Draining from the tranny cooler line - safe or no?

[lairdwd]

So I'm reading widely varying opinions on if draining from the transmission cooler return line is a good idea or not. Several transmission specialists have chimed in to say this is defnitely not a good idea, while the majority seem to think it's fine. One guy indicated that the different PSI with the line on and off could damage some components of the transmission.

So what say you? Safe to use or unsafe? My car is a 2000 accord. Here is a video of a guy doing it.

using transmission itself to drain from transmission cooler line

 

[SteveSRT8]

I have been performing this operation for almost a decade on a variety of cars and trucks. Never an issue.

As long as you do not allow the pump to run dry I do not see how anything could be damaged. The trans is simply not moving, only the pump is.

 

[willix]

That's basically how its done when you pay to have it flushed. I did mine GM not to long ago that way although not really recommend to Because of questionable practices with the machine change outfits use may just filter it or use the wrong type fluid.

On my 4T80e trans. Its a dry sump & internal filter cannot be accessed for in car service.

I was told a procedure to pull the line and start the engine & drain till it spits. I would fill a gallon jug before it spitting & I would shut down and ad a gallon.

I repeated this for 3 gallons & 1/2 a qt to get the cold level.
All is good.

 

[Kestas]

LOTS of people on this site have been doing it with no problem.

The return line dumps fluid into the open sump, so I don't see how the pressure can drop significantly. This argument doesn't hold water with me.

Some argue that air bubbles can be a problem, but the parts are still wet with atf lubricant, so a momentary introduction of air doesn't worry me.

 

[onion]

I got into a big argument over the issue on this forum a few years ago. At the time, I contended that the practice was not safe, because the transmission was being run without lube oil. Other people on this site convinced me, though, that this really isn't a big issue- as most of the transmission isn't turning while in park.

Generally speaking, when a vehicle is in park, the input shaft, pump, and half of the forward clutch will be turning. Pump lubrication is a non-issue unless you run the trans. completely dry. So I think the only components that could possibly be damaged using this method are the input shaft and the bushing(s) in the forward clutch. Personally, I think that if you're only running the engine this way for a minute or so, then it's a non-issue... but you ARE taking a chance.

I've done a cooler line flush on a couple of GM transmissions. One never had any issues. The other suffered complete and utter failure (no fluid movement whatsoever from the pump)at 182,000 miles (about 40k after the cooler line flush). I doubt it had anything to do with said flush... but nobody will likely ever know.

 

[unDummy]

Onion, you are not suppose to run it without lube. At no time during a cooler line flush is the transmission without lube.

A cooler line flush is obsolutely harmless if done correctly.

 

[Steve S]

The oil comes from the torque converter that is what the big heat generator is in an auto trans. It will not hurt a thing.
do not run the engine when the oil stops .

 

[Jim Allen]

I too vote for the cooler line fluid change. I also have a 2000 Accord, if you recall, ... and it's got 20K on the last one and still works great. Avoid letting it run out of oil, but even so, a momentary lapse in the process is not a worry. Most of the transmission is not in operation while in park

WAR STORY: I've seen what a blocked cooler line can do, stopping lube flow.. which is what happens when there's no oil return from the cooler line. It took about 20 miles of DRIVING at 45 mph on a 700R4 in a Blazer to kill it. It operated normally but began making strange noises about 2/3s of the way through. Never really did stop operating but the planetary gears were wasted when I tore it down.

 

[onion]

Originally Posted By: unDummy
Onion, you are not suppose to run it without lube. At no time during a cooler line flush is the transmission without lube.

A cooler line flush is obsolutely harmless if done correctly.



Yes, when you do a cooler line flush, the transmission is being run without lube oil. With most transmissions during normal operation, oil flows from the pump through the main circuit where it's used for the clutches and such... then through the torque converter, then through the cooler, then back into the transmission as LUBE. If you disconnect a transmission cooler line and run it into a bucket, then you are running the transmission without proper lube.

Again, I think this is pretty much a non-issue as long as you only run the engine for a minute or so. But this IS running the trans. without proper lube, and this DOES entail at least some (small?) level of risk. To claim otherwise is just incorrect.

 

[Texan4Life]

I just changed out my transmission fluid. I just took my auxiliary trans filter off and had a helper start the engine while I watched. The second the fluid stopped gushing from the filter head I signaled the helper to turn the engine off.

Went from ST dex 3 to M1 synthetic ATF.

 

[lairdwd]

Here is one reply from a mechanic on the youtube link:

Quote:
As a Master ASE Technician, I highly encourage to NOT use this procedure for automatic transmissions. By removing the supply to the cooler, the cooler obviously has no return fluid now. This may be alright in theory for some Honda automatics (due to the return line "dumps" back into the trans pan; it doesn't feed into any circuits), but for transmissions like Hydramatics (GM's 4T60, 4T80, 4L60) it feeds the lubrication circuit for the transmission.

 

[unDummy]

And, while in park at idle with no load, what exactly in the lubrication circuit has an issue? Please, the facts and not internet hearsay.

Again, a cooler line flush is absolutely harmless when done correctly.

 

[mechtech2]

Like kestas said, the return fluid is simply allowed to return to the pan/sump.
If you have your drain/fill system set up right, and are careful, I see no harm.
It is a good way to perform a 'flush'.

 

[rslifkin]

To avoid the issue of lack of lube on transmissions that feed lube circuits from the cooler return, you could always install a y valve setup in the line, and bleed off about 1/2 the flow, letting some still go back to the transmission. Under no load, at idle, in park, this would still be plenty of lube.

 

[tinmanSC]

Originally Posted By: rslifkin
To avoid the issue of lack of lube on transmissions that feed lube circuits from the cooler return, you could always install a y valve setup in the line, and bleed off about 1/2 the flow, letting some still go back to the transmission. Under no load, at idle, in park, this would still be plenty of lube.

But this isn't a flush, or at least not >50% flush. If you are letting 50% of your fluid back into the pan then you are just contaminating your new fluid. You would need to "flush" it like this several times to get, say, 90% new fluid.

 

[calvin1]

Use the biggest OD tubing you can stuff down the dip stick and either siphon (SLOW, especially when cold) or vacuum (I've rigged a ghetto extractor with a shop vac and a 6 gallon gas can as an accumulator). This gets more fluid and avoids the risk of running the transmission with no fluid.

 

[KrisZ]

I don't know about other transmissions, but on my Mazda 3 I drained about 3 qts at a time from a cooler line while the engine was idling and the transmission in PARK. I assume most transmissions will do the same. So how was the transmission running?

 

[cp3]

calvin1, how is sucking fluid out through the tube going to get more fluid? You wouldn't pull any from the converter would you?

 

[Jim Allen]

Originally Posted By: cp3
calvin1, how is sucking fluid out through the tube going to get more fluid? You wouldn't pull any from the converter would you?


+1 A fluid exchange gets ALL the fluid. Or virtually at least.You waste some of the new, though flushing out the old but you can minimize that by draining the sump and refilling it with fresh fluid first. I woulnd't do a fluid suck on a car that had a pan because you may be leaving a lot of junk on a magnet below, not to mention the pan filter.

As to trans damage, I can't speak to every trans out there. There may be some that could be harmed, but none I know about. I takes no more than about five minutes to do the job, or less, and I can't see that being much harm. The parts lubed are the ones normally in operation when the car is moving... eg the planetary gears. WIth the car just sitting there, it's pretty much just the front pump and converter, which are full of oil, and the input shaft to some point.

Some advocate running until the fluid runs dry and I would advise against that because there is a slim chance of damaging the front pump running it dry. That's why the best way, in my estimation, is to be filling via the dipstick at the same rate as it's pumping out, which, as I said before is < 1 GPM on most transmissions and easy to keep up with. I've done it alone every time, but it wouldn't hurt to have a person in the driver's seat ready to shut down in an instant

 

[AzFireGuy79]

You obviously don't understand the theory behind automatic Transmissions onion. The sump where the pump sucks up atf is always topped off with fluid, therefore the transmission is never without lube. End of story. You must have missed that in hydramatics 101......