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Old 01-20-2009, 05:51 AM   #1
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Test Drive Question??

HELLO ALL: We have test driven a nice TJ Unlimited. It is an auto with 40 K miles, stock suspension and tires, etc. It drove out very well with one exception:

(???) When it drops into overdrive at 50 mph there appears a low pitched hum/rumble. The noise is obvious, and there is just a slight bit of high pitched vibration to go with it. I wanted to think exhaust resonance, but it really seems to be a bit much for just that. It is not there if you lock out OD and turn additional RPM. It goes away as you accelerate on to about 60 mph. It is only there under power/acceleration.

Can anyone offer their experience, help, or suggestions for further analysis?

THANKS!!! THIS IS A GREAT FORUM, AND HAS ALREADY BEEN A SUPER RESOURCE AS WE LOOK FOR A VEHICLE.

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Old 01-20-2009, 11:11 AM   #2
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hey welcome to the forums , my tj does the same thing as for the high pitch sound could be the tranny is it a zingy sound?

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Old 01-20-2009, 12:45 PM   #3
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Yeah mine does the same exact thing when overdrive kicks on
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Old 01-20-2009, 01:07 PM   #4
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What size tires were on it? Were they slightly bigger than the usual factory size? The reason I ask is that the overdrive ratio in that transmission is pretty aggressive in that it drops the rpms quite a bit and with larger tires, the engine could be lugging a tad at low rpms until you get up to faster highway speeds.
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Old 01-20-2009, 02:15 PM   #5
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Mine has done the same thing since it was new. I'm told that it is "normal".

Welcome to the forum by the way.... Lots of good resources here.
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Old 01-20-2009, 02:29 PM   #6
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That's the newer 4-speed for you. Every single one that I have driven did the same.
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Old 01-20-2009, 04:06 PM   #7
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?? More experience - and thanks!!!

Thanks to all those who responded (what a GREAT FORUM!!!). Looks like I have a consistent answer from a good number of sources. Also, I found another auto LJ for sale today at a dealer, and the wife and I drove it. Although it was a softop and noisier, I could still feel and hear the same thing going on. It (the hum and vibration) was just quite a bit more aggressive in both vehicles than what I would have expected as normal.

I have one more question for those who responded with their experience. It really is an AGGRESSIVE overdrive gear, dropping RPM to a little less than 1500 when it shifts in at 50mph. WILL THE OD PULL BIGGER TIRES WITHOUT BOGGING OR HUNTING? Anyone using larger tires (?size?) with reasonable satisfaction? I know I'll have to change some things to go larger on tires, but I'd really rather avoid an axle ratio change to go along with all the rest that must be done.

THANKS TO ALL... AND TO ANY WHO CAN HELP WITH FURTHER ENLIGHTENMENT!!!
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Old 01-20-2009, 04:12 PM   #8
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I think most of the Unlimited came with 3.73's... so larger tires up to 35" shouldn't be much of an issue. If it's a Rubicon, it has 4.11's and those are nice for larger tires as well.
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Old 01-20-2009, 04:15 PM   #9
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I believe standard LJs come with 3.73 gears, Rubicon LJs come stock with 4.11s. I run 33" tires on my standard LJ with no transmission problems at all. I cruise at 70-75 in overdrive on the freeway all the time at just under 2,000 rpm. The only thing I've really lost with the bigger tires is some of the low end.

I've owned my '04 unlimited (LJ) since it was new and have nothing but great things to say about it. If I were to do it over again, I think I'd go with an '05 Rubicon for the lockers and the gearing.

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