The "lock" feature mimics a part-time 4x4 mechanically locking center differential by engaging electromagnetic plates to lock the front and rear axle in 4WD. It does not act like a locking rear or front differential.

Jeep has BLD (brake lock differentials) which engage your brakes when you lose traction...acting similar to a limited-slip. The brakes independently apply pressure to a slipping wheel, temporarily slowing/ceasing the spin and sending power across the "axle" or to the opposite wheel. From my experience with my '09 Liberty it is a good system, but DOES NOT replace a locker.

 

Thanks for the information on the FDII. I'm trying to figure out if I want to buy an FDII and I just want to learn more about this new non-traditional 4wd system. I have an XJ and love it to pieces, but 20 mpg in the city with a lift is unbeatable!!!

 

The FD-II 4wd system is continuous 4wd and can be locked into place too.

Well, it's a continuous AWD. In good traction conditions only front wheels get power, but if the computer thinks the front wheels could use a little help it can engage the ECC, in other words going into four wheel drive mode on its own.

That chrome handle called 4WD lock can be used to manually lock the Jeep into four wheel drive mode. A center differential lock, in other words.

Does this ABS act like a limited slip diff.

In FDII, on the rear axle, yes.

Does the Pat FDII have a locking differential when locked in 4wd mode?

Only the center differential. Both axles are open - but with FDII you get the Brake Lock Differential which mimics a limited slip differential.

All I want to know is if the FDII system locks all 4 wheels when in 4wd or if it is just like any other 4wd system and only 1 front and 1 rear wheel drive it at a time.

The latter. No lockers beside the center differential one (just the limited slip in the rear - which as I'm sure you know is not as effective as a 100% lock).

Please let me know so I can decide If I can give up my XJ for the gas millage!!!epper:

I dunno, from my FDI point-of-view a modified XJ would make a much better off-roading rig... The FDII is more capable than the FDI though. Besides, if you're going to do any serious off-roading, everyone and their dog says it is much better to have a separate rig than have to worry about repairs to your daily driver.

(Me, I've been dreaming of buying an XJ for off-roading use, so for me the idea of getting rid of one seems weird. )

 

Tony1911

Gotta play devils advocate for a moment. If the vehicle is only sending power to one wheel on the rear axle or one on the front axle when I am locked, why do I have an electronic program that can stop one wheel from turning with the brakes when it senses slip.

Staight logic is confusing me on this one.

Maybe you can explain it to me, but this is why I am confused.

If I am in 4 wheel drive lock on my FDII, I have brake traction control. This is supposed to allow me to stop one wheel from spinning on an axle for example if one wheel is off the ground. Why would I need this if both wheels weren't spinning. Is that how it switches the power across the vehicle? Wouldn't it take more, like an additional sensor, to send that power across?

Are you sure that all four wheels never recieve power at the same time? I don't know, I am just asking, how sure are you? 100%

It could be poor wording choice by Jeep, but it always says in the manual and materials that when the front wheels (wheels with an "s" at the end) lose traction that 50% of the power is sent to the rear wheels (again wheels with an "s,") and not 50% of the power is sent to a rear wheel.

I think Homac posted that exert from the manual a while back. Not to make this point, but just in general.

Anyway, you are probably right, I am just trying to understand the system better.

 

The Patriot has open differentials on the front and back. Click here for an explaination of an open diff http://auto.howstuffworks.com/differential2.htm
With an open diff, the drive shaft transfers power to both wheels equally as long as they both have traction. With the open diff on our Pats, the power transfer takes the path of least resistance....if one wheel is off the ground, it will get all of the power. That's where the TCS kicks in with the brake locking the spinning wheel, which allows the open diff to transfer some power to the wheel with traction.
If you have 4x4 locked, and have traction, all 4 wheels will be pushing the vehicle equally.

 

This should help:

http://blog.chryslerllc.com/blog.do?p=entry&id=270

 

Thanks, really it just confused me more, but I will re-read it a few times to see if I can make sense of it. I think becaus the article isn't patriot specific that I don't know if any or all of it applies to us.

Hey, if anyone wants to try and put that article into laymans terms feel free.

It does do a good job of enhancing my general knowledge, but didn't help me much with the pat's system.

 

It's my understanding that the computer cannot control power directly between left/right wheels on the same axle (front/rear - pretend there's an actual axle there).

It can't even directly control power front to rear. 100% power is just sent to the center differential. By controlling a clutch of some sorts, the power is either sent entirely to the front wheels (clutch is open), or engage the clutch which splits the power 50/50 front and rear.

If the computer detects slippage, it can control that clutch, the main transmission, and actual engine RPM to carefully feather back the actual power to the wheels. That is why if you are on a slippery hill, you don't move at all because it is cutting back the power to nothing because the wheels are slipping.

Left and right slippage is controlled with the Brake-Lock-Differential. If the left is spinning faster than the right, the brakes are applied to the left to stop the spinning. In this case you have to think about torque to each wheel instead of speed, slipping, etc.
With the open front and rear differentials, the torque applied to the left/right wheels is always the same. Always. Period. If one wheel is off the ground, a very small amount of torque applied to it gets it spinning madly. Therefore the same small amount of torque is applied to the other wheel on the ground - and so it doesn't move at all.
If you then apply brakes to the spinning wheel, the torque needed to keep it spinning goes up - which means the torque to the other wheel goes up. The harder the brakes are applied, the more torque needed to overcome the breaks, and the more torque applied to the other wheel - eventually enough to get you moving again.

The problem is that the engine has to provide double the torque to keep it going.

 

Okay, so what does all this mean. Is there or not a time when all four wheels are getting power at the same time? It kind of sounds like they are all spinning by the above desription. If one of the rear wheels is spinning faster than the other, then there must be at least 3 wheels spinning (right) and probably 4. Right? Or am I in left field?

Lets say you are locked then you have up a 50/50 power split front to back. If only one wheel is spinning at a time, why would one need brake traction control.

If it is as stated above then both wheels are getting power or otherwise one would always be getting more power/torque,wheel speed and thus constantly braked right and conversely why would we need the brake to hold one wheel from spinning if power was sent to the other side and only one side can use power at a time?

Am I making any sense? The only other way I can see it working is if a sensor senses extreme wheel speed (slippage) brakes that wheel and that braking action trips some mechanism or additional sensor that then sends power to the other side.

I guess it doesn't really matter, cause knowing how it works doesn't change anything about how it works. I am just curious if anybody knows.

If the computer can not control power left to right, then they both have to be moving always?

It was mentioned above that the brake lock sends the power across the axle in another post. How can a brake send power across the axle. I am not arguing it can't, I just want to know how it does it.

I am more confused than ever. However, that doesn't always take much.

 

Thanks Hitzy. Your explanation finally made it 'make sense.' That is a great website (http://auto.howstuffworks.com/differential2.htm) A picture is worth a thousand words right....

 

That is a good explanation Hitzy. There is a lot of confusion about the Patriot 4wd and traction systems, even among owners. The alphabet soup of different but related subsystems doesn't help.

One function of the traction control system (TCS) is often called "brake lock differential", or BLD. It accomplishes a similar effect to a limited slip axle differential, but gets there by a different route.

BLD is side-side. So on either the front or rear axle, if one wheel is spinning and the other is not moving, the ABS is applied on the spinning wheel. This increases resistance on the spining wheel, making the wheel with traction the path of lower resistance, so this transfers torque to the wheel with traction. This is independent on each axle: the system can apply the BLD on the front right wheel and the rear left wheel at the same time.

Both FDI and FDII are full-time four wheel drive systems, that allow wheels on the same axle to rotate at different speeds and can be driven on dry, hard surfaces without driveline binding (aka "tire scrub" or "wheel hop").

The Patriot does not have a mechanical center differential but it does have an electronic center coupling (ECC). This functions similarly to a locking (mechanical) center differential; 4wd lock mode engages the ECC and forces 50% of torque to the rear axle. When the system is not in 4WD lock mode, it will still automatically send torque to the rear wheels when needed, by engaging the ECC on demand; in this mode the % of torque transferred to the rear axle is variable.

That all works the same on FDI and FDII. The info from Jeep is that the main difference between the TCS system in FDI and FDII is that the FDII is optimized for off-road, which I guess implies that FDI TCS is more optimized for the highway.

The other factor that affects TCS is the ESP mode. Someone above mentioned that one of the TCS functions is to reduce engine power when wheel spin is detected, in some situations this helps to re-establish traction. Engine power reduction only occurs when the ESP mode is "on" (default). When ESP mode is partially off or completely off, engine power reduction does not occur. BLD is always on, regardless of 4wd or ESP mode.

http://www.jeep.com/en/4x4/how_systems_work/freedom_drive/

http://www.jeep.com/en/4x4/how_systems_work/freedom_drive_2/

 

The Patriot does not have a mechanical center differential but it does have an electronic center coupling (ECC). This functions similarly to a locking (mechanical) center differential; 4wd lock mode engages the ECC and forces 50% of torque to the rear axle. When the system is not in 4WD lock mode, it will still automatically send torque to the rear wheels when needed, by engaging the ECC on demand; in this mode the % of torque transferred to the rear axle is variable.

Can anyone confirm that the torque is variable in automatic mode. Some said that when ECC is engaged automatically, only 50% of torque is transfered to the rear wheels. Others said that between 0% and 70% of torque is transfered.

I would like to have a confirmation once for all with proofs, like images of clutch that engages the rear wheels ;-)

Thank you!

 

Sorry for the long winded explanation UPKEV. I'm an engineer and sometimes I get ahead of myself.

Technically the open diff at each axle is a torque equalizer... but practically it's like Hitzy said: path of least resistance.

By braking on the wheel that's off the ground, it adds resistance to that wheel, and eventually (once the braking is hard enough) the wheel that's on the ground is actually "lower resistance" and the vehicle moves.

 

From http://www.jeep.com/en/4x4/how_systems_work/freedom_drive_2/

"The electronic control module processes signals from the wheel speed sensors and the accelerator pedal to determine how much torque to transfer to the rear wheels, then sends an electrical current to the ECC that is proportional to the amount of torque required."

If you listen to the audio on that page, it clearly explains that torque gets transferred to the rear axle under hard acceleration or tight turns.

The web page indicates that 50% of torque is directed to the rear axle in 4wd lock mode, but does not address front/rear torque distribution in automatic mode. I also can't find any discussion of torque distribution in the 2008 manual.

The 2008 Patriot manual (pp 130-132) implies than any vehicle with ESP has TCS. The explanation of TCS (p 130) does not suggest that the BLD function is absent in any transaxle version.

Under "ESP 2WD Operating Modes" (pp 135-136), the manual states: "When in “Partial Off” mode, the TCS portion of ESP, except for the “limited slip” feature described in the TCS section, has been disabled," which I take as an indication that all vehicles with ESP are also equipped with TCS. Also, under "ESP 4WD Operating Modes: "all 4WD equipped vehicles can also choose [ESP Off]" and "In this mode, ESP and TCS, except for the 'limited slip' feature described in the TCS section, are turned off until the vehicle reaches a speed of 35 mph (56 km/h). At 35 mph (56 km/h) the normal ESP stability function returns with the exception of engine power reduction. TCS remains off. When the vehicle speed drops below 30 mph (48 km/h) the ESP system shuts off."

The description of FDII (p 244) states that "Off-road Brake Traction Control" is an additional feature of FDII. On page 245, it states "Off-Road Brake Traction Control – When conditions warrant, the vehicle braking system transfers torque from side to side. It does not control throttle input unlike normal traction control which is active in normal drive mode" [emphasis added]. This is pretty vague, but sounds exactly like regular TCS with ESP partially off. On my current CVT2L, when 4wd lock is engaged and the gear selector is in L or R, the ESP indicator is lit solid (indicating ESP in partial off mode). AFAIK, the sources don't discuss any other modifications to FDII TCS that would constitute optimizations for off-road use. There may well be algorithmic differences, or "Off-road TCS" might just be marketing-speak for "'Off road mode' automatically switches ESP partially off".

 

It's a fact for FDI: When one wheel on each axle looses traction, the only thing that can move the Patriot is gravity or momentum. I've been there and got really embarassed in the eyes of my Wrangler companions. I don't know how the FDII would act in a similar situation.

 

APT, I know Tony1911 will violently disagree with me, but I believe the 2007 4x4 FDI MK's with a PRNDL shift were more capable than the newer FDI's that have the autostick.

I too have observed behavior that indicates that I have some BLD operation when in 4x4 + L + ESP partial. Many of the other FDI drivers who have observed otherwise seem to have autosticks.

 

Very interesting, spaulesl. The 2007 FDI CVT Patriots all had the same CVT modes vs. the 2008+ that have either autostick or PRNDL. I suspect we man trans owners behave more like the autostick.

 

Would the 19:1 low range not make the TCS act more agressively?

 

BLD is not mentioned in the owner's manual. I think that this TLA entered the jeeppatriot.com lexicon from this blog posting from a Jeep engineer. The blog entry is not specifically about MKs, FDI or FDII systems, but if you read the followup comments, the engineer specifically states "Yes, your FWD Compass has BLD as part of the traction control system. "

I don't have an FDI, so I am not sure how it behaves in the real world. I do have FDII and have seen BLD action on both front and rear axles at the same time on a cross-axle problem.

What is TCS other than BLD + engine power reduction? The blog post above discusses that "off-road" BLD is is tuned to ensure that both wheels on a driven axle turn at the same rate, regardless of what that rate is. But that does not mean that there is no attempt to limit slip in "on-road" modes. Off-road TCS is probably "BLD" and on-road TCS is probably better described as "BLSD".

 

BLD is not mentioned in the owner's manual. I think that this TLA entered the jeeppatriot.com lexicon from this blog posting from a Jeep engineer.

Actually, the term BLD was used by jeep.com in its description of the off-road package. That is where at least one poster picked up the term. I tried to look for it a few days ago but couldn't see it then - it could be that they've removed the acronym from the description.

I do have FDII and have seen BLD action on both front and rear axles at the same time on a cross-axle problem.

Cool! I was worried that the BLD would only be active on the rear axle from what I read (from somewhere, forget the source) but if you guys have that system active on both axles, that's just twice as neat. :smiley_thumbs_up:

What is TCS other than BLD + engine power reduction? The blog post above discusses that "off-road" BLD is is tuned to ensure that both wheels on a driven axle turn at the same rate, regardless of what that rate is. But that does not mean that there is no attempt to limit slip in "on-road" modes. Off-road TCS is probably "BLD" and on-road TCS is probably better described as "BLSD".

Umm... From that last sentence I seem to gather that you think the BLD is a 100% lock. I don't think so - it is, after all, "only" a brake clamping down on a spinning wheel. A limited slip differential-type behavior created with the brakes instead of having a more complicated differential on the axle.

As for "what is TCS", it is a good question, but if you'll just look at the videos you'll notice that for off-roading, it is nowhere near as efficient as the FDII brake locking system. An wheel that has lost traction appears to simply spin freely. Engine power reduction does not even enter into it, there simply doesn't appear to be any discernible effort on the part of the vehicle to stop that one wheel from spinning so that its opposite would get some torque.

 

Given the peculiar nature of the patriot's torque distribution throughout the drivetrain, would installing a rear locker or limited slip even be worthwhile? I've noticed while offroad that on (ice/snow) OR (loose sand) hills that the electronics become ineffective (even with BFG MTs) and traction becomes nonexistant. It seems to me that a rear lsd or locker would be the easiest solution in regards to installation, although somehow locking the front transaxle would be the best traction wise. But who knows how to lock up a transaxle?! Especially something like whatever the Patriot has. In the rear, given the worst set of curcumstances, you could always replace the differential with a more common differential. But the front?

 

When I was high-centered in snow, all 4 wheels spun about the same.

And someone mentioned that the FDII when in Low off-road mode would turn ESP off, but that's a no. I have to turn off the ESP with the button, even in off road low gear.

The only thing I wonder about is the down hill crawl being able to turn off. I was told that with ESP full off the down hill crawl would not work, but it seems to always be on when in off road low gear. I really don't need it on the hills I'm on and would like it to be off, but I can live with it if it has to remain on.

 

Dear friends,

Please take a look at my post:

http://www.jeeppatriot.com/forum/showthread.php?t=96899

Are you able to help me?