Will operating without overdrive regularly to compensate for gearing harm my transmission long term?

[DaveC]

First off, my apologies for 'another re-gearing thread', but while I've read a number of threads on the topic and taken in a ton of great info and perspective on what would be the ideal choice for my TJ if I were to re-gear, my question is, do I really need to in order to prevent unnecessary wear on the transmission and what could happen if I don't?

My TJ has the 4.0, with the 42RLE transmission and 3.73 gearing. The Jeep is a weekend fun vehicle; driven 75% of the time on the road and the rest in mild to moderate off-roading. I may consider lockers at some point in the future, but want to keep it 'stock-ish' - simple and capable. I'm on 31s and won't ever go larger; maybe 32's simply to fill out the wheel wells a bit more when the current tires are ready to be replaced, but that's it.

I keep the overdrive disengaged virtually 100% of the time (thanks to all those who suggested this little trick), and I'm fine with the acceleration as it is now...after all, it's a Jeep, not a sports car. So my question is, will continuing to follow the practice of operating without overdrive do any harm to the transmission in the long run?

 

[Steel City 06]

No it shouldn’t. In fact, it could even protect it.

The vast majority of heat in automatic transmissions comes from the torque converter when it is unlocked. The torque converter locks up at high speeds and low throttles to reduce generated heat and improve fuel economy.

Driving in OD with poor gearing will cause the torque converter to unlock often, resulting in reduced fuel economy and increased heat in the transmission. How much of an effect this has depends on the actual loads and speeds seen by the transmission.

Turning OD off will cause the torque converter to remain locked at a much wider range of throttle, thus reducing heat generated and potentially increasing fuel economy.

In addition, the higher transmission speeds will pump the fluid through the transmission coolers faster, increasing the overall rate of heat dissipation. The net effect is that the transmission runs cooler with OD off under moderate loads.

Some owners manuals for certain 4-6 speed automatic vehicles (Ford E-series comes to mind) actually recommend turning OD off while towing for that reason. Towing with OD on could cause the transmission to overheat and fail.

This said, you will still want to regear eventually regardless. Acceleration from a standstill will require significantly more torque converter slippage than with proper gearing, and puts additional stress on the driveline. Gearing acts as a lever. Deeper gearing gives the transmission mechanical advantage versus the road, but tall gearing gives the road more mechanical advantage versus the transmission.

So while it doesn’t hurt to turn OD off, the effect of reduced mechanical advantage during acceleration will likely shorten the life of the transmission.

 

[DaveC]

No it shouldn’t. In fact, it could even protect it.

The vast majority of heat in automatic transmissions comes from the torque converter when it is unlocked. The torque converter locks up at high speeds and low throttles to reduce generated heat and improve fuel economy.

Driving in OD with poor gearing will cause the torque converter to unlock often, resulting in reduced fuel economy and increased heat in the transmission. How much of an effect this has depends on the actual loads and speeds seen by the transmission.

Turning OD off will cause the torque converter to remain locked at a much wider range of throttle, thus reducing heat generated and potentially increasing fuel economy.

In addition, the higher transmission speeds will pump the fluid through the transmission coolers faster, increasing the overall rate of heat dissipation. The net effect is that the transmission runs cooler with OD off under moderate loads.

Some owners manuals for certain 4-6 speed automatic vehicles (Ford E-series comes to mind) actually recommend turning OD off while towing for that reason. Towing with OD on could cause the transmission to overheat and fail.

This said, you will still want to regear eventually regardless. Acceleration from a standstill will require significantly more torque converter slippage than with proper gearing, and puts additional stress on the driveline. Gearing acts as a lever. Deeper gearing gives the transmission mechanical advantage versus the road, but tall gearing gives the road more mechanical advantage versus the transmission.

So while it doesn’t hurt to turn OD off, the effect of reduced mechanical advantage during acceleration will likely shorten the life of the transmission.

Great info! I've towed for as long as I've been driving and have always turned off overdrive for the climbs. I always understood the concept behind getting better pulling power but never knew the other aspects, although I've definitely seen the mileage improve with the OD off.

Again, good stuff and thanks!

 

[Brianj5600]

I drove my 2004 with 35s, and 42rle for about a year with 4.10s. Under heavy load it shifted from OD to 3rd simultaneously unlocking the converter. As load dropped the converter locked back up. It seems like it unlocked and shifted to OD then locked again.

I think I liked the 4.10s better than the 5.38s on the interstate at 70 mph. OD was useless with 4.10s, but 3rd gear was 2700 rpm vs 2500 rpm in OD with 5.38s. The 4.10s at 2700 rpm had no trouble on the hills around middle Tennessee. The 5.38s struggle to maintain speed.

I don't see any issues with not using OD. The converter locks up in 3rd so no heat worries. I never felt under geared offroad with a 4:1 tcase, but bot sure with a 2.72:1. I also liked the 4.10s better than the 5.38s in 4 low.

 

[Steel City 06]

I drove my 2004 with 35s, and 42rle for about a year with 4.10s. Under heavy load it shifted from OD to 3rd simultaneously unlocking the converter. As load dropped the converter locked back up. It seems like it unlocked and shifted to OD then locked again.

I think I liked the 4.10s better than the 5.38s on the interstate at 70 mph. OD was useless with 4.10s, but 3rd gear was 2700 rpm vs 2500 rpm in OD with 5.38s. The 4.10s at 2700 rpm had no trouble on the hills around middle Tennessee. The 5.38s struggle to maintain speed.

I don't see any issues with not using OD. The converter locks up in 3rd so no heat worries. I never felt under geared offroad with a 4:1 tcase, but bot sure with a 2.72:1. I also liked the 4.10s better than the 5.38s in 4 low.

Have you considered 5.89?
Not sure it’s worth the cost, but it is available

 

[Brianj5600]

I ordered everything for a stroker 3 days ago. I think it will take care of it.