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Automatic OD Tranny Question


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Mornin gang, While driving near the rolling foothills of the Smokey Mountains the other day with a lot of ups and downs (no long steep hard pulls, just a lot of up and down with tons of OD in/out shifting) my Chevy 4L80E Automatic Overdrive Tranny acted up for a period. It as usual shifted down (out of Overdrive) up the hills, but when starting down and coasting IT DIDNT SHIFT BACK UP TO OD AS IT NORMALLY DID??? I think its somewhat temp related as it ONLY occurred in the up and down (lots of OD to 3rd mode) but it wasnt very hot and the engine temp never exceeded 190. We stopped to get gas and after that it started acting normally and shifted into OD and stayed there unless starting up a hill when it would again downshift but as soon as we crested the hill, back to OD again ALL WAS FINE. YES IT DOES HAVE A HUGE FRONT AUXILIARY TRANNY COOLER only 45K miles has always shifted smooth and perfect and still does, except for OD yesterday.

 

MY PLAN when I get home is have my mechanic buddy use his power tranny machine to completely flush and drain (I'm sure it gets torque converter fluid also) and refill with SYNTHETIC TRANNY FLUID and if it has a pan bottom screen filter to, of course, change that also. Maybe SYNTHETIC fluid plus a filter (if it has one) will cure it??? I may add some Lucas Tranny Treatment while at it.

 

Your thoughts, opinions and experience please

 

Ol John T live from Jekyll Island Georgia

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It sounds like the transmission overheated because of the excessive shifting in and out of overdrive. Changing the fluid is probably a good idea, and synthetic fluid is more tolerant of high temperatures. But I agree with Stanley in that the best thing to do in cases like that is to simply lock out overdrive until you get back onto flatter ground to avoid the excessive heating.

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Most of the modern automatic transmissions will do this when the transmission temperature exceeds a certain number. The engineers do it so the transmission will not self destruct.

 

I have found the Edge CTS to be very helpful in knowing what the engine/transmission are doing.

 

Now, they have the Edge CTS 2.

 

AttitudeCTS2_1.png

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Thanks yall for the inputs. I think if I power flush and re fill with synthetic fluid (and filter if so equipped) it wont hurt and maybe improve any rising temp problems. Normally it doesn't set and hunt and shift over and over and over but in those particular hills I was in (Tennessee and North Carolina near Smokies) it did indeed shift more from 3rd to OD and back to 3rd then usual, so I think I will take Stanleys advice if I get in that situation again and just keep it in 3rd to avoid all that shifting into OD.

 

I knew I would get some good info here, thanks again yall

 

John T

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I thought I posted this yesterday but don't see it??

 

Thanks gang, I think I will go ahead and power flush and re fill with Synthetic Tranny Fluid (and filter if so equipped) and take Stanley's good advice and just put it in 3rd to avoid OD if theres a bunch of constant short up and down hills where it shifts so much from 3rd to OD. Other then short up and down for miles and miles as I encountered the other day, it never over hunts or shifts excessively. Its clean tight and dry under the tranny, has a huge aux cooler, fluid is clean and red with no burned smell or color, and always works fine beside the other day until we stopped for gas, so I want to take good care of it.

 

Safe travels

 

John T

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The fact that the fluid is still clean and red is a good sign. It doesn't sound like you overheated it badly and likely didn't cause any permanent damage. I got one so hot on a long steep grade once that it spit a bunch of fluid out of the front seal. Messy!!

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Do some research about the synthetic transmission fluid. In an aftermarket heavily built transmission- Garmons Transmissions in Georgia- the synthetic fluid is actually too slick to let the clutches (steels and friction rings) lock up properly creating more heat than necessary. In the Dodge I had, with a 48re, (I know it was a pos until the rebuild) there was a big difference in the tightness of the shifting when using regular hydrulic fluid or the synthetic version.

Another idea, when towing in the mountains or any area where the trans wants to "Hunt", take it out of overdrive. Every time the trans shifts up and down, the torque converter unlocks, trans shifts, torque converter relocks. This happens in the split second of the shift, but creates mucho heat in the process.

When your mechanic does the flush, the torque converter flushes with the rest of the transmission fluid. It's all one big circuit.

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A thought on transmission temp, there are two main points you can check the fluid temp, in the pan and as it is leaving the transmission. The pan temp is very comforting as the fluid there has gone through the cooler and there is enough of it to mask any temp spikes from shifting. The output temp is a lot less comforting as you'll see spikes in the temp at every shift, you'll also see a fast rise if something is slipping so I really prefer that location.

 

The trans fluid doesn't degrade from heat at the average temp, it does at the peak temp so that is another point in favor of monitoring at the transmission outlet.

 

Another big plus for outlet temp monitoring in my opinion is that you'll see a fast rise in temp if you are pulling hard and the torque converter unlocks. Our Ford did that to us a lot, we would hit a hill, drop to 3rd and floor it to build boost and keep our speed up. At some point the computer would decide our RPMs were too low for the power we were making and unlock the converter to let the RPMs climb a bit. Great for making power but it made huge amounts of trans heat fast. I'd try to pay attention to the speedometer and tach so I'd be aware of the change but sometimes I'd miss it. Having the temp where the wife could keep an eye on it never failed, she would say we are getting hot and I'd drop to 2nd where the converter would lock back up and the trans would start cooling off again.

 

Of course you can have two sensors and either two gauges or a dual needle gauge which is something I'd consider next time.

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Alie and Jim, Thanks for that information.

 

I was under the impression Synthetic fluid held up and performed better then other oil at higher temps. This will be easy for me to determine, since I know I had an issue with regular oil I plan to change over to full Synthetic and then see what happens. If it performs worse, I will change back to regular oil, if it perfroms better I will continue its use, piece of cake and then I will know for sure. HOWEVER the minor problem I encountered will likely go away if when I'm in constant up and down up n down hills I simply don't use OD.

 

John T

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