'97 XLT 4x4 Modes: What am I Actually Getting?

[Lonnie-S]

I've been trying to understand exactly what I'm getting using the various 4x4 modes in my truck. It's an XLT with SOHC, 4.0L V6 with automatic transmission and 4.11 limited-slip rear axle. I assume the front differential is 4.11 too, but the open type. Maybe others have the same question(s) in their mind. Here's what I think I'm getting. I could very well be wrong.

4x4x Auto: It's really in 2-wheel, rear drive, but if the system senses a loss of traction on the rear wheels, power is sent to the front differential via the transfer case. If that's right, how does it "sense" the rear axle loss of traction and what logic (percent of power to transfer, for how long, etc.) is used to make this happen and which vehicle component makes the decision?

4x4 High: Power is supplied to both front and rear differentials all the time. If so, what percentage goes to the front and rear? Is the percentage changeable by the 4x4 system dynamically as conditions change or is it always fixed?

4x4 Low: Same as 4x4 High, but uses low range gear of transfer case.

Secondarily, I'd like to know if a locking differential at front or back, or both, would be compatible with the stock 4x4 system or just cause problems. In particular, is it possible to "lock" a limited slip differential at all? Has anyone had experience with lockers on an otherwise stock 4x4 system?

Thanks,

Lonnie

 

[imp]

One Step at a Time

4x4x Auto: It's really in 2-wheel, rear drive, but if the system senses a loss of traction on the rear wheels, power is sent to the front differential via the transfer case. If that's right, how does it "sense" the rear axle loss of traction and what logic (percent of power to transfer, for how long, etc.) is used to make this happen and which vehicle component makes the decision?

Yes. 4X4 AUTO is driving only the rear wheels. The front drive-line is turning all the time, though, disconnected from power by an electronic clutch within the transfer case.

The signal(s) from the ABS (Anti-lock Brake System) in rear are continually being compared to those of the front wheels. In the case of vehicles having 2 rear sensors, one at each wheel, any difference between the speeds of the 2 rear wheels will also be interpreted as a need for 4X4. When such speed discrepancy is detected, the PCM (computer) instantly energizes the E. Clutch and power is then directed to the front wheels by it. How long it remains engaged is dependent on two things: 1. Is speed difference continuing after engage (as on ice, mud, snow, loose sand), then it will remain engaged until wheel speed difference is basically zero for a programmed time, how long, just a guess, maybe 2-4 seconds. So, it's possible for the clutch to be applied/released quickly and repetitively, a situation you will not like: clunk, clunk. Then you switch to 4X4 HIGH, which turns ON the clutch and it stays on, no PCM input to it.

4x4 High: Power is supplied to both front and rear differentials all the time. If so, what percentage goes to the front and rear? Is the percentage changeable by the 4x4 system dynamically as conditions change or is it always fixed?

The power flow is said to be about 65% rear, 35% front, never changes, determined by gearing in the transfer case.

4x4 Low: Same as 4x4 High, but uses low range gear of transfer case.

Yes. However, more is going on than meets the eye. In 4X4 LOW, the shift schedules are altered to allow less jerking when shifts occur, for one thing.

Secondarily, I'd like to know if a locking differential at front or back, or both, would be compatible with the stock 4x4 system or just cause problems. In particular, is it possible to "lock" a limited slip differential at all? Has anyone had experience with lockers on an otherwise stock 4x4 system?

In general, "locking" differentials are of two types: positive drive to both wheels until mechanically "unlock ked" (Detroit Locker type, or air-controlled type, etc.) and clutch-type which allow "slippage" to occur between wheels at some pre-determined torque level. Either type are usable in 4X4s in rear, but neither are recommended for front. Fronts ARE possible using the "Torsen" type worm-gear differential, are said to work very smoothly, and well. No personal experience there, sorry. I DID use a Detroit Locker in rear of my 1970 Bronco equipped with a '91 5.0L HO Mustang engine and T-5 5-speed. imp

 

[XLTrunner]

http://www.rsgear.com/articles/2002_09_Gears.pdf

Here is a good tech article that describes how a Borg-Warner 4405 works. It's important to note that, despite prevailing belief that 4Auto is a 2WD mode until slippage is detected, there is actually a small (article uses the term "minimal") percentage of power always directed to the front wheels in this mode. I've read between 4%-8%. It's also important to note that things such as uneven tire diameters, uneven tire pressures and worn front suspension parts (causing misalignment) can cause premature failure of the t-case.

 

[Lonnie-S]

[MENTION=126284]imp[/MENTION]
Thank you, sir, for the detailed reply.

[MENTION=282941]XLTrunner[/MENTION]
Thank you very much for that article. I'm impressed. The Explorer 4x4 system is much more sophisticated than I thought. That's a great article, but I wish the tables and drawings were more readable. The text is very helpful and informative, however

Cheers,

 

[2TimingTom]

[MENTION=282941]XLTrunner[/MENTION]: that's interesting.

And why I love my manual transfer case even more.

 

[2TimingTom]

To add on what [MENTION=126284]imp[/MENTION] said about lockers, a front locker (when locked) will behave a lot like a power steering pump that is not working- meaning the steering will be VERY heavy. Furthermore, a traction device like a locker or limited slip (like an Eaton Truetrac) on IFS..... I would be highly cautious. Plus having it in the rear will get you through nearly everything you encounter.

Depending on what you want to do, the factory limited slip might be plenty. It might need to be rebuilt or adjusted though.

 

[Lonnie-S]

To add on what [MENTION=126284]imp[/MENTION] said about lockers, a front locker (when locked) will behave a lot like a power steering pump that is not working- meaning the steering will be VERY heavy. Furthermore, a traction device like a locker or limited slip (like an Eaton Truetrac) on IFS..... I would be highly cautious. Plus having it in the rear will get you through nearly everything you encounter.

Depending on what you want to do, the factory limited slip might be plenty. It might need to be rebuilt or adjusted though.

The reason I was interested in locking the front is the idea that an open diff at the front would allow one wheel to be undriven in poor traction situations, even while in 4 wheel drive. Having the front locked would ensure they both drive all the time. I would only be using 4x4 in low speed situations anyway, so maybe the high steering effort wouldn't be so bad?

When I first bought my truck, I went through an off-road driving school. The instructor insisted that those who had locked differentials leave them unlocked during the first half of the course because otherwise, "It makes the course too easy." Me, I like easy! I'm more of a back road explorer than a true off-road guy. So, I won't ever have the driving skills of someone who off-roads a lot. If I can get my equipment to compensate for me in some way, that would be great.

Cheers,

 

[2TimingTom]

In 4lo with open diffs, you really have 2 wheel drive when the going gets really tough: one front wheel and one rear wheel. But that only happens if the suspension is flexed out and tires are significantly unloaded- and of course the 2 wheels that are getting power are the 2 that don't need the power.

You can put a locker in the rear. There are units available: ARB, OX and some others. The front I'm not sure if that diff is supported by any lockers. If there is one for the front, it shouldn't mess up the 4wd system as long as you only engaged the locker when you were in either 4hi or 4lo (the "auto" I'm unsure of).

There was a similar thread on my local 4x4 forum where someone was asking roughly this same question. A bunch of guys with Jeeps chimed in and said their front locker rarely gets used- like once every few years, and usually only in the snow. And all said that the rear alone will get you through just about anything or get you REALLY stuck.

I have Eaton Truetracs in my Explorer front and rear (my front axle is from a Jeep Cherokee). They are torsion style worm gear limited slips. They are cheaper than an ARB and don't have an air compressor and air lines to break down. They also work better than a clutch type limited slip. They work invisibly to my knowledge: I just get through stuff. The downside with a Truetrac is when you have a tire in the air or has very little weight on it, the diff will behave just like an open diff. But there are tricks to get through it. Usually you can still move in one direction when your truck is like that, go that direction (probably down the hill) slightly, then try the other direction again but while riding the brake with your left foot. This applies a torque to that unloaded wheel and the diff sends power to both wheels. I've only done it once, but it worked.

 

[Lonnie-S]

[MENTION=156828]2TimingTom[/MENTION]
You say, "Truetrac" and "my front axle is from a Jeep Cherokee", so I'm going to assume you've switched to a solid front axle? Any photos of your rig on this system?

I aware of the general operation of a Torson (sp?) type differential, but really don't know much about them technically. Truetrac is an Eaton system, isn't it? Do they sell into the aftermarket too, or just OEM?

Cheers,

 

[imp]

"Truetrac" and "Torsen" are one and the same. The Torsen name was applied by one of the several owners of the system's patents (if still in effect). Eaton currently makes them using the Truetrac name. Not made for every possible axle design. Maybe not for some of the smaller Dana axles. Available for Ford 9-inch.

Search on-line under Truetrac for more history, availability, companies selling them. No doubt in my mind one of the best ways to go- other than a "spool", not for 4X4 front! imp

 

[Lonnie-S]

I did some Google searching last night and found quite a bit of material. Here's something that appears to be based on the same Torsen technology, but engagement is electric from the driver's seat, not mechanical or pneumatic. It's probably old news to many of you, but new to me. They stress the use of front and rear lockers independently or together, all selectable by the driver.

The first half is basically interesting, visual, sales propaganda, but there's some elementary technical info near the end.

Eaton ELocker differentials ==> https://www.youtube.com/watch?v=85-8IozvwVg

Cheers,