[S10 Racer]

I've heard this from more than one tranny mechanic, is there any truth that changing your fluid on a high mileage tranny will cause it to start slipping or leaking??

[SSedan64]

It can cause problems, a Flush & Fill usually does cause problems.
New/Fresh fluid will have fresh Detergents that can dislodge sludge/sediment that can get into places it shouldn't be.
If you do anything a Filter & Fluid that's required to refill is all I'd do.
Don't use Synthetic Fluid in a High Miler or Trans that the Internals are questionable either. If the Clutches/Bands have any Glazing, Syn will cause slippage and leaks past old Seals.

[malc]

Here in Spain I had no option but to use synthetic fluid in my TH700,
for some reason no other type is available any more.
So far, touch wood and whistle, nothing bad has happened.

[malc]

Dual post due to web gremlins.............

[joshp83]

id do he filter but ive seen a lot of negitives from flushing

[SSedan64]

Originally Posted by malc

Here in Spain I had no option but to use synthetic fluid in my TH700,
for some reason no other type is available any more.
So far, touch wood and whistle, nothing bad has happened.

Wasn't Spain one of the first to go thru the massive GREEN mistake. May be why you have no Mineral base fluids.

[kc0kfg]

Originally Posted by S10 Racer

I've heard this from more than one tranny mechanic, is there any truth that changing your fluid on a high mileage tranny will cause it to start slipping or leaking??

I trust my trans guy and here is how he tells it, if you stay on top of filters and fluid changes you will extend the life of a auto trans, period.

This old story of them failing after a change often stems from when the fluid and filter was changed.

Often people don't do it until they notice the trans slipping or otherwise not working right, they flush/filter and fill and the trans fails soon after so it must have been because of the new fluid and filter, when it was going to fail with or without the fluid /filter change.

Most likely if its slipping or acting odd its not going to be fixed with new fluid.

[66SSImp454]

I would rather do a flush than a filter change on any transmission. I have had more issues after a filter change than a fluid flush. Plus the filter is nothing more than a screen.

The transmission would have to be pretty bad in order to create slipping in either situation. If the fluid it burnt, good chance the trans is or will soon be.

[bentwings]

In the auto shop days we changed trans fluid on many of the used car lot cars. Most were already burned up and we could count on having plenty of trans work. On our regular customers service a routine trans filter and fluid change was not a problem. I guess when the trans failed it was easy to look back and say 'well trans fluid was changed and 2 weeks later the trans failed so changing fluid was not a good idea."

today.. do I change my streetrod trans fluid....not on a bet. haha it's nice and red and has never been over 200 deg other than when it is 99 deg outside. I don't even like to change the oil if it is clean.

[75gmck25]

I have a 4R70w in my Ford Expedition, and it has about 170k miles on it. This is Ford's heavy duty overdrive trans, and I have always used a very large stacked plate external cooler. The first few times I had a dealer flush the transmission (which was recommended by Ford as the right service), and then at about 150k I dropped the pan and replaced the filter. The filter did not appear to be very dirty, despite the mileage.

Flush/fill/change transmission fluid doesn't seem to have any negative effect on my high mileage transmission. However, its been done about every 40k miles, which is different than trying the same thing on a transmission that has been neglected for years.

Bruce

[malc]

As well as the cooler get a remote cannister type filter.

[S10 Racer]

Thanks for all the replies. The reason I asked was my son's Allison 1000 tranny in his duramax was slipping and putting the truck in limp mode. I was going to change the fluid but instead just changed the screw on filter, topped off the fluid and it still slips in 4th & 5th gear. It pulls good in 1st thru 3rd but then slips bad when it shifts to 4th. I think we are just going to buy a Suncoast tranny that can handle the increased HP since he installed a chip.