10L80 Fluid Leak: Common Spots and How to Confirm Each One

A customer drops off a 10L80 truck with a spot on the driveway and says "it's leaking somewhere back there." That sentence covers at least four different repairs on this platform, and GM has written up three of them separately because the parts, the access, and the fix are not interchangeable. Guess wrong and you pull a crossmember for a plug that was never leaking, or you chase a welch plug when the real culprit is the output seal six inches away. Clean it, dry it, and make the fluid show you exactly where it is coming from before you order a single part.

This covers the spots GM itself has documented on the 10L80 (and in one case the shared 10L90), what each bulletin actually says to do, and the order to check them in so you are not working blind.

Recommended 10L80 parts for a leak repair

The filter and pan gasket kit for the most common leak point, the OE-style cooler lines when the fitting itself is damaged, and a proper filler adapter so the refill goes in clean instead of down the side of the case. Ships same day from the USA. Secure checkout.

10L60/10L80/10L90 Transmission Filter and Pan Gasket Kit

$30.99View part

Silverado 1500 Transmission Cooler Lines Inlet and Outlet 85043951

$256.99View part

GM ATF Filler Adapter 6L80 8L90 10L80 Fluid Fill Tool

$36.99View part

Four spots, four different jobs

Before the detail, here is the map. Front to back on the case:

Three of these four have a GM document behind them because GM's own field data showed them recurring enough to write up. The fourth, the pan and filter, is just a gasket doing gasket things after years of heat cycles and is not a defect bulletin candidate. Work the list in the order it is written, because each one is faster to rule out than the one after it.

The cooler line fitting bolt (bulletin 18-NA-026)

This is the earliest-documented 10L80 leak point and it sits on the left side of the transmission case, at the retaining bolt that holds the oil cooler line fitting in place. GM's bulletin covers 2018 Escalade, Tahoe and Yukon with the 10L80 (RPO MF6), the 2019 Silverado 1500 New and Sierra 1500 New with the 10L80 (RPO MQB), and the 2017-2018 Camaro with the shared 10L90 (RPO MGL).

GM's stated cause is specific: the case area near the threaded bolt passage may contain porosity from the casting process, and that porosity can let fluid seep past the retaining bolt even when the bolt itself is tight. The tell is fluid at the bolt head, not a wide wet area — wipe it dry, run the truck, and check the bolt head specifically rather than the whole quadrant of the case.

GM's fix does not involve splitting the case. Install a new oil cooler line fitting seal, part number 23135703, onto the cooler line, then clean and reseal the retaining bolt with medium-strength threadlocker (blue), part number 19333511 (Canada 10953489), before torquing it back down. The bulletin's own note warns not to use brake-clean solvent in the threaded passage before reinstalling — wash and dry it properly instead. This is a same-day repair once you have the seal in hand.

The rear case plugs (bulletin PIP5872)

Move to the rear of the case and this one shows up as a puddle under the middle-rear of the truck. GM's bulletin PIP5872, published October 2022, covers 2022 model year 10L80 trucks and SUVs across nearly the whole lineup — Escalade, Suburban, Tahoe, Yukon, Yukon XL, Silverado, Sierra and the Cheyenne export badge, on RPOs MHS, MHT, MQB and MQC.

The cause is plain: one or more of the two rear case plugs may be leaking. GM's own diagnostic trick is worth copying — aim a light from the right side of the transmission, into the gap between the crossmember and the case, and you can usually spot which plug is wet without removing anything yet.

Once you have confirmed it, the crossmember support and mount come off (GM's photos show both 2WD and 4WD, and the procedure is the same either way) to get access to the plugs. The bulletin is explicit on scope: only replace the plug that is actually leaking, not both as a pair. Remove and discard the leaking plug, clean the threaded hole with a lint-free shop towel (again, no brake-clean solvent in the hole), apply a small bead of AC Delco Thread Sealant 19333512 (Canada 10953480) to the new plug — part number 11548770 — and torque it to 11 lb-ft (15 Nm). Reinstall the crossmember, then check and correct the fluid level per Service Information.

Red fluid at the rear: output seal or welch plug? (bulletin 22-NA-225)

This is the one that trips people up, because two different leak sources sit almost on top of each other at the rear of a 4WD or AWD truck: the transmission/transfer case output seal and the propeller shaft front slip yoke welch plug. GM's bulletin 22-NA-225 (revised October 2023) covers a wide span — 2015 through 2024 across Escalade, Colorado/Canyon, Express/Savana, Silverado and Sierra 1500 in every generation back to 2014, the 2500HD/3500HD, Suburban, Tahoe and Yukon — wherever RPO F48 (AWD/4WD with driver select) is present. For 10L80-equipped trucks specifically, GM's own additional notes flag T1 LD (Silverado/Sierra 1500) as affected on every configuration except 2WD trucks still running the old 8-speed.

GM's procedure is a clean-and-look method, and it works: clean the area of concern with mineral spirits, then inspect two points in sequence.

1. Visually check the output seal and the boot area right where the shaft enters the case for ATF.
2. If that area is dry, check the welch plug on the propeller shaft's front slip yoke, just aft of the output seal.
3. If both areas are wet and you cannot tell which one is the actual source, dry everything with a soft rag and do a series of short drives around the block to let the leak reappear, then go back to step one.

The fix branches from there. If the output seal is the source, replace the output seal per the Propeller Shaft Front Slip Yoke Replacement procedure in Service Information, and GM's bulletin says no further work is required. If the welch plug is leaking instead, replace the propeller shaft front slip yoke — the welch plug itself is not sold or serviced separately. Do not skip the mineral-spirits clean step; the two leak points are close enough that a casual wipe-down will send you after the wrong part.

The pan gasket and filter: the one without a bulletin

The fourth common spot is the most ordinary one and the reason it has no GM bulletin is that it is not a defect, it is wear. The 10L80 pan gasket and the filter's sealing points age out over time like any other automatic transmission gasket, and heat cycling plus years of exposure to road salt and grime eventually lets a weep start at the pan rail. If the leak is low on the case, running down the pan rather than dripping from a specific bolt or boot up near the output shaft, the pan gasket is the suspect, not either of the two case bulletins above.

Service it the same way you would any pan-and-filter job: drop the pan, inspect what comes out in the filter and the fluid for metal or friction material (that points to internal wear, not a seal issue), replace the filter and gasket as a set rather than reusing either one, clean both mating surfaces completely before the new gasket goes on, and snug the pan bolts evenly in a crisscross pattern rather than cranking one side down first. A warped or unevenly torqued pan flange is the single most common cause of a new gasket still weeping two weeks after the service.

Checking the fluid and the fill after any of these repairs

Every one of these jobs ends the same way: confirm the fluid level and refill with the right spec. GM's spec for the 10L80 is DEXRON ULV, part number 19352619 in the US (19352620 in Canada) — nothing else goes in this unit, and regular DEXRON VI is not a substitute. The published fluid change capacity is 7.7 liters (8.1 quarts) across the MQB, MQC, MHT, MI2 and MHS RPO codes, which is the change figure, not a dry fill, so expect to add more after any job that drains the cooler lines or the pan. There is no dipstick on this transmission; level gets set at the check plug per the Service Information procedure at the specified fluid temperature, not by a fixed quart count poured in and called good.

Repair order that actually saves time

1. Clean the whole underside of the transmission and the transfer case joint with mineral spirits or an equivalent degreaser. A fresh leak cannot hide from a clean, dry surface.
2. Drive the truck a short distance (or idle it to operating temperature) and inspect cold, then recheck after the drive. Some leaks only show up once the fluid is hot and pressurized.
3. Check the cooler line fitting bolt on the left side of the case first — it is visible without removing anything.
4. Use a light between the crossmember and the case to check the two rear case plugs.
5. If the truck is AWD/4WD and the wet area is further back, clean with mineral spirits and separately inspect the output seal/boot and the propeller shaft welch plug per 22-NA-225's method.
6. If none of the above are wet and the fluid is tracking down from the pan rail, it is the pan gasket and filter — drop the pan last, since it is the most labor-intensive of the four.
7. Repair only the confirmed source, refill with DEXRON ULV to the correct level at the check plug, and road test before calling it done.

FAQ

Where does a 10L80 leak fluid most often?

GM has published bulletins for three specific spots: the oil cooler line fitting retaining bolt on the left side of the case (bulletin 18-NA-026), one or both rear case plugs (bulletin PIP5872), and the rear output seal where the transmission meets the transfer case, which GM distinguishes from a leaking propeller shaft welch plug in the same area (bulletin 22-NA-225). A worn pan gasket or a dropped filter O-ring is the fourth common point, and it is not tied to a bulletin because it is ordinary wear, not a design issue.

How do I tell a transmission leak from a transfer case leak at the back of a 10L80 truck?

Per GM bulletin 22-NA-225, clean the area with mineral spirits first, then look at two specific points: the output seal and boot where the shaft enters the case, and the welch plug on the propeller shaft's front slip yoke just behind it. If one is dry and the other is wet, you have your answer. If both look wet, dry everything with a shop rag and drive the truck around the block a few times to let the leak show itself again, then recheck both points before you order parts.

Is a leak at the front of a 10L80 case always the cooler line fitting?

It is the documented cause on 2017-2019 10L80 and 10L90 trucks and SUVs per GM bulletin 18-NA-026, where porosity in the casting near the oil cooler line retaining bolt passage lets fluid seep past the bolt. Wipe the area dry and recheck at the bolt head specifically. If it is wet there, GM's fix is a new cooler line fitting seal and a bolt reinstalled with blue threadlocker, not a case replacement.

Do the rear case plugs on a 10L80 need to be replaced as a pair?

No. GM's bulletin PIP5872 is explicit on this: only replace the plug that is actually leaking. Use a light from the right side of the transmission, between the crossmember and the case, to identify which of the two plugs is wet. Pulling a plug that is not leaking just adds a sealing joint that did not need to be disturbed.

What fluid does a 10L80 take and how much after a leak repair drains it?

GM specifies DEXRON ULV, part number 19352619 in the US, and nothing else. The published fluid change capacity is 7.7 liters, or 8.1 quarts, across the MQB, MQC, MHT, MI2 and MHS RPO codes. That figure is the change capacity, not a dry fill, so top off at the check plug per the Service Information procedure rather than dumping in a fixed amount and calling it done.

Need parts for this job?

Filter and pan gasket kits, OE-style cooler lines and the fill tools to refill it correctly are on our 10L80 transmission parts page, priced and in stock, with checkout on our own site. Match the part to the confirmed leak point above before you buy — the four repairs covered here do not share parts.