Thoughts on doing a "Trans flush"...
Frmr Ford Trans Engr
Joined: Aug 1999
Posts: 24,712
Likes: 2,638
From: SE Florida
Many people seem to think the bypass is to warm up the transmission. Not true. There is no thermostat in there, only a ball and a spring. If the pressure gets too high in the cooler line due to a restriction the bypass opens. It will not close again until the restriction is repaired. It is only there to keep flow to the rear lube system in case the cooler line gets restricted. It doesn't do anything to help warm the trans or keep it at temperature.
Hotshot
Joined: May 2005
Posts: 13,484
Likes: 42
From: Stevens Point, WI
Yes, I'm sure about that, and no, you didn't know what that bypass is for.
Many people seem to think the bypass is to warm up the transmission. Not true. There is no thermostat in there, only a ball and a spring. If the pressure gets too high in the cooler line due to a restriction the bypass opens. It will not close again until the restriction is repaired. It is only there to keep flow to the rear lube system in case the cooler line gets restricted. It doesn't do anything to help warm the trans or keep it at temperature.
Many people seem to think the bypass is to warm up the transmission. Not true. There is no thermostat in there, only a ball and a spring. If the pressure gets too high in the cooler line due to a restriction the bypass opens. It will not close again until the restriction is repaired. It is only there to keep flow to the rear lube system in case the cooler line gets restricted. It doesn't do anything to help warm the trans or keep it at temperature.
This all seems very familiar like we've been threw this before. How come I keep forgetting its a bypass?
My 03 doesn't have it. Would that bypass be part of the 4r100 problems in earlier years? (I've probably asked this before as well and forgot, must have Alzheimer.)
Frmr Ford Trans Engr
Joined: Aug 1999
Posts: 24,712
Likes: 2,638
From: SE Florida
Then someone removed it.
What problems do you mean?
What problems do you mean?
Postmaster
Joined: Jun 2007
Posts: 2,621
Likes: 1
From: Sweetwater Texas
I did a "flush" on a buddies 02, I just dropped the pan and pulled the plug on the tc. The truck still had the orginal filter and fluid with 300 k on it. The fluid was completly black and filter was clogged pretty good. After wards she went back to shiffting properly and a year later, still no probs. Dunno if this helps you any.
Hotshot
Joined: May 2005
Posts: 13,484
Likes: 42
From: Stevens Point, WI
Yes it is all coming back to me now, this conversation is coming along nearly identical to what I remember we've been threw before. lol
No one removed the bypass, it is factory like it is.
The troubles I am talking about are simply the 4r100's going out, burning up, not working like they should. I am wondering if it is because of the bypass. Say the cooler plugs up while towing and the bypass opens up just dumping scalding hot fluid back into the trans burning up the trans.
Cargo Master
Joined: Mar 2006
Posts: 2,502
Likes: 2
From: Bend Oregon
If the bypass was not in place when the cooler circuit plugged you would instantaneously lose the transmission due to lack of lubrication. With the bypass in place and if you have a temp gauge your transmission has a fighting chance of surviving a plugged cooler circuit.
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
AEV FXL Super Duty - the Super Duty Raptor Ford Doesn't Make
Brett Foote
Lobo Vs Lobo: Proof the F-150 Lobo Should Be Even Lower!
Michael S. Palmer
Ford's 2001 Explorer Sportsman Concept Looks For a New Home
Verdad Gallardo
10 Best Ford Truck Engines We Miss the Most!
Joe Kucinski
2026 Shelby F-150 Off-Road: Better Than a Raptor R?
Brett Foote
2027 Super Duty Carhartt Package First Look: 12 Things You NEED to Know!
Michael S. Palmer
10 Most Surprising 2026 Ford Truck Features!
Joe Kucinski
Top 10 Ford Trucks Coming to Mecum Indy 2026
Brett Foote
5 Best / 5 Worst Ford Truck Wheels of All Time
Joe Kucinski
Molon Labe
Joined: Dec 2006
Posts: 5,272
Likes: 1
From: Alaska
Did you purchase it new? I talked to a few different trans shops here in FL, and they all wanted to remove the bypass with a rebuild.
Which I totally disagree with!
Which I totally disagree with!
Senior User
Joined: Sep 2011
Posts: 118
Likes: 0
From: Montana
Changing the fluid is maintenance. A trans flush is just that. If thats why you are doing it than go for it. If you are having an issue and hope that it will fix it, it may not. It may make it worse. Ive been a mechanic for over twenty years and have heard alot of opinion on this. If your trans is in such shape that changing the fluid causes it to fail, it was going to fail anyway.
Hotshot
Joined: May 2005
Posts: 13,484
Likes: 42
From: Stevens Point, WI
If the bypass was not in place when the cooler circuit plugged you would instantaneously lose the transmission due to lack of lubrication. With the bypass in place and if you have a temp gauge your transmission has a fighting chance of surviving a plugged cooler circuit.
But would it plug if the pressurized fluid didn't have anywhere else to go?
Hotshot
Joined: May 2005
Posts: 13,484
Likes: 42
From: Stevens Point, WI
I am the second owner, I also have all the service records for it and there is nothing on the transmission. No replacement, no fluid change, no service, nothing.
Molon Labe
Joined: Dec 2006
Posts: 5,272
Likes: 1
From: Alaska
Cargo Master
Joined: Jul 2011
Posts: 2,658
Likes: 968
From: Walpole, MA
Are you sure about that? When I did the flush on my 03 7.3 X the trans lines were strait in and strait out, easy flush, no problems.
Then when I went to help a buddy with his flush on his 02 7.3 F350 the trans lines had a bypass line between the front and rear lines. I knew right away what this line was for and told him we have to drive the truck around for awhile and get it warm before doing the flush.
Then when I went to help a buddy with his flush on his 02 7.3 F350 the trans lines had a bypass line between the front and rear lines. I knew right away what this line was for and told him we have to drive the truck around for awhile and get it warm before doing the flush.
Mark's cooler line exchange works great. I have done it on 2 vehicles. My 2002 with 60k old ATF from the prior owner and my mother's Mercury mountaineer which had 140k and 14 year old Mercon on it. Both trans function perfectly now. Easy as pie and the don't flush an old trans stuff I agree with but frankly I would only flush a trans that needs a power flush, I would recommend a cooler exchange to anyone.
Post Fiend
Joined: Sep 2003
Posts: 6,096
Likes: 26
From: Mesa AZ
To me that's like saying "the engine oil shouldn't be changed if it's got over 75k on it" it's just Plain silly! sorry my opinion! Ask him to give you 1 good substantiated reason as to why he would not change the oil, aside from selling a transmission rebuild!
Jim & fat Monty
Jim & fat Monty
I once spoke with a repair shop owner who had the same policy....he won't change fluid on transmissions with high miles unless he has serviced it before or the owner can document that it's had regular fluid changes. His logic is that 95% of the customers that bring in a vehicle and request a transmission flush is because they are having some sort of issue already. He changes the fluid and then the customer brings the vehicle back claiming that he caused the unit to fail.
I am not sure I agree but I guess I can see his point....I know a lot of people that never THINK about ever changing their trans fluid until they begin having problems.