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Follow up Transmission Question


oldjohnt

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A while back I posted a question regarding auto tranny (GM 4L80E Auto OD) heating when driving in hilly country with way too many 3rd to 4 OD shifts taking place (just keep it in 3rd now if a lot of hills). Next question, in mountain country on long downgrades I shift my tranny down to 3 or 2 or even 1 for the good engine braking it provides DOES THAT HEAT UP THE TRANNY?? I really like it to save on brakes and wouldn't feel as comfortable without using it. I have since had it professionally power flushed and re filled with full synthetic fluid even though the old was neither burned or discolored or had any burned smell. It has an aux tranny cooler and I have room to add yet one more if needed and plan to add a temp gauge also.

 

Thanks yall

 

John T

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The shifts will cause a bit of heat from the clutches releasing and grabbing but as long as you don't shift often that won't be an issue.

 

Being in gear, regardless of which one isn't a big heat source, you might see a tiny bit more heat in one gear over another due to gear friction and the power path but it is going to be tiny.

 

Where you will see a LOT of heat is if the gear you pick causes your torque converter to unlock, the lock/unlock cycle creates heat similar to shifting but the converter when unlocked is a major heat generator and can push the transmission (even with good cooling) out of the safe zone in a minute or so.

 

A temp gauge on the transmission outlet is the gold standard for knowing when things are heating up which then allows you to take action to stop the heat source from cooking things. A gauge in the pan or on the inlet is too slow to react to be much more than a "yep, you toasted it" indicator. Some different situations:

 

Shift, you may see a small temp spike. The more power applied when you shift the bigger the spike.

 

Shift and cause the converter to unlock, you'll see the spike from the shift followed by a steady rise in temp as the converter heats up. Again the more power the faster the heating.

 

Don't shift but let the engine lug to the point the converter unlocks and you'll see the rise as above. Best bet is to downshift quickly to stop the heat then decide on a new road speed, either comfortable in your new gear or enough faster than your original speed that the converter will stay locked.

 

Bottom line pick a gear and speed to keep the temp under control. If you can't pull off and idle the engine in neutral to let things cool. Idling in gear cause heat from the converter so don't do that.

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X2on Stan post about unlocked torque converter. You need a lot of air and lot of surface to cool that puppy down.

 

Too cold is also bad. Run it through the cooler, then if you have the radiator heat exchanged through that. That way, if your over cooled, you just pick up some engine heat.

 

I've used these in the past. - http://www.haydenauto.com/upload/HaydenAuto/Documents/Cat_Hayden/2007-hayden-trans-oil-coolers.pdf

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Going to your external cooler first is good, it sheds a lot of heat there before it gets to your radiator that may well be stressing from engine heat already and as Bill mentioned it warms the fluid back up a bit on really cold days which can make for smoother shifts although with synthetic that is less of an issue than with dyno based stuff.

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To answer the other part of your question, it's unlikely you'll overheat the transmission while using it for downhill braking. You will generate heat from the torque converter slippage, but the engine braking power is substantially less than what is produced during heavy throttle operation, so there's little or no chance of generating enough heat to harm the transmission.

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THANKS ALL, and Bill B I saved that Hayden info, great reference material. And they have tranny temp gauges available.

 

One thing I'm unsure of is if my GM 4L80E, which has an Automatic Overdrive, accomplishes that higher "gear" apparent feel by actually shifting into a higher mechanical gear or by locking up the torque converter??? It feels like an actual higher gear ratio shift versus a torque converter lock but what do I know grrrrrrrrrrrrrrr.

 

I do know in hilly country with lots of up n down up n down I'm going to just put in in 3 so it stops shifting up into OD and then back down to 3rd all the time. THATS WHAT HEATS IT UP IT !!!!!!!!!!!!!!

 

Based on what was said, I think I will continue to use lower gears for engine braking down long steep downgrades as that reallyyyyyyyyyyy helps hold me back

 

 

Thanks again, reason for asking is I'm headed West in the near future, lots of Colorado and Utah mountain driving.

 

John T

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