Hello All,
I introduced myself last week in the introductory section. I have completed a modification which defaults the JK with auto trans to Overdrive-Off and also installs an alternate (easier to reach) OD- select switch.

I have stock 32" tires with auto trans and 3.73 rear end. I find driveability is best if OD is kept off with speeds up to 55 MPH. RPM does not exceed 2000 by much, and is really where the 3.8 L six wants to be. With OD on, it will tend to shift out of 3rd around 40-45 mph if driving easy. At 45-50 MPH, a moderate hill or acceleration will cause a downshift from OD to 3rd. I find staying in 3rd up to 55 MPH saves shifts, allows the torque converter to stay locked, gives better response, and does not harm fuel mileage. Now others with different gear ratio, tires, etc. may feel differently.

If anyone is interested, I have posted the pdf file detailing the overdrive off default modification. The file was updated on 3/1/09 to include a protection diode. Even though you think of lighter circuit as "conventional" power, it routes through the TIPM, so the diode is cheap insurance to protect the vehicle computer from surges. This came to mind as I was drawing things out for my fog lamp control ckt mod. Click on the link below:

http://home.comcast.net/~kingslea1/Jeep/ODMod.pdf

 

You would have to host the pdf file somewhere and then link to it from here.

 

That's an interesting mod. I, for one, am looking forward to seeing it. 11 pages, I'm a bit scared.

 

Cruiser1027, no need to be scared. It is 11 pages due to the pics and the diagrams. Just holler if you have any questions. Electrically it is not challenging. The worst part, like any vehicle, is taking the stuff off to get to the wires.

 

Quite a write up...Nice work! I have been thinking about this mod as well.... I would be happy if they would have put the OD button on the shift handle....not sure why they thought we should lean forward and reach around the shifter to get to this button?

 

Redrunner, GREAT JOB!!!! From what I gather, you are using a timed relay to make the contact as if you were doing it manually. Then adding a momentary switch to override the relay if you want to engage overdrive. PLEASE correct me if I'm wrong.

 

Cruiser, you have it, the relay is closing a contact in parallel with the factory OD switch just as if you pressed the switch. The additional switch is just for an easier to reach switch to enable or disable OD. The factory switch will still work also

 

Thank you. Happy wheeling!

 

That's just it.........THEY DIDN'T THINK....!!!

What a crock...!!!

 

Are you sure it does not effect fuel mileage? Has anyone figured out the exact rpms for best mileage?

And that brings me to another question I have been wanting to ask all you engineers out there....

What uses more fuel......to lug (torque) up a hill, or to downshift, (or OD off) to keep rpms up to climb the hill? I realize that if you had a vacuum gauge and could monitor it all the time and compare mileage at certain vacuum then you would know for sure. Has anybody done this with their JK?