Auto Tranny Experts....Question?

[DEWFPO]

Scenario : Long uphill grade of several miles. Transmission is in 3rd gear with no lockup, engine rpm 2,800. Obviously this is going to build up heat in the trans because of the slippage (w/o lockup).

Two questions about the above scenario :

1.) Is less heat built up in the trans with HIGHER rpm (i.e. less slip in the trans, than with lower rpm going uphill)?

2.) On the flip side of the grade... (car coasting down steep grade, engine rpm 2,800, auto trans in 3rd gear with no lockup.) Isn't the trans still building up heat in the fluid for the same reason as above going down hill.

Thanks for your thoughts. I drive the mountains every day (and always manually shift the auto trans for best performance when on grades, up and down). For some reason yesterday coming down a long, steep, mountain grade I was wondering if the transmission fluid was still building heat on the downslope side of the grades.

DEWFPO

 

[MolaKule]

If you're holding the transmission in third gear up the mountain, then heat is building since you're inputting more horsepower (energy per second) to the transmission. Additionally, the engine is generating more heat and putting that into the coolant, which increases the transmission heat as well, since the transmission is cooled via the heat exchanger in the radiator. The torque convertor (and fluid) in this situation is the component generating the most heat since a torque convertor is a constant slip clutch.

Now on the downslope, engine is producing little heat, the coolant temp will drop, and so will the transmission temps.

The heat killer for an auto transmission comes from constant shifting, especially on the up shifts.

Since you hold the transmission in third gear, I think the transmission will last longer.

 

[mrchecker]

I wouldn't call myself an expert, but at 2800 RPM there's not a heck of a lot of slippage. The torque converter doesn't build much heat above its stall speed because the slippage is pretty low at high speed (around 5%). Remember that automatic transmissions got along fine for 30 years before lockup torque converters came along. Also remember that the direct drive in 3rd produces zero heat compared to the 4th gear overdrive. I agree with Molakule that the thing to worry about is constant up and down shifting, that's what makes heat and wears out transmissions. Keep doin' what you're doin'.

 

[theguru]

You may also consider an aftermarket trans oil cooler. And I agree at the 2800 revs there will be virtualy no slip. Just keep that oil cool.