4Lo to Cool Trans?

[GX470_Prado]

So I am very new to all this so this is most likely a dumb question:

I was watching a video of this group of guys doing a nice Overland Expedition and during one of the clips one of the rigs with an autotrans gets a warning light about the transmission oil temp... One of the more experanced members suggested putting the rig in 4Lo to cool the trans, my questions is that valid and why?

Again I know nothing of offroading but I know if I am towing I either limit what the trans can use to try to avoid it "searching" back and forth or once in the highest gear try to keep a very light foot to keep it from downshifting more often than needed. In short I try to avoid making (or allowing) it to change gears.

 

[tanglefoot]

For off-road, low-speed trail exploration, low-range can certainly help the transmission keep its cool. A auto-transmission equipped vehicle can traverse fairly gnarly terrain with the transfer case in high range and copious amounts of throttle but in the process, the transmission needs to slip quite a bit in order to allow the engine to produce enough torque to move the vehicle over the obstacles and up the climbs. Just like the clutch behind a manual transmission, slipping creates heat.

Shifting said automatic-transmission-equipped vehicle into low-range introduces a gear reduction behind the transmission (usually somewhere around 2.5:1) that makes moving the vehicle over the terrain that ~2.5 times easier than in high-range, so that the transmission needs to slip much less and the engine doesn't need to work as hard. It also helps save the brakes on the descents as the engine braking is multiplied by ~2.5 when in low-range. It's just a good thing all around for challenging terrain.

On-road, unless the desired speed is about 15 mph or less and you have some heavy hauling to do, low-range will not be a benefit, as it's pretty difficult to cover ground very quickly or efficiently with that gear reduction in place.

When towing, I don't think the gear-hunting itself raises transmission temp too much. It's more a matter of the amount of throttle that is being applied and the resulting amount of torque the transmission needs to transmit. Using moderate amounts of throttle on long hill-climbs allows the transmission to stay much cooler than keeping the pedal mashed the entire way to the top. Many auto transmissions are reluctant to shift down, causing loss of speed and gear-hunting. Manually keeping the transmission in a lower gear (a gear that will keep the engine somewhere around 3000 rpm), makes for much more pleasant hill-climbing.

 

[GX470_Prado]

Tanglefoot,

Thanks, that make perfect sense... I should have thought about it that way.

 

[the kid]

its allowing the lower gearing to do the work instead of the transmission making up for the differences or pressure between the motor and the tires...

yes the guy in the video is correct in his idea/suggestion

 

[Martinjmpr]

Having a transmission temp gauge on my 4runner via the Scangauge II has let me see what activities heat up the tranny and which ones cause it to cool down. Running in OD in hilly terrain the temps climb quite fast as the transmission "hunts" for the right gear. I've noticed that just dropping the transmission from D to 4 (on my 5 speed unit) will immediately cause the temp to drop by almost 10 degrees (f.) Also, if I'm climbing a long, steep hill in 4th and the engine speed is low, dropping it down to 3rd will also cause the temp to drop. This is particuarly true when pulling the trailer.

Off-road I am very rarely in D. My thought is that by keeping the transmission in 3 or even 2, I am saving wear and tear on the brakes and also on the transmission, since it's not constantly shifting back and forth between 5 gears.

I probably shift my automatic as much as some people shift their manual transmissions, it's one of the reasons I would never buy a vehicle with a column-mounted automatic transmission shifter. Even on the street, when I pull up to a stop light or a railroad crossing, I'll normally bump the shifter up into N as I don't like to sit at a stop with the transmission in gear and the brake on.