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Discussion Starter #1
Can somebody tell me how critical it is to have identical tires in a JGC 98 Limited 5.9L with Quadra-trac? By "identical" I mean size, brand and wear.

Car manual says all tires shall be same size. Do they mean NOMINAL size as I always thought? I have been changing tires in pairs so one set is normally more worn out than the other. After having to change the front differential twice and several other repairs in the fron axle I am wondering if the tire size has to do with all this. According to Firestone, it does not (they may be justifying themselves since they have been the ones changing my tires in pairs and never teld me it was an incorrect practice. But recently a Jeep dealer service guy told me that all tires shall be the same diameter, he even said that if one tire goes wrong, I should replace all of them even if the others still have 50% of life on them! Wow!

Is this an issue with the Quadra-trac or is it an issue with other types of transfer cases?
Sorry, I am pretty ignorant about this, but according to the Jeep dealer is is my fault, according to Firestone it is OK to change tires in pairs... all I know is that I am the one paying the bills, differentials do not come cheap.:brickwall:
 

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You should always replace all 4 tires at the same time if you can. You can cause things to wear out prematurely or the worst case scenario which is damaging your differential like you have if you don't change all 4. You can't forget, a person who changes tires is not necessarily a mechanic and more than likely does not know jack about your jeeps drivetrain system.
 

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If you got two tires wearing faster than the others than you either aren't getting your tires rotated often enough or one or more of the tires are out of balance and wearing unevenly. Tires that wear really fast up front may be signs of bad alignment or worn suspension/steering components.

keep the tires in the same diameter and rotate frequently.
 

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Discussion Starter #4 (Edited)
If you got two tires wearing faster than the others than you either aren't getting your tires rotated often enough or one or more of the tires are out of balance and wearing unevenly. Tires that wear really fast up front may be signs of bad alignment or worn suspension/steering components.
I had a flat tire that could not be replaced, so I ended with two tires new and two with about 50%. Since then I have been changing tires in pairs, mainly due to ignorance from my part that this was important. The car manual does not stress this point, it just says "use same tire size" nothing about replacing all tires at same time. Neither did Firestone or Wal-Mart who replaced the tires or CAR-X who replaced the differential told me anything and in fact yesterday I called them to discuss my new problem and they insisted "it was not required", totally different from what I am reading here .

I am pretty ignorant on this matter, and I think some AWD and 4WD have different transfer cases / differentials and this issue is more significant for some than for others. Is the Quadra-Trac II on of the "sensitive" ones?

I am really pissed of, about my ignorance but mainly the fact that nobody (including the car manual) told me about this and I went through 4 repairs of the front train in 8 years. I would imagine a car like this would be sturdy enough for to allow changing tires in pairs and if not, a detail WARNING in the manual to make this totally clear...
 

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I had a flat tire that could not be replaced, so I ended with two tires new and two with about 50%. Since then I have been changing tires in pairs, mainly due to ignorance from my part that this was important. The car manual does not stress this point, it just says "use same tire size" nothing about replacing all tires at same time. Neither did Firestone or Wal-Mart who replaced the tires or CAR-X who replaced the differential told me anything and in fact yesterday I called them to discuss my new problem and they insisted "it was not required", totally different from what I am reading here .

I am pretty ignorant on this matter, and I think some AWD and 4WD have different transfer cases / differentials and this issue is more significant for some than for others. Is the Quadra-Trac II on of the "sensitive" ones?

I am really pissed of, about my ignorance but mainly the fact that nobody (including the car manual) told me about this and I went through 4 repairs of the front train in 8 years. I would imagine a car like this would be sturdy enough for to allow changing tires in pairs and if not, a detail WARNING in the manual to make this totally clear...
A 2wd car would not require having 4 of the same sized tires cuz the drivetrain is up front (or rear). A 4wd vehicle that uses 4wd part time or full time would require equal sized tires at all four corners. I don't think a 1/2" difference between the tires would be a big deal but 1"+ would cause drivetrain stress.

You said 4 repairs of the front train in 8 years. What repairs were done?
 

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Discussion Starter #8 (Edited)
You said 4 repairs of the front train in 8 years. What repairs were done?
2001 Ring and pinion bearings for front differential
2007 Complete front differential (one bearing locked in and damaged the casing)
2009 CV joint (prior to current trip to TX)
2009 I have not done it but dealer told me I need to get the ring and pinion bearings for front differential replaced (same as in 2001). Since I do not trust this not breaking as in 2007 (need to make my trip back to KC Saturday) I had the shaft from the transfer case removed today. Differential will still work but not transmit torque which should be better.

All this problems happened in long trips, and my beloved Jeep (that I am starting to hate) left me stranded. In 8 years I made 100K miles (had 45,000 when I bought it).
 

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Discussion Starter #9 (Edited)
My owners manual says that all tires should be within 1/4 inch of each other
Is yours a Grand Cherokee? What year? Does it have Quadra-Trac? I would be very interested to know if this is something Jeep added later in the Manual when failures starting to pile up.
I have talked with several car mechanics (Car-X, a former Jeep dealer, Firestone) and most of them are totally UNAWARE of this fact (equaly worn tires / replacing four at a time). Today the guys at the Firestone in Brownsville, TX even laughed at me when I told them why I wanted to change two 50% worn tires so they would be equal to the brand new ones.
Something smells bad in Denmark....
 

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Discussion Starter #10
You can't forget, a person who changes tires is not necessarily a mechanic and more than likely does not know jack about your jeeps drivetrain system.
I may be very naive, but I DO expect a company dedicated to change tires to KNOW if 4 tires NEED to be replaced and never in pairs in a AWD car. Who else otherwise? Soccer mom? This is not clearly said in the car manual even assuming car owners read them. I read it multiple times and still it is not clear what they intended to say by "all tires shall be same size". To me this means they all shall be P225/70 R16, no reference to wear. If in fact it is a requirement, it should be in red capital leters in the manual and most big tire shops (Firestone, Wal-Mart, Tires Plus) should be aware of it and either not change 2 tires or have you sign a disclaimer that they told you the risks.
 

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Is yours a Grand Cherokee? What year? Does it have Quadra-Trac? I would be very interested to know if this is something Jeep added later in the Manual when failures starting to pile up.
I have talked with several car mechanics (Car-X, a former Jeep dealer, Firestone) and most of them are totally UNAWARE of this fact (equaly worn tires / replacing four at a time). Today the guys at the Firestone in Brownsville, TX even laughed at me when I told them why I wanted to change two 50% worn tires so they would be equal to the brand new ones.
Something smells bad in Denmark....
Mine is a 98 ZJ. I have the 242 transfer case not the quadra trac. A friend of mine had a 78 Blazer that had fulltime 4wd and he always had to replace 4 tires at a time. He also had to make sure the spare was the same size. I don't mean to offend anyone but alot of the so called mechanics at these tire places are not much better than the ones at a Jiffy lube. I am the type person that when I am buying something I tell them what I want, most of them have no idea. If they were good mechanics they probably would not be changing tires. I also do the same thing at Advance Auto or Autozone.
 

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Here is the Heart of the problems.

Meet the NP 249 Viscous Coupler. This is a very High fail rate part in the Transfer case 249 AWD


How it works will help you under stand why you front and rear Tires need to be the same size.

Unlike the 242 in all time 4wd which funtions like an open differntal, the 249 uses this clutch type pack that is encased in a fluid. as the rear tires begin to spin or loose traction thus turning at a different speed then the front the fluid heats and becomes a solid locking the clutches up to transfer tourqe to the front drive shaft. If your front tires are a different size the tires will turn at a differnt rate from each other causing your transfer case to act in a locked manner . this puts a tremendous amount of strain on you front drive train while on pavment.
All v-8s had this transfer case, and some I-6 . It is very common for the fluid to dry up or harden over time.
This is also why a 242 is such a common swap for us ,it is a cheap and easy swap.
This problem is also why there are so many of the Grand Cherokees out there with no front drive shaft.;)
 

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Discussion Starter #13
All v-8s had this transfer case, and some I-6 . It is very common for the fluid to dry up or harden over time.
This is also why a 242 is such a common swap for us ,it is a cheap and easy swap.
This problem is also why there are so many of the Grand Cherokees out there with no front drive shaft.
I am a little confused, please be patient with my ignorance. Is the 249 the Quadra-trac?
Is the 242 an upgrade to the 249 or the other way around? Are you suggesting the Quadra-Trac is a bad design? (I would not be surprised).
Last, what happens when the shaft from the transfer case to the front differential is removed? Will the transfer case then work "locked"? Is this OK or further damage will be done?
I am asking this because when the front differential started making noise a mechanic removed the shaft from the Quadra-Trac to the front axle, so now I am driving as 2D until I can get back from my trip and can make the lengthy and expensive repairs. I know, the differential is still rotating, but we thought that if not transmitting torque the bearings would be less likely to fully fail and freeze (I was told the pinion and/or ring bearings are bad and making the noise).
 

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249 is the quadratrac. The 242 is an upgrade.(I'm sure the guys with 231's will disagree) With the 242 you get 2wd, fulltime 4wd,parttime 4wd and low 4wd. I like the 242 because with the fulltime I can run it all the time if I want or I can use 2wd. The ultimate for offroad use would be a 231, the aftermarket has all kinds of things like 2wd low kits, Tera low kits and chain upgrades.
 

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Discussion Starter #15
If your front tires are a different size the tires will turn at a different rate from each other causing your transfer case to act in a locked manner
So what happens when because of failure of the front differential the shaft from the transfer case is removed to run the car as a 2WD? Will this make the transfer case to run "locked"? Is this OK or can damage it?
 

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So what happens when because of failure of the front differential the shaft from the transfer case is removed to run the car as a 2WD? Will this make the transfer case to run "locked"? Is this OK or can damage it?
It shouldn't hurt your transfer case to drive without the front shaft on, but if your carrier or pinion bearings are going out, you should not be driving your jeep around because your front differential is still spinning which is still going to damage your front end whether or not you are transferring power to it through your driveline. The only way your front end could last longer would be if you had hubs that you could unlock and that would in turn disconnect your outer axle shafts from your from the hub and not turn the differential.

Like Draken said, a NP242 transfer case is a GREAT swap for the NP249 if you want the fulltime option and most do. You could also swap in a NP231, which is more stout and has more aftermarket support for it but it may not be practical for the way you use your jeep.
 
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