Overfilled or Underfilled Transmission Fluid Causing Poor Shifting?

[zer0Limitz]

Hey,

So I have a 97 2dr Yukon GT with pushbutton 4x4. My truck sometimes doesn't seem to downshift very well. If im going 50 and I let go of the gas it seems the truck slows down and doesn't downshift right. Well my question is I have heard having too much transmission fluid in your truck or to little can cause the transmission not to shift properly? Is that true or false?

 

[Big Buttons]

true

 

[OR VietVet]

It is true but you may also have a warped valve body that the filter bolts up to and maybe a cracked sleeve on one of the valves. I am not a trans specialist by any means but have heard the same scenario when these things were taking place.

 

[SunlitComet]

when slowing down you may not down shift and notice it easily. The down shift waits until you are close near base idle in that gear before dropping down to the next. The only way it will do it at a higher idle is if you are doing say 70mph and you shift into 1st. That will cycle thru the gears going down into the next one at the safest rpm the next one can deal with without over rev.

Low fluid can cause lack of pressure which can wreck all kinds of havoc and overheating.

 

[zer0Limitz]

I have been wanting to change/flush the transmission fluid in my truck for awhile. Ive gone to a few places but they turn me down cuz I has over 100k or 150k miles. So im thinking I should just do it myself but ive heard changing the tranny fluid can make the transmission fail or get f'ed up? I figured it would be better for it?

 

[SunlitComet]

If you have reached that much mileage it is better left alone at this point. Was is the condition of the fluid tho?

 

[OR VietVet]

There you go, try that but like the shops said, "If you have too many miles on the trans fluid in there you may be opening up a can of worms but that typically happens with a full fluid flush.

 

[zer0Limitz]

Well over those 218k miles on my truck the tranny has been rebuilt twice. I think the fluid is a little over filled cuz I remember I put a bottle of Lucas treatment in it. I dunno what to do. Why with so many miles and doing a flush cause issues? (what kind of issues?)

 

[OR VietVet]

The total miles is not so much a problem as a lot of miles on the fluid. I am guessing the existing fluid is as old as the last trans rebuild. You did not say how long ago that has been. The clutches in the trans are in danger of coming apart if gobs of miles, over 100k, gets on them without the fluid and filter change in that time period. The trans fluid is high in detergents and if you have old fluid in there for so long the new fluid, when it is all changed during a flush, can be such a shock to the clutches that they come apart. So, knowing that, can you supply some detail as to how old the fluid is in that trans? When was it last serviced? Are you saying the last rebuild was 100k or 150k ago and that is the last time any fluid has been addressed in the trans besides a simple top off? Specific details would help.

 

[mattluttrell]

Yes an overfill and underfill can cause problems. Yes you should change the fluid. The manual says every 50K if its a taxi or in extreme heat. 100K if normal wear.

You can not change it if you like rebuilding the transmission.