Home | Mailing List | Specifications | Care and Feeding | Modifications | Vendors | Literature


ATX Fluid Flow & Tranny Coolers

New 02/09/2005

I am unsure about the amount of transmission fluid going thru the cooler. Is it the full amount of volume that the transmission is using or just a percentage of the flow? I am planning to install a three way valve so that in winter I can direct the flow to the "factory" cooler or direct the flow to the installed "Aux" cooler in the summer. My transmission temp gauge just barely gets over 100 even at highway speeds in the winter time. Is there a optimum temp for the transmission to run at?
Best thing to do is to get an oil t-stat that will keep the fluid around 190-200F. This will keep the water out of it.

Paul Nimz
I'm not sure why you'd bypass the factory cooler. Most aux cooler
makers suggest installing their coolers to work in conjunction with
all the factory cooling, not instead of.

Here's some FAQ's from Hayden's website:

Q. Should the cooler be installed before or after the radiator?
A. Hayden recommends installing the auxiliary cooler after the
radiator to return the coolest fluid directly to the transmission.
Installing the cooler before the radiator will still provide
additional cooling and may be necessary in some difficult access

Q. Can you over cool the transmission fluid?
A. Transmissions are not highly sensitive to cool operating
temperatures. However, in sub-zero (20-30 F) weather conditions
transmission fluid can actually gel up in an external cooler and cease
to flow, causing damage. Use of the radiator cooler actually helps
warm the fluid under these conditions. It is critical in extreme cold
conditions to use the original equipment cooler in series with the
auxiliary cooler and allow the vehicle to warm up before driving.

I've been looking into getting a TOC (transmission oil cooler) for my
wife's Odyssey. I found the Tru-Cool brand offers a unique (as far as
I can tell) bypass design, where thick, cold fluid bypasses the
cooling portion of the stacked-plate design TOC. Once the fluid warms
up the fluid goes through the cooling passages.

Maybe Paul will elaborate on his oil t-stat and how it would be
installed. I've never seen this done before!

Good luck,

Steve Tatro
B&M's coolers also have the bypass built into them.

While we are on the subject . . .

I just got through asking a very reputable gentlemen this, while I await the

Who here thinks I should get an aux cooler NOW as opposed to waiting until
tranny starts acting up?
to be more precise, what I mean is, if this tranny is supposed to fail at
some point no matter what I do, maybe it would be prudent just to get the
aux cooler with my impending tranny rebuild?
or, how many miles could I be adding to the life of the original tranny
should I get a cooler now when I am swapping out fluid as suggested ?

'98 white 90k welded
Richmond, VA
My Tru cool unit has the bypass under the driver's side headlight. attached
to the bottom of the bumper bar/rad cradle
The fluid comes out of the factory tube cooler to the valve then dependant
on temps goes through the true cool unit or back to the transmission.

The first valve that came with the kit leaked at a seam.
Called them and the sent one out n/c

been 2 winters with it and no problems.
plus the 23x4x3/4 inch size is easy to fit into the car.

Clare - Who is Shift Key Challenged
I vote for getting it NOW! I installed my cooler at 35,000 miles. Tranny is
still going strong at 128k. I also change the fluid every year.

Glen Murdock
If you are going to do it anyway, do it sooner rather than
later. I put on my cooler at about 80,000 miles. There is
no way really, to tell if it will extend the life of the
tranny but chances are it will. I know that the fluid now
always looks and smells new, even after racing and towing a
small trailer over mountains.

Don Mallinson
20K on my ATF and it still looks really good. I did the "paper towel
test" (comparing a drop of the fluid from the dipstick to a drop of
ATF straight from the bottle) and it is actually hard to even see an
appreciable difference in color. I will still be replacing my ATF in
the spring. I installed the B&M cooler at the last change, and I'm
thinking about getting one of the FPS kits this time around and
putting the B&M on the SLO. I credit the cooler and the 15K miles of
driving with firmer shifts (Apten chip) for the good condition of the
ATF and trans. Of course, those firmer shifts have cost me a motor
mount, but c'est la vie. Do the math--shorter ATF change intervals
might cost you an extra $80 every two years (assuming you are changing
it every year rather than every other year) + Chip cost ~$300 + Cooler
cost ~$50 + Motor mount ~$70 = ~$500--a lot cheaper than an ATX
rebuild. It's not a foolproof plan, but if it gives me an extra 20K
miles before the ATX rebuild, I think it's money well spent.

Dan Carman
When I put in the 3.98 final drive one problem that surfaced is the TC
did not lock up after an hours drive. Even when it was locked it was
still slipping and lead to a lot of heat build up. Prior to going to
California last summer I installed an additional cooler to the OEM aux
cooler and the large oil cooler I already had for the tranny. I also
took the radiator cooler out of the loop. During that trip when going
up the mountains the tranny fluid temp got to over 240F with coolant
temps around 220F.

I now know that the coolant high temp is because of an externally
clogged radiator ala Larry, and I also know now that the radiator
will add little if any heat to the ATX fluid in the winter. I've
since taken out the two coolers I had and installed a large Hayden
tank style cooler along with the OEM aux cooler and the radiator still
out of the loop. Still if the TC does not lock up the temps will rise
to 175F even in 20F temps. As soon as the TC locks the temp of the
fluid will drop dramatically.

Doug feels the fluid should be around 190F to keep the moisture driven
out and I agree with him. The moisture comes from condensation on
cool down as the tranny is vented. FPS offers an oil stat that will
do this, it can also be found at the bigger car parts places. I also
have seen that the PS fluid never gets very hot. Even after a hard
track session in the 100F ambient heat at Thunderhill the PS fluid was
only 145F. This too I feel would benefit from the oil stat.

At this time I have not installed either as I am waiting for better
weather. But I do feel that with the Hayden and pathetic aux OEM ATX
cooler as long as the TC locks up then cooling is not an issue. But
the stock setup is lacking especially if the coolant get over 220F
The ATX fluid if kept near 190F will give better gas mileage while
stiff well within the safe range. BTW I use Amsoil ATX fluid and all
my thinking excludes dino ATX fluid.

Paul Nimz
On my SHO, Doug cut the lines to the factory cooler
and didn't use it. I'm not sure why. You'd have to
ask him. His Powerstroke write-up adds a cooler and
uses the factory cooler.

How long your ATX lasts might depend on what any
previous owner's) did to it. After my ATX experience
I'd say a temp gauge to keep an eye on things and a
big cooler would both a good investment. I suspect
the ATX I just had replaced had been overheating for
some time and I just didn't notice.

Now that I have a new FPS ATX, cooler and shift kit to
compare to, I'm convinced there was definitely already
something wrong with the ATX in my car when I bought
it with 45,000. Early warning signs would be vague
rumbles, rubbing noises, vibrations, and other
rotational noises that can't be traced to wheel
bearings, strut bearings, ball joints, CV joints, or
anything else that you can test or inspect. Anything
odd with the 1-2 shift and an unexplained drop-off in
gas mileage would be two more. I chased the undefined
rotational noises with no success through 3 years, new
front brakes and rotors, new tires, and 3 ATX fluid
changes. Finally I had low ATX fluid and obvious
damage to the ATX at 57,000. Even if the dealer
somehow shorted me a quart, there were no leaks and 2
quarts missing so it must have burned off.

After watching that ATX overheat, boil, smoke, stink,
and grind in anything but 4th gear from WV to Atlanta
I suspect overheating was why I got the inconsistent
fluid level readings depending on when/where I checked
it. I saw even more fluid level variety on the way to
Atlanta. I had anything from slightly high with gray
sludge to foam all the way up the stick and
overflowing the tube. 4th worked great but all other
gears shifted erratically and overheated. Oddly
enough, ATX never did slip much. Maybe they don't
until the very end.

Amy Patterson
'98 black
My opinion is that heat is what kills the transmissions. Put a cooler in now and prolong the life of the transmission. Most of the heat by the way is generated when the Torque Converter continually lock and unlocks with the AC compressor. I proved this to myself at least when my TC refused to lock after the previous transmission replacement and on a 75 mile trip the fluid started bubbling out of the filler tube. When I got it back home I mounted an auxiliary cooler and it never happened again. The tranny survived another 14,000 miles before it expired from fluid starvation caused by a failed fitting on the transmission flexible line.

Carter Fuji
I have also eliminated the factory cooler all together. I believe this will explain the basis of my decision
SOWHAT9 - The Complete Story of What Happened to Her Engine

Make sure you pay close attention to the picture of what we found in the tranny pan.


Contact Information