Thank you for the detailed info!

It's interesting that you were seeing such high temps even with the stock bumper. I do have a new radiator/cap installed but maybe I should look into replacing the thermostat? I'll track the engine temps on the way up the mountain on Friday and that should shed some additional light on my situation.

Either way, it sounds like adding an aux transmission cooler is a no-brainer but it would be nice to isolate/fix any other issues that might be contributing to my high temps.

Probably the trans cooler made the most difference, but it's hard to really say. Get both!

My trans cooler is plumbed in-line with the OEM radiator trans cooler, for what it's worth.

Its all about keeping the torque converter locked as much as possible. It will only lock in the top 'available' gear (generally, though sometimes it will lock 3rd with OD on in the right conditions). A slipping torque converter makes a lot of heat to reject in the cooler.

OD on will only lock the converter starting at around 50mph (stock gears/tires) and quickly unlock with throttle application. OD off will lock the converter at 45mph reliably and keep it locked under much higher loads. That's the difference. It will never lock in 1st or 2nd on A340 trans.

-Charlie

Thank you again for the detailed explanation. Can you tell me what is meant by a "slipping" torque converter? Are you saying that even if the transmission is staying in 4th, the torque converter may still not be fully locked causing it to generate more heat? My apologies if its a silly question but I don't know much about automatic transmissions.

Haha! Yeah I probably will end up doing both but adding holes to the bumper is going to be messy since the bedliner coating was already applied. I asked my wielder friend who built the bumper for me and he says he can add holes behind the decorative slots in the front plate but he said it may not be pretty so I was hoping I might be able to avoid going that route by installing an aux trans cooler. But there's still a lot of radiator and AC condenser down there as well that could also use more airflow so I'll probably do it.

Yes, that's exactly it. Pay attention while accelerating on to the freeway - you'll actually feel/see 4 'shifts' on a 4-speed transmission. The last 'shift' you feel is the torque converter locking, and acts like a '5th' gear. At that point, applying more or less throttle (to a point) will keep the engine RPM locked to road speed. When the torque converter is unlocked, engine speed can/will go up and down related to throttle, not just road speed.

And unlocked (slipping) torque converter is how the engine stays running while stopped in 1st gear, how gear changes are smoothed out, etc. The slipping is good for smoothness/control, but bad for efficiency (most modern high-gear-count auto transmissions lock in every gear but 1st).

-Charlie

Oh wow, that is super good to know, thank you again! I'll definitely keep the OD off on my next trip up the mountain on Friday. It'll be a loud trip with the 4.88 re-gear and my already-embarrassing gas mileage will likely suffer but it'll be worth it if I can keep those transmission temps in a more reasonable range.

I'm not sure if you mentioned your engine coolant temp earlier, but it's part of the puzzle and well worth popping to 3 bolts on the water inlet and making sure the jiggle valve is at the bottom of the TStat. I know it gets argued about a lot and doesn't seem to have much effect for some (I take their word for it), but I dropped 12 degrees (coolant) immediately and for evermore.....

Attached Images

 

Excellent explanation @phattyduck .

Listen and feel for the lock/unlock.

With the torque converter "unlocked", power transmission occurs only by fluid coupling. Pretty neat how it works really. It also allows for "torque multiplication."

With it "locked" it behaves much like a clutch in a manual transmission. It's a physical, direct, and very efficient method of transmitting torque when underway in certain conditions.

If you ever have an automatic transmission vehicle buck violently when coming to a stop, the converter is failing to unlock appropriately.

These are the cuts I made. I made the slot first, and later added round holes where the red circles are. I'm sure there's a lot of stagnant air behind the bumper's bash plate at road speeds but at least the fan can pull air through.

The OD off stuff phattyduck is talking about will also make a HUGE difference on grades and in traffic.

Attached Images

 

Bypassed cooler in radiator using the B&M 70264, use OE skid plate I cut 6 1-1/4" holes in leading edge
As others have mentioned make sure thermostat is clocked correctly.

I always try to drive at a speed to get lockup if I can. With 4.10’s lockup is 48mph in 4th or around 41mph in 3rd.

I’m partial to the TrueCool 4589 for bypassing the Rad. But it will require relocating the horn.


Sent from my iPhone using Tapatalk

3rd with the torque converter locked is only a few hundred RPM higher than 4th unlocked - it won't be much different in sound. Obviously on the flats and mild hills, leave OD on...

Under load, a little more RPM is almost always a good thing - the water pump and fans spin faster (more cooling), the engine torque requirement is lower (hp = RPM x torque) which is better for engine, and it is better for transmission heat (no or less torque converter losses). 3rd locked will be *better* for mileage than OD unlocked - along with being better for the rest of the drivetrain.

For the same set of reasons, 4LO is better when going slow offroad.

-Charlie

Oh dang, sounds like OD off is pretty much good for everything when traveling a consistent incline. Any idea if ECT does any good or harm in that situation? Since I re-geared to 4.88, I've been thinking of leaving it on all the time since it feels a little better when accelerating from a stopped position (it feels like it jumps into 2nd gear a little too quickly and ECT seems to help a bit with that).

BTW, thank you again for all you super-informative answers! Without helpful people like you and the other posters on this forum, I'd probably be dealing with a busted transmission in the near future.

I was actually thinking of cutting holes in my front skid plate and the bumper itself to increase air flow, but I'm leaving that as a last resort until I see how well the Koyorad aluminum radiator performs in adverse conditions.