4L80E 6AN PTFE Transmission Cooler Line Kit, 70 Inch

Rusted factory cooler lines weeping ATF onto the frame of an old 2500 or 3500, a 4L80E swap that needs plumbing to the radiator, or an auxiliary cooler going onto a tow rig? This kit gives you a single 70 inch (5.8 ft) run of -6AN hose with a PTFE liner and a stainless steel braid, plus the end fittings to thread it into the transmission case. It is built around the GM 4L80E and also fits the 700R4, 200-4R, TH350 and TH400, the Ford AOD, 4R70W and 4R100, and the TR6060.

What the cooler lines do on a 4L80E

A 4L80E pushes hot fluid out of the converter circuit, through the cooler and back into the case, where it feeds the lube circuit for the bushings and gear train. The lines are part of that loop. If one leaks, the unit loses fluid; if one kinks or collapses, cooler flow drops and the lube circuit is starved even though the dipstick reads full. GM puts the normal 4L80E operating range at roughly 180 to 200 F and warns that the truck will go into a protection mode as temperatures approach 265 F. A heavy truck that tows will see that ceiling fast if cooler flow is restricted, and heat is what kills the frictions and the converter clutch in this transmission.

Why PTFE hose instead of rubber

A PTFE liner does not swell, soften or break down in ATF the way rubber hose eventually does. This hose is rated from -76 F to 500 F, with 3000 psi working pressure and a 10000 psi burst rating, so it holds up routed near an exhaust pipe or a hot catalytic converter. The stainless braid protects it from abrasion where it passes crossmembers and brackets. At -6AN the hose has a 5/16 inch inside diameter and a 0.45 inch outside diameter.

Signs your cooler lines need replacing

• ATF dripping from a line, a fitting or a rusted crimp along the frame rail
• Fluid level dropping between checks with no leak at the pan or seals
• A TRANSMISSION HOT message or high temp reading while towing
• Kinked, flattened or rubbed-through lines after suspension or exhaust work
• Late-shift or converter clutch slip complaints that only show up when the unit is hot

How to confirm before buying

Clean the lines and fittings, run the truck to temperature and look for fresh fluid. Then check flow: disconnect the return line at the cooler, run it into a container, and start the engine in Park for a few seconds. A strong, steady stream means the circuit is open; a weak trickle means a blocked cooler, a kinked line or a converter or pump problem. If the truck already threw slip or pressure codes, read our 4L80E common problems guide and the Sierra 2500 problems guide before you assume it is only a line issue.

Fitment

The included fittings cover the common case port threads: 9/16-18 UNF, 3/8-18 NPT and 1/4-18 NPT. Pull one of your old fittings or check the case port before you start so you use the right adapter. A 4L60E is different: its case uses a 1/4 inch NPS quick-connect style port, so it needs a separate -6AN to 1/4 NPS short adapter that is not in this kit. On a 4L80E the kit fits the 1991-2009 Chevrolet and GMC 2500 and 3500 pickups, Suburban 2500, Express and Savana vans and Hummer H1, plus swap builds. Ask with your year and VIN if you are not sure. Everything else for this transmission is on the 4L80E collection page.

Install notes

• Measure the full run from the case to the cooler or radiator tank, following the factory routing and staying clear of moving steering and suspension parts. 70 inches covers one line on most trucks; plan on two runs if you are replacing both supply and return.
• Tape the hose where you will cut it, then cut cleanly with a cutoff wheel or a fine tooth blade so the braid does not fray.
• Seat the olive and socket fully on the hose end before tightening the AN nut. Use thread sealant on the pipe-thread fittings only, never on the AN flare.
• Secure the line with clamps every couple of feet so it cannot rub through on a frame rail.
• Top off with Dexron VI, run the engine to temperature, recheck the level hot, and look for weeps after the first heat cycle.
• If the old lines leaked long enough to run the unit low and hot, check line pressure and shift quality before calling the job done. A unit that slips afterward may need a 4L80E overhaul kit, and a weak pressure control solenoid can show up after a hot, low-fluid episode.

Will this cooler line kit fit my 4L60E?

Only with an adapter. The 4L60E case uses a 1/4 inch NPS quick-connect style port, so you need a separate -6AN to 1/4 NPS short adapter, which is not included.

Is 70 inches enough for both lines?

It is one 70 inch run. Most trucks need a separate run for supply and return, so measure both routes before ordering.

What thread sizes do the fittings cover?

9/16-18 UNF, 3/8-18 NPT and 1/4-18 NPT, which are the common case port threads on the GM and Ford units listed.

Can I run this hose near the exhaust?

The PTFE liner is rated to 500 F, so it tolerates exhaust heat far better than rubber, but still keep clearance and use clamps so it cannot touch a pipe or rub on the frame.

What fluid should I use after replacing the lines on a 4L80E?

Dexron VI. Refill, run the unit to operating temperature and check the level hot on the dipstick.

• 4L80E Common Problems and Fixes: The Six Failures That Actually Come Through the Door
• 4L80E Sierra 2500 Common Problems and Fixes
• 4L80E Silverado 2500 Common Problems and Fixes
• 4L80E Express Van Common Problems and Fixes