Transmission Line Fix-n-Flush on a Chevy Truck

This work was done on a 2004 Z71 4x4 Chevy Tahoe with the towing package.  Should be similar on Sierra/Tahoe/Suburban/Silverado trucks of the same generation.  Trucks without the towing package should have two hydraulic lines.  Mine has three.  Haven't seen a pictorial of this repair so I wanted to throw this out there.  Comments welcome.  This is my implementation (misinterpretation?) of a repair discussed on several GMC light truck forums.  I'm not by any means qualified to write this and you are a fool to follow any of my mistakes.

Problem

The transmission cooler lines start leaking where the hard lines attach to rubber at the pneumatic fitting.  My truck started doing this at 120K miles on one of the three lines.  This is what it looked like.

Approach

Lots of of forum users see this much earlier than I did and reported the replacement parts also generate this leak. Forum users also report success converting from the pneumatic fittings to traditional hose clamps on flared ended hard lines.  Since I tow an airstream, I wanted to make sure I don't end up with a seized up transmission on a excursion so I opted to flare and hose clamp rather than replace with parts with potential manufacturing problems.  Pneumatic lines like this shouldn't ever leak under the weak pressure of a transmission pump!  I will say that the replacement lines are really affordable, under $100 for a whole set, so make your pick.  120K miles on soft tubing isn't bad from my perspective.

Since I'm opening up the lubrication system, might as well replace the filter and fluid while I'm at it.  A transmission fluid/filter service at the dealer only replaces about 5 out of 11 quarts of fluid in your transmission leaving some old dirty oil in your transmission.  On the Mercedes forums, owners  like to replace all of the fluid and having the hose clamps makes this trivial, so I'll show how I did this at the bottom.

Parts and Supplies

Since my truck has the towing transmission cooler option, there are three lines to "upgrade" (or downgrade as pneumatic fittings should in theory be better technology than hose clamps).

• Hayden 106 3/8" inner diameter transmission fluid line
• 6 Hose clamps
• Doorman 800-801 retainer spring set for the clip that you lose
• 5 Gallons of Dextron III compatible transmission fluid 
• Loctite 
• Carb Cleaner
• AC Delco 24208576 Transmission Filter Kit.  Check part number for your truck!
• Lots of rags, this job is messy 

Special Tools

• Slide Hammer Pilot Bearing Puller if you replace the transmission filter seal
• Either
• 3/8" flare tool.  I love the Eastwood Professional Brake Tubing Flare
•   -- and/or --
• Dremel and bench grinder
• 10mm and 13mm U-Jointed socket
• 18mm deep well socket
• Transmission Fill Funnel
• Transmission Drain Funnel
• Hooked mechanics dental pick
• Right angled mechanics dental pick
• Mirror, magnet, and light to find hydraulic line retainer clips that fly off
• Mighty vac and tubing to suck out an overly filled transmission

System 

In the photo below, you can partially see the three lines.  This is looking down on the passenger side behind the radiator with the fan and fan shroud removed.  Degreaser took the paint off my front crossmember!

The cooled transmission fluid returns to the transmission from the heat exchanger (small radiator) behind the bowtie Chevy emblem on the front of the truck.  The return line is via the straight hydraulic line going from right to left across the top of the  photo.  Going upstream of the flow, it goes through the passenger side of the main radiator.  Not sure why the fluid goes through two heat exchangers.  Seems like one will warm and one will cool.  My leaky line was the output line from the transmission pump which travels just to below the return line in the photo turning downward in the photo mounting at the lower right side radiator mount point.  This line comes with a chaffe guard.  It is very hard to squeeze the larger diameter hayden line into this chafe guard, but I was able to do it.

Notes on transmission filter swap

I didn't take any photos as this is a pretty common service item of a vehicle.  I use long extensions to get out of the way of the hot exhaust pipe and fluid.  A U-Jointed 13mm socket is your friend here. You do need to remove the shifter cable along the driver's side of the transmission which isn't fun when the system is hot.  The shifter bracket bolts bolt downward and they are torx T40 head screws, not hex key or hex head bolts.  There is room to get a socket wrench in there.  Pop the ball/socket cam lever cable connection with a screwdriver and unbolt the shifter cable.  I didn't need to remove the heat shield like the Haynes manual recommended.

Once you drop the pan and clean up the fluid you failed to collect (d'oh), the Haynes manual shows a hooked tool removing the filter mount seal from the transmission.  No freaking way will this thing come out without a slide hammer pilot bearing puller.  If you don't have one, just reuse the old one.  Pop the filter in and reinstall the pan and shifter.  Now you have just 1/3 to 1/2 of the fluid out.

Pulling the lines

Removing all the cowling around the radiator and the front grill is really easy on these trucks and should be done first.  I cut the lines between the radiator and engine pulley to drain most of the fluid out of the lines.

Now to popping the line lines out.  Go ahead and remove the heat shield to the transmission now.  The retainer clips that hold the lines into their fittings are shrouded by a plastic collared cover.  Simply rotate these collars 1/3 of a turn and pull away from the fitting, no problem.  Then, I used a hooked and right angled pick pull the retainer clips out.  Start at the front and move back.  The ones that connect into the transmission I think would be impossible without the hooked mechanics pick.  When you do get it off, the clip will bounce off spring loaded to God-Knows-Where.  I spent two hours looking for one before I went to the parts store and obtained the Doorman 800-801 clip set on a Sunday.  Try to use the GM clips as the plastic collars don't fit as well on the Doorman clips.

Getting the long lines out of the truck past the front differential takes patience.  Go forward, twist, come back, twist, wiggle left/right, go forward, etc.  I managed to get them out through the front without stress on the lines, but it took some time and some self hypnosis.  About mid way I pulled the battery cable and snuck the line underneath the starter electrical cable which seems to got me some much needed room.

Converting to Flared/Clamped

To flare I used my Eastwood Professional flaring tool.  That tool is amazing.  There is a good video on the Eastwood site on how it works.  However, it needs over an inch of straight pipe and I didn't want longer than necessary soft line so I "stole" the existing flare from the old pneumatic fitting on the pipes that had a pneumatic connection close to a bend.  To do this, I cut the collar on each side with a Dremel.

Pry off the collar then cutoff the extra tubing...

Then with a bench grinder carefully grind down to the first flare.  Looks pretty good!  Clean the lines up with some carb cleaner and rinse thoroughly and set aside to dry.  (I wonder how much I spend on carb cleaner every year?)  You don't want to introduce metal shavings into your transmission.  Here is one repaired line before final assembly...

Reinstall

Okay, this is why I'm spending the time with the write-up....  I spent four hours trying to get the clip into the top fitting of the transmission and couldn't get it done.  This task is easy on the other connectors and I thought no problem, just push it, so what if its hard to see and the angles is bad.  What worked was getting an 18mm deep well socket and removing the fittings off the side of the transmission and bench installing the clips.  Clean the threads up, put on some lock tight and reinstall.  The hydraulic hard lines can be popped in with the clips pre-installed.  I didn't know this when I started but tested this on the front fittings and it works great...  So don't wast your time, pull the transmission fittings out.

 Reinstall the fittings...  Remember the case is aluminum so don't go crazy with the torque.

Pop in the hard lines snuggly into their fittings.  Note, to thread the fitting into the transmission, I came from the back, not from underneath as the angle is odd from underneath...  This is what it looks like from the rear/right...

 

A bit closer in...

Its pretty easy to get a hand in the right spot from this angle.

Transmission flush

With everything hooked up its time to fill up.  Decided with 120K miles it was time for a complete fluid flush.  You can use the transmission pump to push the fluid out through the lines and heat exchangers up to the return line into a container.  I learned this trick from the Mercedes forums where Mercedes reversed its position on "sealed for life" transmission and started recommending a fluid swap much to the surprise of many Mercedes owners!  Chevy's partial replace is fine, but a complete swap is cheap and easy with this method.  Here is the setup...  Notice the soap bottle with the soft transmission line stuck into it at the bottom.

 

I collected 5 quartz of fluid from the pan and cut-up tranny lines, so I added 5 quartz to the filler tube (slowly). Reinstall the air intake between the air mass flow sensor and the throttle body so the motor will run.  Start the engine for 10 seconds or so until you get 2 quartz of fluid into the container and stop the motor.   This is easier with two people obviously.  Now put 2 quarts of clean fluid in and repeat.  Do this over and over until the fluid is no longer brownish red and starts running red/pink depending on which brand of fluid you pour in.  I pushed in 4 and 1/2 gallons...  Well actually 4 and 3/4 gallons.    I overfilled it and instead of reopening the system, I sucked a quart out the tube with a mighty vac.

Look forward to your feedback!