# Transmission Questions



## seville009 (Sep 9, 2002)

I'm no mechanic, but have gained alot of knowledge about my truck's (1999 Expedition) transmissions/axles/etc from this site and from the process of putting a plow on.

Some questions that I have about automatics:

(a) I understand that heat is bad for the transmission, but when a transmission "burns out", what, exactly, is breaking in the tranmission? Is it the mechanical things that are actually moving the pieces around? What exactly is the "weak link" in the transmission.


(b) Why would shifting from reverse to drive frequently cause the transmission to fail? I understand about not shifting while it is motion, as the gears/teeth would be "slammed". I had first hand experience with this back in the 1970's when I got stuck in a snowbank with a 1976 Plymouth Grand Fury and ruined the transmission trying to get out..


(c) I had the Ford dealer change the transmission fluid about a year ago - first time in this truck; was at about 38,000 miles. After they did that, it won't shift into overdrive until the truck has warmed up for about 15 minutes. I first noticed this when, after having the fluid changed, my mileage dropped almost 1 mph (from about 12.6 to 11.6 per the little computer that's in the truck). That was because I'd being doing 65mph on the highway and the truck wouldn;'t shift into OD (even if I kicked the accelerator or turned to OD on and off). The problem went away during the summer, but is coming back again now that it's getting colder out. My guess is that the dealer put too heavy of fluid in there. I have not had any other problems with the transmission, so I don't think that they did anything to the tranmission itself, just that the fluid is too thick. Dealer recommendations aside, what is better for the transmission fluid for plowing - thick or thin fluid?


I ask because if I have a better understanding of the mechanics of how it works, I'll do a better job of keeping it in good shape in terms of my driving habits.

Thanks.


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## wyldman (Jan 18, 2001)

An auto trans is very sensitive to temperature.As the temps rise,the life of the trans is shortened.When a trans "burns out" it's usually a clutch pack or band that wears out it's friction material,or is incorrectly applied,and begins to slip.When this happens,it wears quickly and creates even more heat.Eventually it fails,and the vehicle will lose a gear or not even move.The excessive heat and debris created can also damage a lot of other components.When the trans is rebuilt,it's usually just wear parts that are replaces.Most of the other mechanicals are reused unless damaged.

Shifting to fast from drive to reverse to fast,or continuously can damage the trans,as it's slipping the clutches a little bit every time it shifts.The clutches have to slip,or it would slam into gear,and shift very harsh.

There are actually no gears that mover,or shift in an auto trans.They use a planetary gearset,which will give you different gear ratios depending on what part of it you drive or hold.The clutches and bands are applied to drive or hold the certain parts of the planetary gearset.

With your truck,it has a temp sensor which should prevent OD and lockup until the fluid has reached a certain temperature.If the fluid isn't getting up to temperature,or there is a problem with the sensor,you may not get OD.I would take it back to them and explain what happened,and maybe they can fix it for you.There is really only one viscosity of trans fluid,so I doubt is they could have actually put fluid in that was too thick.There may also be an updated strategy,or reflash for the PCM,which may cure your problem.Ask them when they have it in to check the trans.


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## seville009 (Sep 9, 2002)

Thanks Chris...


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