high mileage trans flush...stupid?

**Big6ft6** · 03-03-2008, 03:19 PM

I have a 3/4 ton 89 burb with a th400 trans. It has 209,000 miles on it. I bought it for $500 and know nothing of the maintenance history.

I've heard a lot of rumors about changning high mileage trans fluid.

1) Basically I've heard if it hasn't been consistantly changed throughout its life leave it alone. Others say if the fluid looks clean and smells normal, go ahead and change the fluid.

2) Others say do a "slow change", basically just empty the pan, add a few quarts wait a month do it again etc until you've changed it out a bunch.

3) The last option is to use the "power flush" machines at the jiffy lube type places that filter and replace the same fluid.

The th400 is supposed to be such a strong trans, and it works fine now as far as I can tell, but I'd like to do everything I can to keep it working into the future.

I'm looking for opinions / experiences, should I change it? flush it? or leave it alone!

**Pete95Sierra** · 03-03-2008, 07:10 PM

I bought my truck used and since the fluid wasnt changed regularly, I had problems.... I replaced it when it hadnt in prob forever cuz I didnt know that rumor.... rumor was true and ended up having to pay 2 grand for a trans rebuild :(

**GM_Guy** · 03-03-2008, 09:14 PM

'89 should have a 700R4 OD automatic in it

**PontiAddict** · 03-03-2008, 10:04 PM

Here is the best way to put it...

Failure is going to happen, its a certainty. At that mileage, you're already on bought time.

I am a strong believer in flushing transmissions at whatever mileage, no matter how long it has been simply because its going to happen anyways, and if it doesn't fail from the flush, you just prolonged its life.

I've had personal experience with flushing high mileage transmissions and have not personally had any failures from it, I've only come out with a better, smoother shifting, cooler transmission.

Thing is, if you owned it for most of its life, and you didn't maintain it, in my opinion, you don't deserve something easy like doing it later and expecting everything to be alright. However if you don't know the history, I'd go for it without a second thought.

Get it running, full operating temperature, and park on a level surface. Remove the transmission dipstick and record the level, color and smell. Normal fluid is BRIGHT red and clean, but smell varies. The sign of bad fluid is DARK red fluid, anything other than bright, and smells burnt.

New fluid is better than old fluid for any transmission, new or old.

**Gmcsuvguy** · 03-03-2008, 11:32 PM

3/4 ton will have the TH400,1/2 tons would have the 700r4. I agree with everyone, check the fluid condition, if it is brown, drop the pan and see if there is anything in the pan (sand-like material) if there is anything there your trannys not long for this world. If it's clean-ish flush it, I'm a strong believer in servicing the transmission and flushing it is even better. Burnt fluid used to mean money in my pocket...specialized in transmissions when I worked for GM.

**Big6ft6** · 03-04-2008, 09:26 AM

Thanks guys for all the feedback! This isn't my daily driver, and since I bought it for $500 a new transmisison or a rebuild would cost more than twice or three times the cost of my truck. I bought to be a hunting and hauling truck. Maybe one trip a year pulling a flat-bed car hauler...otherwise helping friends move and haulin' a deer out of the woods (if I'm lucky).

Once I get the fuel lines and tank back up under the truck I'll driver her around and then drop the trans pan...I might just have to snap a photo and post here for everyone's opinion..I'd be shocked if there was any red color at all judging from the conditino of the rest of the truck. It will most likely be brown

**PontiAddict** · 03-04-2008, 02:24 PM

Well, as mentioned, brown really isn't going to be a horrible thing. Its not good, and signals that no one has changed it in a while, but as long as there are no metal shavings or other granules of crud in that pan, you should be safe for a flush.

Remember when you drop it to get enough fluid to fill it back up. Seems simple, but you'd be surprised how many people I've seen forget that.

You can also change the filter then if you'd like with the 1/2 new fluid and see how it responds.

Since it looks like you're going to be using this for SOME heavy duty stuff, new fluid in there seems more important than ever.

**collinsperformance** · 03-04-2008, 05:16 PM

one other way if you are worried is to cut in an aftermarket tranny fluid filter ( i highly recomend these and over sized coolers) thenyou can add a new filter and 1-3 quarts for the new cooler and and run a week or two then replace the filter and drop the pan and inside filter and maybe even drin the converter and start fresh............mike