/5 of a turn of the loose or spinning rear wheel. By increasing engaged time in small increments, the system can apply just enough torque to the front axle to give you added traction, while allowing periods of relief for the drive train to prevent binding. These engage/disengage pulses occur several times per second.

Once traction is regained and the wheels are turning at the same speed (the rears are no longer turning faster than the front wheels), the procedure reverses itself, once again in 10% increments. This continues over a period of a couple of minutes until the power distribution is back to the 5% front/ 95% rear ratio.

I hope that I explained this reasonably clearly. I'm no expert, but I have been reading up on the system.

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Tom


[This message has been edited by Tom Wilk (edited 10-09-1999).]

Great explanation Tom! I wasn't sure how that new-fangled system works until now...

thanx !

Damn, I'm impressed.

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Stephen Withrow
---------------
97 Limited
98 Lexus LS400

I have to give credit where it is due. The information on the Control Trac system was derived from a Deja search, as well as a search of the original patent. I cannot recall who wrote the original post describing the system, but I seem to think his name was Vic. Deja also helped me find the original patent number, 5485894. I went to the US patent's web page, whose URL escapes me, and searched for this patent. It gives a very in depth description of how the system works. So, the information is out there, but it isn't always easy to find.

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Tom

Any ideas about the clutch going BANG offroad?? I or my wife have had it pop loose MANY times when the going gets tough - it's disconcerting to say the least. The noise when it pops is bad enough, but the "slip sliding away" from traction loss is equally uncomfortable.

Regards,
Brian in CA

I can only guess here. I would say it's one of 3 possibilities:

1. Problems with the GEM module. I'll have to look it up again, but when I read over the patent on the Control Trac, I seem to recall that it mentioned steering position as part of the information input that the computer would read. I'm not sure, and I'll try to look it up again. (the patent was very long, but very informative). Still, in 4-lo, it should be locked regardless of what the computer is told.

2. By design. I wonder if the clutch would disengage when torque differences were too great for the system to handle. Sort of like blowing a fuse instead of damaging wiring in an electrical circuit. I don't recall reading anything about the system sensing torque or force; just speed differences between the wheels.

3. Too much torque for the system. Maybe the clutch can only handle so much load before it "gives". Still, I would think that it would just start slipping a bit, not let go with a bang.

My thoughts are with #1. I will try to read the patent more thoroughly to see if there is any hint of how this could happen. (I only skimmed it before, as it is very long)

I would probably have the dealer look at the GEM, if it is still under warranty.

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Tom

Hi all, your knowledge impresses me!
I have simple question though....
I have a 96XLT with control trac (2wd,4wd auto, and 4wd low options) i regularly turn the switch to 4WD Auto during rain and other inclement weather, not to mention if I am in any moderate off road situations. I understand how things work but I am still concerned, is it ok to run the vehicle in 4wd auto on regular pavement, wet or dry, or otherwise? I was told that ford recommends putting the vehicle in 4wd on occasion regardless of weather, just to keep things working correctly. Again, I know I may be way off. but if someone could just clarify or confirm that I am ok, or not ok in doing that, I'd be hyper-appreciative!
Thanks,
Kent

Well, the further I look into this, the more complicated it gets. http://www.4x4central.com/ubb/smile.gif I've found more patents that may or may not cover the control trac system including patent no.'s:
5485894
5605201
5609219
5704444
3 of these are very similar, but one appears to be a system used in addition to the automatic system that overrides in extreme conditions.

That being said, I'll try to share what little I've gathered from these documents.

Brian,

After reading a bit more, I'm more inclined to think that your problem may be in the GEM module (or at least that would appear to be the best place to look). Does the system disengage and "bang" when you turn the steering wheel while in 4-Lo? Someone in another thread posted that this happened to them. The system does utilize steering wheel position as one of the inputs to help it decide on how much to engage the clutch in Auto mode. I'm thinking that the manual override that is provided by the dashboard switch is still letting the steering position sensor information bias the computer so that it may not stay at full engagement, even with the switch in 4-lo. If that is the case, hopefully, a reprogammed GEM will fix the problem.

Kent,
The manual is probably vague (as most manuals are), but it should be OK to use Auto on the road. It's kind of interesting that you have a 2wd position on yours. The newer ones have 4-Auto, 4-High, and 4-Low positions only. Do you get any binding when you turn while in 4-Auto? If the thing is locked in 4wd, you'll feel it in the steering wheel the minute you try to turn. Steering will get heavy, and if you turn even a little bit sharp, the truck will buck and you'll hear wheels slip as they try to maintain equal speed. If this happens, DO NOT try to use 4-auto on the street. Personally, I would stick to 2wd on the road, wet or dry, unless I needed the extra traction, though you should engage 4wd occasionally to keep the parts moving.

I'm curious here, does your system use the automatic hubs in conjunction with 4wd? The patents I've read show that the system can be used with auto hubs or a fully engaged (to the wheels) front drive train.

I hope this has helped a bit.

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Tom

WOW, you are either really smart, have too much time on your hands, or both. in any event, thanks for the explanation! i've wondered how control trac works.

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Tom
96 XLT

My wifes sister has a 96 sport and it has a 2wd position, 4 Auto and 4LO positions. My 98XLT has 4 auto, 4Hi, and 4LO positions. I was told that in the 98 version combined the 2wd and 4auto positions. The auto mode changes to 2wd after 45MPH is reached. I am not sure if it is 100% to the rear or if it maintains the 95/5 split. I would assume this was to help the mileage.
I am curious as to the website describing these patents. Sounds like interesting rainy-day reading. Something else to therorize over.

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Exploring CA
98 XLT 4X4 4.0L SOHC

Damn, I'm impressed again.

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Stephen Withrow
---------------
97 Limited
98 Lexus LS400

Jim F,

The US patent web site is: http://www/uspto.gov/patft/index.html It has the ability to search by patent number or key word. Use the patent numbers above to find some info on the system. You'll need a very long rainy day to read all that stuff.

Also, you'll need a plug-in to view the diagrams (.tiff files). Click on help if you have a problem viewing, and they will give you the necessary instructions on finding the free plug-in. It works fine with IE 5.0, but I can't speak for Netscape.

Tom,

It's not that I'm particularly smart, nor do I have too much time. It's just that I'm avoiding other chores that I have including some studying for a midterm next monday night. There's a host of info out on the web, but its sometimes hard to seperate the wheat from the chaff. I've found that deja.com is a good source for searching the newsgroups, where some, but by no means all people do have a good understanding of things.

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Tom

Tom - Whoa! Thanks for the info on 4 x4. I was told that for my '99 XLT that I should use AUTO mode ALL the time unless I REALLLY, REALLY need to use the other two modes (if conditions are intolerable). I will, however,upon the advice I have read herein, now engage it briefly during bad weather just to keep it from getting 'rusty'.

JUST CURIOUS>>>>you mention a GEM module from time to time in writing about the 4 x 4 mode. Is this the same GEM module that was needed to fix my 'phantom' windshield wiper problem? The dealer did say that it was important that the GEM module he used to fix my wiper problem be designated for a 4 x 4 vehicle. Thanks!!!

Thanks for the website regarding the patents, however the link has a slash in place of a dot.

Here a clickable correction:

http://www.uspto.gov/patft/index.html


Thanks again

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Exploring CA
98 XLT 4X4 4.0L SOHC

hey tom I have a 98 with Auto,4high,4low. I was wondering if I am on some trails or in some mud is it ok to be in auto.What I am trying to say is that when do I really need to switch out of auto into high. can using the control trac offroad damage it

really what are the limits of the control trac

You can offroad in auto and switch to 4x4 hi on the fly w/o a problem. In Auto, there will be a lag in the fronts getting power based on the computer's readings. Also remember, to switch to low you need to be stopped, in neutral w/your foot on the brake. Not sure if you would want to run into a mudhole in auto, get stuck and then try to get yourself into low and out while your Ex starts sinking......

Yeah I know, but I love control trac , Ive gotten in some pretty hairy trails and havent gotten stuck in high. I think it awesome.I like my SOHC. Will be lifted in a little while. Ohh, and when you mean its a little slower in auto what do you mean?

I think he means the time it takes for the power to be transfered to the gripping wheel(s) is slower.

I do not see why someone would tell you to use the AUTO position all the time. In my personal opinion it may engauge at some time while driving and if you aren't paying too much attention it could possibly cause a wreck. I know that when I had my X in AUTO mode the first snow we had when the roads were plowed but spots of packed snow was still on the highway my AUTO would engauge when the rear wheels slipped and the X jerked forward, now if that same thing would happen on that slushy kinda snow it could make you jerk to one side or the other, but that is my personal opinion, it may not be possible but hey.........I'm just trying to start an argument :)

I dont want to use auto all the time, I just wanted to know when I should or should or shouldnt use it , and Tplynch already answered that

I always start off in Auto. If I feel the front wheels engaging frequently and if the surface warrants it (snow, mud or sand) I'll switch to High. When I drive at higher than normal speeds on dirt covered, washboard roads I'll often switch it to High since it prevents the rear end from skipping out.

I asked some people on another post and they didnt know what this was. Alot of times when make a some what slow turn I fell kind of like a jolting coming from seems like the steering. I dont know what the heck this is. All I can think off is that the 4wd is still engaged and its slipping some how. I dont know if I am damaging some thing or not but the jolting and vibrating get me annoyed and worried. It only happens for a second or two in forward and reverse in the turns(most of the time when the terrain is slightly inclined and not when the wheel is straight). My brother thinks its coming from the rear but I think its coming from the front. If you guys have had this happen to you or heard this happen to someone else I could really use the info. Thanks, Matt

It could be that your rear speed sensor is working marginally. It might be fooling your computer into thinking that your rear wheels are slipping thereby forcing some torque to the front wheels. I'm not sure how to check them, but the rear sensor is easy to remove and clean. If that doesn't work, I think they only cost about $35.12 each. Might be worth it to put a new one in. If yours is actually transfering torque to the front wheels on dry pavement, that isn't good and won't last very long. Everyone has a different definition for "clunk" so it's hard saying if it is an actual problem without being there and feeling it. If it is something that concerns you, then at the least I would have it looked into.

On 8/17/2002:

The rear sensor (1L2Z-7F293-AB) cost $32.59 plus $2.53 tax (Sac Co, CA 7.75% sales tax) for a total of $35.12.

You're off by $0.12, Robert. :D

What do you mean?


:p

My bad, I could have sworn it said $35 ten minutes ago. :D

I think one of the reasons the fronts get 5% power in the Auto position in control-trac is that it combats sway during moderate cornering.

I think that a couple of people have mentioned that it helps improve handling, but I have never disabled mine to see the effect.

I have notice the difference when I have my auto turned off. Why would that little bit of power up front provide better handling?

because of when you turn, the front track will be PULLING the front of the car into the turn, instead of the rear pushing the front which puts most of the turning force on the front outside wheel.

my theory, anyway.