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Discussion Starter #1
Hi all,

I'm a Commander owner from Canada.

I did a lift and tires on my Commander, and feel as though the effectiveness of the "diff locking" system has decreased.

So now i'm thinking out loud here....

The differential locking system on both the QuadraTrac II and QuadraDrive II is based upon wheel speed. As we all know, QTII uses brake force to simulate a LSD or locking diff, QDII uses electronically locking LSD's.

Either way, both systems are ELECTRONIC and based upon wheel speed. As wheel speed differs, the computer will send brake force - or lock the diff.

What if your tire size has increased over stock??? That would effectively reduce the reaction time of the computer - in the locking process of the traction system.

So what if we were to use a programmer, to over boast the tire size? Not only could we bring our traction aid's back to a stock manner.... But by suggesting to the computer that we have bigger tires than we actually do, we could in turn speed up the process at which the tires/ axles/ brake force system begin to lock.



My problem: my Commander started out life in a rental car fleet. That being said, it has all kinds of special codes and accessories spec'ed by the rental company - that most Commanders don't. The wife's brother is a service tech at a Chrysler Jeep dealer, and before I bought it - he ran it by the service and sales guys to see what they thought.

Many of them did not recognize the tire and suspension codes that it had on the build sheet. It was really LOW to the ground, and came with 235/60/17's on it! That was the size spec'ed from factory. However it had Mopar skid plates, all the limited accessories (leather, sunroof, etc) and extended range fuel tank right. The lift and tires I could handle. Being an ex rental it was CHEAP.

Story goes on. Bought it, drove it. QTII worked awesome. I wondered what the fuss over QDII was.Two wheels on the ground, one front opposite rear, and zero hesitation it would pull through anything.

I then put in factory suspension, and 245/70/17's. QTII got worse. It would lag and lag and then finally hook up.

I then put in a 2.25 lift and 265/75/17's. QTII is super laggy now, and it takes a good jab of the brake pedal/ yank of the e-brake to get it activated.


My Thought: The computer is calibrated for 235/60/17's. What if I plug in a programmer, and tell the ECU that it has 33's on it. Hell, say 35's. I should in turn get the sensitivity back that I once had in the factory system?? My speedo might be out, but I only put on 5000 km per year (purely recreational) I've even thought of high performance brake pads, (hawk, ebc, etc)

Thoughts, ideas? :icon_cool:
 

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Welcome!

Contrary to what you mentioned, QD-II is the only drivetrain with differential lockers, whereas the QT-II system has open diffs. The only thing they share is the transfercase.

Now for the good news, if you have QT-II, you can install front and rear lockers to make a QD-II equivalent driveline. This has already been done by 07JeepXK; in the rear is a powertrax no slip locker and in the front is a QD-II donor diff with factory locker already installed. The front diff just bolts in and the locker can be hooked up to a manual switch.

Here's the thread detailing Adam's build:http://www.jeepcommander.com/forums/showthread.php?t=19328
 

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Discussion Starter #3
Thanks!

I did not mention that QTII has locking diffs. It uses brake force distribution to simulate an lsd or locker.

I'm fully aware of lockers and diff swaps, and just am not interested....

The function of the QTII served me very well, in its original form. I'm wondering if by fooling the computers, we can get that function back?
 

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Adding a lift and bigger tires to your jeep wouldn't cause your 4wd system to not work properly or change how it worked when it was stock. Well mine didn't change anything and I've run many different tire sizes/lift combinations since I bought it brand new.

I was under the assumption that all Commanders with 17 inch wheels came stock with 245/65R17's. What does the sticker on the drivers side door jam say it came with?

Also you should have to trick the computer to think it has larger tires than it really does. You should have programmed the size you are actually running.
 

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Discussion Starter #5 (Edited)
Sticker says 235/60/17. Ex rental. Oddness. But no I've never reprogrammed the Speedo.

The factory qdii system is based on rev/mi of the tire vs rotations at the brake rotor.

If factory tires are allowed one rotation before brake force is applied, a much larger tire could potentially get out 4 rotations.

I'm going to reprogram for a size a little larger than what I have now, it should fix me right up.

Thanks for the help?
 

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Again I have no issues with my 4wd system and I run a 305/70R17 and I'm programmed to the correct tire size.
 

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Sticker says 235/60/17. Ex rental. Oddness. But no I've never reprogrammed the Speedo.

The factory qdii system is based on rev/mi of the tire vs rotations at the brake rotor.

If factory tires are allowed one rotation before brake force is applied, a much larger tire could potentially get out 4 rotations.

I'm going to reprogram for a size a little larger than what I have now, it should fix me right up.

Thanks for the help?
I am not sure I understand how it could get out by 4 rotations. It would add somewhere around 4.5-5" less spin or around 10%. 235's may be slightly more. Its possible that the braking lags slightly (to stop the spinning wheel) due to additional tire weight but that would surprise me if it were noticeable. Let us know if changing makes a difference.
 
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