Torque converter lock up

**hotrodfeguy** · 10-16-2019

But also if you unlock it. Any jolts to power-train. IE wheel hop and bumps(think of when you are jerked around by the 20k trailer your towing) when loaded down going up grade. That jerking and loading and unloading of the tires makes a shock to the drive train witch breaks things. Like transfer cases u joints ect. Something has to give someplace in that equation. It tends to find the weakest link. Unlocked the fluid will slip(be the cushion) instead of a clutch/drive-train in 5/6 or at the convertor lock up clutch. Granted you may loose MPG with it unlocked. Witch lets be honest is why the lock up convertor was created. To help with EPA and average MGP across all cars/trucks sold from the manufacturer. With that slip in the fluid creates heat(this is the cost). As the fluid is the only connection between engine and your trans input shaft. Now I would think with a extra cooler you could keep heat in check. I have always felt the lock up creates the 5/6 slip/wear of the clutch pack. If you look back at pre lock ups. Like a C-6 / TH 400 / 727 power glide They took insane beating and kept going. But back then MPG was not on the top of the list of priority. I just wish a tuner could come up with this. As it is not a emission part . But a drive train "part/fix"

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**Aj Grubb** · 10-17-2019

Locking the converter is not causing increased wear and tear on the u-joints or transfer case. It is also not the cause of the failure prone 5/6 gears in the 68RFE.

All the old bulletproof transmissions you mentioned were never subjected to the loads or power delivery challenges present in a heavy duty truck with a 6.7L Cummins making at least 800ftlbs of torque at extremely low RPM while moving 15 tons. Also none of them had overdrive, let alone two overdrive gears like the 68RFE.

You rarely see burnt clutches for any of the gears in the 68RFE other than the overdrive gears, the overdrive gears have smaller clutch packs than the lower gears.

Also much of the burnt clutch issues seen in the 68RFE are related to valve body issues, particularly cross leaks which allows there to be hydraulic pressure where there shouldn't be. This allows the overdrive clutches to partially apply when they should not be, which causes them to drag, overheat and wear out quickly.

The lock up converter is your best friend and the best friend of this transmission. It keeps fluid temps down and puts power to the wheels instead of wastefully converting it to heat. The heavier you are loaded the more often you want the converter locked up. That is why it gets locked up earlier and more often in tow haul mode. The HD Allison Transmissions in the meduim duty trucks keep the converter locked pretty much constantly. If you drive one across the parking lot you will feel the converter lock. That is what you really want. The reason our trucks keep it unlocked so much at low speeds is mostly for driver comfort, most new truck buyers don't even want to be able to tell that the transmission is shifting. A very smooth driving experience is nice but there is a price to pay for that when moving heavy loads.

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**hotrodfeguy** · 10-25-2019

Ever wonder why the manual trans doesn't carry the same load rating? Ever wonder why when rebuilding a manual trans engine vs a auto the thrust bearing on the main cap has more wear on the manual trans? 800TQ on one end locked via drive train/ground. Thats why guys buy ladder bars and such. Wheel hop via bumps while loaded at 20k is hard on stuff. I myself will take the unlocked convertor. Allowing the fluid to take the jolting/shocking.

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**MC250r719** · 10-25-2019

Originally Posted by hotrodfeguy

Ever wonder why the manual trans doesn't carry the same load rating? Ever wonder why when rebuilding a manual trans engine vs a auto the thrust bearing on the main cap has more wear on the manual trans? 800TQ on one end locked via drive train/ground. Thats why guys buy ladder bars and such. Wheel hop via bumps while loaded at 20k is hard on stuff. I myself will take the unlocked convertor. Allowing the fluid to take the jolting/shocking.

The clutch is the weak point and will slip with any power increase (good job FCA

), of course the thrust bearings are going to wear different when you compare 660lbft of torque to 930lbft of torque (specs from 2018 ram 3500). I have seen one or two guys who tow heavy use traction bars but most who have them either drag race or do truck/tractor pulls. It is a good idea to have them to protect from axle wrap.

As far as the unlocked TC, I would have to imagine you will burn up your trans pretty quick towing 20k.

I'd take a broken u-joint any day of the week over a burnt up transmission.

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**hotrodfeguy** · 10-26-2019

Well I have not had many cummins blocks come through my machine shop, to know the thrust wear. Between manual trans and auto. But have done many older gas. Where I can do apples to apples. On the thrust wear. Yes thats the point of traction bars. What I am trying to say is with large loads and bumps the wheel "hops" and puts a jolt on the drivetrain the same way as a wheel hop. The unlocked convertor would cushion that chatter. Yes your fluid temp would be higher. But like I said if you ran an extra cooler when unlocked I think you would be ok.

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