6L90 Silverado 2500 Slipping: Causes and Fix

A gas Silverado 2500HD that slips almost never does it in the driveway. It slips at 60 mph on a grade with eight or ten thousand pounds on the hitch, the tach climbs 300 rpm, and the truck feels like it is in neutral for a second before it catches. By the time it reaches the shop, it drives fine empty. That is why a slipping 2500HD needs its own diagnostic plan: the load that makes it slip is the load you cannot reproduce in the bay, so you have to get the answer out of the scan data, the gear pattern, and a pressure gauge.

This page stays on the heavy-duty gas trucks. For the 6L90 in general (vans, Suburbans, half-tons), read 6L90 slipping causes and fix first. Here we cover what towing does to this unit, which clutch it usually takes out, and the GM bulletins that apply to the HD trucks.

Recommended 6L90 parts

Matched to a towing slip: a master rebuild kit for a burnt 4-5-6 or 3-5-R pack, a bonded piston set for pistons that leak once they have been cooked, and a cooler bypass kit to keep fluid temperature down on long pulls. Secure checkout.

6L90 Master Rebuild Kit

$314.99View part

6L80E/6L90 Bonded Clutch Piston Set, 5-Piece 2006-Up

$201.99View part

Superior STL010 Sure Cool Cooler Bypass Upgrade Kit for GM 6L80 / 6L90 / 8L90

$71.99View part

Which 2500HD Trucks This Covers

Read the RPO label in the glove box and look for MYD. That is the 6L90. GM's 2016 Silverado HD specification sheet pairs the Hydra-Matic 6L90 with the Vortec 6.0L gas V8 and puts the Allison 1000 behind the Duramax. When the HD trucks were redesigned for 2020, GM TechLink lists the new 6.6L gas V8 (RPO L8T, 401 hp and 464 lb-ft) with the 6L90 six-speed, RPO MYD, while the Duramax moved to the Allison 10L1000 ten-speed. If the truck is a diesel, nothing here applies.

That L8T number matters. GM's own TechLink writeup says the 6.6L makes 22 percent more torque than the 6.0L it replaced, and it is still feeding a six-speed with the same basic clutch layout. More torque through the same clutch packs, pulling heavier trailers, is exactly the recipe that finds a weak clutch or a pressure problem.

Use the Gear Pattern to Name the Clutch

The 6L90 is clutch-to-clutch. No bands, and every gear is held by two clutches, so the gears that slip tell you which clutch is giving up. GM's gear ratios for this unit are 4.03, 2.36, 1.53, 1.15, 0.85 and 0.67, with Reverse at 3.06.

• Slips in 4th, 5th and 6th, first through third are fine: 4-5-6 clutch. This is the classic towing failure.
• Slips in 3rd, 5th and Reverse: 3-5-R clutch, or the feed into it.
• Slips in 1st through 4th, 5th and 6th are fine: 1-2-3-4 clutch.
• Slips in 2nd and 6th: 2-6 clutch.
• Slips everywhere, worse hot: not one clutch. Fluid level, pump, line pressure, or a converter that is not locking.

Catch It on the Scan Tool Under Load

You need data recorded while the truck is actually working. Hook up a scan tool that can log, and if you cannot hitch the customer's trailer, a loaded test drive up the steepest grade nearby is the next best thing. Record these parameters:

• Input shaft speed and output shaft speed. Divide input by output. In 5th it should sit at 0.85, in 6th at 0.67. If you are in 6th at 2,000 rpm output speed, input should read about 1,340 rpm. Input at 1,500 with the same output speed is a clutch slipping, not a tire or a converter.
• Commanded gear and attained gear. Slip only matters against what the TCM thinks it is holding.
• TCC slip speed. With the converter commanded on, a converter that is really locked holds low, steady slip. Large slip with the converter commanded on is a converter or TCC apply issue, and it feels like a transmission slip to the driver. Sort that out first; our 6L90 converter shudder guide covers it.
• Transmission fluid temperature. Note where the slip starts. A unit that only slips after fluid temp climbs on a long pull is telling you about clutch capacity at hot, thin fluid, or about a cooling problem.

Pull codes too. GM's PI1393B ties a distressed, slipping 4-5-6 clutch to P0796. Bulletin 09-07-30-004J lists P0776, P2715 and P2723 with slip in Reverse, slip in fifth, and slip "on acceleration when passing" on early trucks with a cracked 1-2-3-4 / 3-5-R housing. Those code numbers point you to the right clutch circuit. They do not tell you whether the clutch burned or the pressure feeding it was low, and that is the next step.

Check the Heat Path Before the Clutches

On a towing truck, heat is the first suspect. Two GM bulletins cover the HD trucks directly:

• 22-NA-182 lists 2021 Silverado and Sierra 2500HD/3500HD with the L8T and MYD among the trucks that can see harsh shifts, shudder, surge, neutral or flare, and overheating from a twisted cooler line, usually on low-mileage trucks. Look at the rubber section of the cooler lines, and pull the under-body panels if you have to. A twisted line that will not hold its shape gets replaced.
• The same bulletin states the 6-speed's external thermal bypass valve opens at 158 F (70 C). Bulletin 21-NA-199 covers 2020-2021 Silverado and Sierra 2500HD/3500HD and lists "slip" among its conditions. The revised valve is stamped "70" and fully opens at 158 F instead of the old 194 F (90 C). The HD part number is 85547828, and GM says to install it once during any transmission repair.

Then check the level. It has to be set hot, at the check plug, with the procedure in service information. GM's capacity table lists the 6L90 (MYD) at 5.7 L / 6.0 qt for a pan and filter service using DEXRON VI. A truck that is a quart low may never slip empty, but it can pull the filter dry on a grade with fluid hot and expanded. If the fluid is dark, smells burnt, or has friction material in the pan, you are already past the point where a fluid service fixes it.

Put a Gauge on Line Pressure

This is the step that separates a clutch problem from a supply problem, and on these trucks it often turns out to be supply. GM's PI1393B tells technicians to check line pressure as part of diagnosis. Its reasoning: when converter clutch lining comes loose, it passes through the pump, scores the pump pocket or cover, and drops line pressure. The bulletin says that low pressure distresses clutches and names the 4-5-6 clutch as the one most likely to fail. If line pressure is below spec, GM says the pump and the clutches have to come apart for inspection.

Sonnax's 6L45-6L90 tech article gives you two readings to look for on the gauge:

• Line pressure stuck around 60-68 psi with no rise as rpm and throttle go up points to the line pressure control solenoid in the TEHCM stuck in the exhaust position. A towing truck that cannot raise line pressure under load will slip the 4-5-6 clutch every time.
• Pump pressure under 30 psi at idle can be a missing boost valve roll pin, which is a rebuild assembly error.

Compare your readings with the line pressure table in service information for the truck's year. The point is that pressure has to rise with throttle. If it does not, a new clutch pack will burn just like the old one.

When It Really Is the 4-5-6 Clutch

Once the heat path, the fluid level and line pressure check out, slip in 4th, 5th and 6th on a high-mileage tow truck is a worn 4-5-6 pack. Sonnax has written about why this clutch fails on the 6L80 and 6L90. Their 4-5-6 piston kit page says there are not enough clutches and too little piston-to-plate apply area, and that the backing plate flexes and bows ("coning"), which Sonnax calls a primary cause of clutch failure. Their 104960-10K kit uses a solid backing plate and replaces the OE waved plate with a toothed apply plate. That makes room for one more friction (8 in a 6L90) and raises 4-5-6 capacity by 17 percent.

Be honest with the customer about the tradeoff. Sonnax says the kit is intended for tuned applications and may give a firmer 4-5-6 apply in a truck running the factory calibration. On a dedicated tow rig most owners take that trade. On a daily driver that tows twice a year, a correct OE-spec pack with good pressure is usually enough.

If the 4-5-6 clutch is burnt, do not stop at the clutch. Per PI1393B, inspect the pump for scoring in the pocket or on the cover. If converter lining is the cause, replace the converter and the filter, then flush and flow test the cooler as service information describes. Skip any of that and the debris is still in the circuit when the new pack goes in.

When It Is the 3-5-R Side

Slip in 3rd, 5th and Reverse, especially on a 2007-2008 truck, sends you to bulletin 09-07-30-004J. It covers 2007-2008 Silverado and Sierra with the 6L90 and lists slip in Reverse, delayed Reverse, harsh 2-3, harsh 4-5 or slip in fifth, and slip when passing. The cause is a cracked weld on the 1-2-3-4 / 3-5-R clutch housing. The test is simple: brush soap on the weld, hold a thumb over the bleed passage, blow air into the 3-5-R feed, and look for bubbles. Any bubbles mean a new housing. GM sells complete housing assemblies with the pistons and clutches already installed.

Doing the Repair So It Lives Behind a Trailer

• Gauge it before and after. Line pressure has to rise with throttle before the truck goes back on a trailer.
• Replace the bonded pistons in any overhaul of a heat-damaged unit. A piston that leaks under heat makes the same symptom as a worn pack.
• Converter, filter, and cooler flush whenever there is friction material in the pan (PI1393B).
• Install the "70" TBV (85547828 on HD trucks, per 21-NA-199) and check the cooler lines for twists (22-NA-182).
• DEXRON VI only, level set hot at the check plug.
• Tell the owner to use Tow/Haul and to watch transmission temperature on long grades. Downshifting before the truck lugs is cheaper than a 4-5-6 pack.

FAQ

Why does my Silverado 2500 slip only in 5th and 6th when towing?

5th and 6th both use the 4-5-6 clutch, and so does 4th, so slip in those gears and nowhere else points at that clutch or the pressure feeding it. GM bulletin PI1393B names the 4-5-6 clutch as the one most likely to be distressed when line pressure is low, and a slipping 4-5-6 clutch can set P0796. Check fluid level and line pressure rise before replacing the pack.

Can low fluid make a 6L90 slip only when towing?

Yes. A unit that is a quart low can drive normally empty and then starve the pump on a grade with the fluid hot. GM lists the 6L90 (MYD) at 6.0 quarts of DEXRON VI for a pan and filter service, and the level has to be set hot at the check plug per service information.

Is P0796 always a burnt 4-5-6 clutch?

No. P0796 tells you the 4-5-6 circuit is not holding, not why. Per GM PI1393B, low line pressure from a pump scored by loose converter lining can cause the clutch distress that sets it, so the pump, line pressure and converter have to be checked or the new clutch will fail too.

Will the new 70 thermal bypass valve fix a slipping 2500HD?

It can fix slip caused by heat, not slip from a burnt clutch. GM bulletin 21-NA-199 lists slip among the conditions for 2020-2021 Silverado and Sierra 2500HD/3500HD and replaces the bypass valve with one that fully opens at 158 F instead of 194 F, HD part number 85547828. If the clutches are already burnt, install the valve during the repair, but it will not repair the pack.