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#1792025 - 02/23/10 08:22 AM Transmission Fluid Pressure?
cchase Offline


Registered: 11/03/09
Posts: 4021
Loc: New England
Simple question which I assume is fairly straightforward industry-wide.

What is the pressure of transmission fluid as it is pumped through the radiator? I assume there are variations among makes and transmissions but fairly similar on the whole.

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#1792055 - 02/23/10 08:45 AM Re: Transmission Fluid Pressure? [Re: cchase]
bepperb Offline


Registered: 01/10/08
Posts: 4985
Loc: Milwaukee, WI
Not alot, if you do a fluid exchange it isn't coming out at a significant pressure unless you were to block the hose, which wouldn't happen (the radiator flows unrestricted back to the drain pan).

Actually, though, many trannys vary the pressure going through the tranny via the solenoid pack. So I guess the pressure could be higher in operation, but still next to nothing.


Edited by bepperb (02/23/10 08:46 AM)
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#1792601 - 02/23/10 03:19 PM Re: Transmission Fluid Pressure? [Re: bepperb]
severach Offline


Registered: 02/04/08
Posts: 617
Loc: MI or LA
Originally Posted By: bepperb
(the radiator flows unrestricted back to the drain pan)

I made that assumption too until I thought about how transmissions work and realized that free flow into the pan is unlikely and often impossible.

We'd like to cool the transmission fluid after it has worked. It isn't possible to collect worked fluid under pressure as it squirts out of random bearing surfaces. It just falls into the pan. Engine oil couldn't be cooled this way either.

We could also cool the fluid as it exits the pump. Not possible since the pressure can rise to over 250 psi and the fluid couldn't be held in with low pressure hose. Thin wall radiators would be stressed excessively and any leak or free flow into the pan would result in instant loss of oil pressure. Good design mandates that the area subject to 250 psi be as confined as possible, and certainly not in the rickety lines and radiator. Some transmissions have a high pressure test port and the cooler line is not it.

If the fluid free flowed into the pan, flare connections and high pressure hose would not be needed, just like they are not needed for the power steering return which does free flow into the tank. Manufacturers use flare connections and high pressure line for factory cooler lines though low pressure hose with standard clamps also works, often seen in repairs and aftermarket coolers.

It should be obvious that these problems apply to all automatic transmission manufacturers. Manufacturer coolers equipped with low pressure lines might indicate that the oil flows freely in the pan.

Though I don't know how, GM transmission lines do not free flow into the pan and maintain a steady psi about 35. I've blown into the drain line just to check.

Conjecture:

The cooler line oil must come from the high pressure system. It must be pressure regulated to allow the rest of the system to maintain pressure as a metered amount of oil flows through the cooler. The drain into the pan must be flow regulated through an orifice or it might be used to supply a low pressure system.

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#1792913 - 02/23/10 07:05 PM Re: Transmission Fluid Pressure? [Re: severach]
Texan4Life Offline


Registered: 01/28/10
Posts: 2264
Loc: Texas
not much back pressure... output pressure from the pump(s) is not the same as return line pressure.

Don't know whether transmission uses full flow of bypass systems for coolers.

Return line pressure does not need to be regulated. being free flowing back into the pan is the {"regulator") because it does not allow pressure to build.
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#1792928 - 02/23/10 07:19 PM Re: Transmission Fluid Pressure? [Re: Texan4Life]
mechtech2 Offline


Registered: 09/05/06
Posts: 19479
Loc: Chicago Area
My guess is about 30-80 through the radiator cooler.

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#1792947 - 02/23/10 07:31 PM Re: Transmission Fluid Pressure? [Re: mechtech2]
Texan4Life Offline


Registered: 01/28/10
Posts: 2264
Loc: Texas
so most or all transmissions use bypass cooling?

edit: nvm you were talking about pressure.

you guys are gonna make me hook up a gauge to my return line arn't you?

lol


Edited by Texan4Life (02/23/10 07:32 PM)
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#1793013 - 02/23/10 08:16 PM Re: Transmission Fluid Pressure? [Re: Texan4Life]
cchase Offline


Registered: 11/03/09
Posts: 4021
Loc: New England
The reason I ask is because of the current thread about coolant in someones transmission fluid. My guess would have been that transmission fluid is at a higher pressure than the coolant (which is probably under 20 psi in most applications).

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#1793028 - 02/23/10 08:25 PM Re: Transmission Fluid Pressure? [Re: cchase]
440Magnum Offline


Registered: 02/01/09
Posts: 8754
Loc: Texas
I don't know if there's consistency across brands in that regard or not. I know Mopars generally routed the fluid to the cooler and then to the lubrication points throughout the gear train and bearings, so the pressure through the cooler is on the order of 30-40 PSI. If there are automatics that return cooled fluid directly to the pan instead, the pressure would be significantly lower.

So in this case, the fluid pressure is generally higher than the cooling system, but its not a fail-safe. The instant you turn the engine off the trans fluid pressure drops to zero, but the cooling system pressure can continue to rise. The one time I had a trans cooler fail, things went both ways. There was brown mud in the radiator from oil pumping into the cooling system when the engine was running, and there was brown mud in the transmission pan from coolant getting pushed into the cooler when the engine shut off. Oh, and a quirk of chemistry is that transmission fluid makes the silicates precipitate out of old-style coolants. So it really IS brown mud... sandy mud.
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#1793031 - 02/23/10 08:26 PM Re: Transmission Fluid Pressure? [Re: cchase]
Gary Allan Offline


Registered: 09/28/02
Posts: 39806
Loc: Pottstown, PA
Originally Posted By: cchase
The reason I ask is because of the current thread about coolant in someones transmission fluid. My guess would have been that transmission fluid is at a higher pressure than the coolant (which is probably under 20 psi in most applications).


I agree with the 50lb+/-. Mine was 30lb @ idle semi cold (but not stone cold). It's got limits.


While you would normally reason that you would see ATF in the coolant instead of the other way around, when you shut down the engine, the line pressure is gone immediately, the coolant pressure is not.
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#1793110 - 02/23/10 09:30 PM Re: Transmission Fluid Pressure? [Re: Gary Allan]
cchase Offline


Registered: 11/03/09
Posts: 4021
Loc: New England
Originally Posted By: Gary Allan
While you would normally reason that you would see ATF in the coolant instead of the other way around, when you shut down the engine, the line pressure is gone immediately, the coolant pressure is not.


I think you and 440Magnum nailed it with this.

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#1794073 - 02/24/10 05:48 PM Re: Transmission Fluid Pressure? [Re: cchase]
onion Offline


Registered: 02/04/07
Posts: 2097
Loc: kansastan
Originally Posted By: cchase
Originally Posted By: Gary Allan
While you would normally reason that you would see ATF in the coolant instead of the other way around, when you shut down the engine, the line pressure is gone immediately, the coolant pressure is not.


I think you and 440Magnum nailed it with this.


+1.5
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#1794230 - 02/24/10 07:45 PM Re: Transmission Fluid Pressure? [Re: onion]
JTK Offline


Registered: 08/14/03
Posts: 10569
Loc: Buffalo, NY
I know we've discussed this before, but I'm still not clear on the whole PSIG in the ATF cooler circuit thing. I realize the cooler inlet hose could develop some tremendous pressure given the proper restriction, but the cooler return lines on the transmissions I've flushed or messed with over the years is a no pressure line that just returns to the sump? So basically, any pressure you'd see in that circuit during normal operations would be due to dP/restriction in the system, correct?

Joel


Edited by JTK (02/24/10 07:46 PM)
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#1794541 - 02/25/10 12:39 AM Re: Transmission Fluid Pressure? [Re: cchase]
MacGyver Offline


Registered: 01/31/10
Posts: 37
Loc: Nebraska U.S.A.
Most pressure numbers stated in the other posts are close to correct. The pressure varies with pump RPM and fluid temp. Mods can be done to the valve body to increase flow through the cooling circuit. That brings me to the next point. The return oil from the cooler is a major part of pressurized LUBE CIRCUIT for the entire automatic trans. No cooler flow means no pressurized lube to the rotating components. The oil does not simply return to the pan. This is also the reason that there are factory bypasses in the cooler circuit. Reason being in the event of extreme cold weather the oil is reluctant to flow fast enough through the cooler to get back to the trans to lube the rotating parts. This can and has caused lack of lube failures leading to a low mileage rebuild. Chrysler had this problem with their front wheel drive cars and vans in the early '90s. Bypass valve kits where installed into the cooler lines in the cold weather states to resolve this issue in the field. The newer tranny's had the by pass valve designed internally.
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#1794905 - 02/25/10 11:46 AM Re: Transmission Fluid Pressure? [Re: MacGyver]
JTK Offline


Registered: 08/14/03
Posts: 10569
Loc: Buffalo, NY
Originally Posted By: MacGyver
The return oil from the cooler is a major part of pressurized LUBE CIRCUIT for the entire automatic trans...... The oil does not simply return to the pan.


Understood. So the return oil DOES do some traveling before it trickles down into the sump. I guess that answers my question then in regards to where the pressure comes from in that system. Thank you Sir.

Joel
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2017 Ram 1500 4x4, 3.6L. 2016 Nissan Quest SV (Babe magnet IV)

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#4452070 - 07/07/17 07:42 AM Re: Transmission Fluid Pressure? [Re: cchase]
userfriendly Offline


Registered: 06/03/03
Posts: 2630
Loc: LaFinDuMonde
So... would adding an in-line filter to the cooling lines be a good idea or not?

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