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Old 10-30-2012, 11:43 AM   #1
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05 Auto Transmission OD

I like how my Jeep drives with the OD off.

My question is if the OD is off the trans running on 3 gears like the older models? Since the 4th is suppose to be OD?

I'm not educated on how transmissions work

Thanks!

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Old 10-30-2012, 12:30 PM   #2
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Originally Posted by ninjaturtle0 View Post
My question is if the OD is off the trans running on 3 gears like the older models? Since the 4th is suppose to be OD?
sort of, but sort of not.

the auto transmission is basically a hydraulic pump. there is no direct connection between the engine and the transmission - which is why when you come to a stop the engine doesn't stall like a manual will, without depressing the clutch.

the auto transmission has something called a torque converter...this converts the torque from the engine into torque that can be used by the transmission to propel you. This part is sort of like two fan blades - one fan blade is connected to the engine, one fan blade is connected to the trans. The engine drives one fan blade, and basically pushes the other fan. It's kind of like putting two box fans facing each other, where only one is plugged into power - but the other is driven based on air flow from the first. Well, the same thing is going on here, except we're using oil/trans fluid instead of air.

just like with the two box fans facing each other, there is some loss in speed between the two. basically, the trans does not have to spin at exactly the same RPM as the engine...for this discussion, we'll call it slippage. for example, when you're stopped in drive at the stop light, the engine isn't stalling. when driving, this slippage, or difference in rotation speed generates a lot of heat, which is dissipated by the fluid (so the fluid gets hot). to make the two fans spin at the same speed and reduce heat generation, the trans/torque converter has a "lock-up". this basically forces the fans to spin at very close to the same speed.

so why did I explain all this? and how does it relate to your OD on the 42RLE?

First off, yes, turning the OD off locks out the 4th gear.

Secondly, the other thing it does involves the programming of the trans. with the OD disabled, the transmission forces the torque converter to lock up as soon as possible. It doesn't allow that slippage I mentioned above.

So why do you care? Well, when the torque converter is NOT locked up, you can get an extra ~200-800+ rpms out of your engine. So, for example, when you're climbing a steep hill in 3rd gear, you can be at 2800rpms instead of 2400rpms - and you have a little extra power to get up the hill. When you disable the OD, the transmission will only give you 2400rpms...and you may not have enough power to get up the hill without downshifting into 2nd - which then puts you above 4000rpms with the engine screaming.

The TC slippage basically gives you two or three extra half-gears in between 2nd, 3rd and 4th....depending on the circumstance and trans temperature.

The lack of TC slippage is most annoying below 45mph, when using 2nd and 3rd gears - so when I'm driving below that speed I generally keep the OD engaged.

Of course, being properly geared is a big part of this. You need to throw deep gears at this transmission to get it work properly. 33s need at least 4.88s, 35s need 5.13s or 5.38s.

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Old 10-30-2012, 02:37 PM   #3
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^

Thanks! this clears it up...

I keep it on below 45 MPH but turn the OD off when I need to go above bc I'm still rolling on 3.73 and plan on upgrading later but w/ the 33's my Jeep seems to struggle going 55 mph at 1500 rpm's...

New Jeep just got it in July w/ the 33's already in but still stock axels.

I just had cv shaft and SYE's installed so now I'm saving up to upgrade my 3.73's

Also your right with the OD off I do get a little more RPM and it helps with the hills.

Thanks again!
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