So, after reading all these threads on auto's losing gears, over heating, catching fire and the like, I just want to make sure I understand what mine is doing and if it's bad or not.

Specs: I just put on 35's last week and have had a supercharger for the last 7 months.

Up until the 35's, she'd shift into overdrive at about 40-45 and putz around at ~1500 rpm. Real slugish, but I didn't mind as I was never in a big hurry to get anywhere. Now, with the tires, I feel and see it shift at about 45, but it will go from 2200 rpm down to 1700 and then a final small shift to cruising (at 45mph) at about 1250 rpm. Now I know that's real low, but it doesnt bother me and the tranny doesn't go back and forth to 3rd, it's just that middle ground, the 1700 rpm I'm curious about...

I know 3rd is 2200 as that's what it jumps to when I turn O/D off, but what are the 1700 and 1250? Is it slip in the torque converter? I'm not real savvy with automatics so any dumbing down would be great!

Thanks for the help

 

Did you set the correct tire size in computer. Your jeep might still "think" you have the smaller tires and just not shift correctly.

 

Did you re-gear?

 

I did change the tire size and the speedo and odometer are reading perfect.

And no, still got stock 4.10's...

Any idea's on the tranny?

 

That "partway between" RPM is because the torque converter is not locked up completely. It starts to unlock (or doesn't lock up) when the engine can't pull the load with a locked converter. The converter seems to have a variable vane lockup system where the engine can run at several speeds for a given gear and road speed.

It may take re-gearing to get the Jeep to run in 4th gear with a locked converter. The 3.8 doesn't make much power at 1250 RPM. My auto with 32" tires and 3.73 sits at 1600 RPM at 60 MPH. The torque converter unlocks if the Jeep even thinks there might be a hill - otherwise it stays locked up.

The Jeep will get on down the road OK with the converter unlocked, but it consumes more fuel. Also, an unlocked converter is what builds up too much heat in the autobox. Unless you re-gear, you may want to just leave O.D. off when the converter won't lock up.

 

The second drop in rpms that you are getting is when the torque converter clutch locks up. It shifts in OD with the TCC unlocked, then after being in gear and seeing that the load on the tranny isn't too harsh to drop the rest of the rpms, it locks the TCC up. If you are driving along around 45 you can verify this by letting up completely on the gas and then dropping it right to 1/4 throttle, TCC should unlock, rpms rise up and then relock the TCC once everything is checked as far as load vs throttle.

 

im going to guess you are refering to a shift into lock-up by the torque convertor and a shift into overdrive. 4th is another gear so i guess you are clear on that. the torque convertor is what takes the place of a clutch for a stick shift. now since the torque convertor is fluid driven.think of 2 fans faceing each other one spinning under power of the motor the other being driven by the first fan. the second fan doesnt turn quite as fast as the motor driven one because its not directly connected to the motor.in the transmission its fluid not air driveing these fans.they have put a clutch in the torque convetor to mechanicly join the fans to eliminate this power loss. if you have more questions i would be happy to answer them just pm me

 

I don't think the 42RLE has any variable vanes in the torque converter, if that's what you meant.

However, below 80°F fluid temperature, the torque converter has a partial lock schedule, which would allow for different engine rpm's in a given gear, at a given road speed.

 

Thanks a lot for the quick answers!

That is what I thought was happening from my auto fundamentals book written in the 70's lol. Glad to see the basics haven't changed.

And Freewill, that heating of the tranny from running "unlocked" is what I was worried about. Looks like I'll keep the O/D off around town.

Now to expand on this, and I know there are numerous threads on gearing, but leaving O/D off (.69:1 ratio) and getting 5.13's (4.10/5.13 =~ .79 ratio) would be about the same in terms of final drive ratio, correct? Just trying to see if the cost will be worth it down the road.

And last thing, what is the normal running temp for the tranny? I'd like to check it with my Diablo programmer. Thanks!

 

I have been playing around with this in my head a lot. I just installed 35s. I too have 4:10s. When I have overdrive on I am running about 1900 RPMs at 70 MPH. With over drive off I am about 2700 RPMs at 70MPH. I plan to re-gear to 5:13 which should put us around 2380 RPMs.

I live in Illinois and the dang winds are killing me so I go as far as running with over drive off on the interstate but I keep it at 65MPH and that keeps me pretty close to 2500RPMs. If I don't do that she wants to shift down constantly and I can't keep her at 70MPH. The JK feels real good at that 2500 range.

I will be getting my gears within the next couple weeks.