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A shift flare is the engine RPM jumping up for a beat during a shift instead of dropping straight into the next gear. On a 6L90 you usually catch it on the 2-3 or 3-4 upshift with a trailer on, or on a part-throttle 4-5 in a loaded Express van.
Before you buy anything, understand what a flare actually is: the off-going clutch let go before the on-coming clutch was holding. That is either a hydraulic or electrical problem (fluid level, pressure switches, solenoids, valve bores, line pressure) or a friction problem (a clutch pack that is worn, burned or has a broken wave plate). The first gets fixed in the vehicle. The second does not.
One fitment note: most Tahoes, Yukons and Camaro SS automatics got the 6L80, not the 6L90. Check the RPO sticker in the glovebox or door. MYD is the 6L90, MYC is the 6L80.
Which Clutch Is Involved on Each Shift
The 6L90 is clutch-to-clutch with five clutches: 1-2-3-4, 3-5-R, 4-5-6, 2-6 and low/reverse, plus a low one-way clutch in first. Two clutches are applied in every forward gear. The commonly published apply chart, which you should still confirm against GM service info for your year:
| Gear | Clutches Applied |
|---|---|
| 1st | 1-2-3-4 + low one-way (low/reverse applied for engine braking or manual 1) |
| 2nd | 1-2-3-4 + 2-6 |
| 3rd | 1-2-3-4 + 3-5-R |
| 4th | 1-2-3-4 + 4-5-6 |
| 5th | 3-5-R + 4-5-6 |
| 6th | 2-6 + 4-5-6 |
| Reverse | 3-5-R + low/reverse |
So a 2-3 flare is 2-6 releasing and 3-5-R applying. A 3-4 flare is 3-5-R releasing and 4-5-6 applying. A 4-5 flare is 1-2-3-4 releasing and 3-5-R applying. A 5-6 flare is 3-5-R releasing and 2-6 applying. If the 2-3 and 4-5 both flare and reverse engagement is slow or soft, the 3-5-R circuit is the first suspect. That is the clutch we see fail most on these units, often with a broken wave plate in the pan.
Step One: Check the Fluid Level the GM Way
There is no dipstick. A unit that is a quart low aerates on hard stops and grades, and clutch apply goes soft. Do it right:
- Get the fluid into the temperature window GM lists for your vehicle, read on a scan tool.
- Park on level ground, engine running, cycle through every range, back to Park.
- Pull the check plug on the case. A thin dribble means the level is correct. Nothing means low.
- Fill with Dexron VI only. A GM ATF filler adapter ($36.99) makes adding fluid through the fill port a clean job instead of a mess.
Smell what comes out. Burnt fluid or black friction material in the stream means the clutches are already damaged, and no in-vehicle part will save the unit.
Step Two: Scan Tool, Codes and Fast Learn
Read the TCM directly. P0700 on an engine scan only tells you the TCM is holding a code. The ones that matter here:
- P0731-P0735, P0729, P0736: incorrect gear ratio in 1st through 5th, 6th and reverse. Any of these means measured slip.
- P0751 / P0756: shift solenoid A or B performance. On this unit those usually trace back to the control solenoid valve body or a stuck valve, not a dead coil.
- P0776 / P0777: pressure control solenoid B performance or stuck on. Same neighborhood.
Then run the GM Transmission Adaptive Values Learn, also called Service Fast Learn, with a scan tool that supports it. It clears the stored clutch adapts, runs the clutch fill tests, and then the TCM finishes learning over a drive cycle of light-throttle upshifts. Fast learn is mandatory after a TCM, valve body or transmission replacement. If the flare disappears after a fast learn and creeps back within a few hundred miles, you just proved a friction problem.
The Usual Suspects
Control Solenoid Valve Body (TEHCM)
The TCM, solenoids and pressure switches live in one assembly bolted to the valve body. Pressure switch failures are common, especially on high-mileage HD trucks and vans that live hot, and they give the TCM wrong apply information. If the module tests good electrically, a pressure switch repair kit ($71.99, shared 6L80/6L90 TEHCM part) is the cheap fix. If the module is internally bad, the 6L90 control valve body with TCM ($352.99) replaces it. Plan on programming it to the VIN before the truck will drive right.
Boost Valve and Pressure Regulator Bore Wear
When the pressure regulator valve and its boost valve wear their bore, line pressure drops off under load and every apply gets slower. The 6L80/6L90 pressure regulator valve kit ($100.99) restores it. A TransGo SK6L80 shift kit ($168.99) goes further and corrects factory hydraulic weak points. Neither fixes burned frictions.
Worn Clutches and Wave Plates
The 3-5-R and 4-5-6 packs take the most abuse on towing rigs and vans. Broken wave plate pieces in the pan, combined with a ratio code, is a guaranteed teardown.
Symptom to Fix: Quick Reference
| Symptom | Likely Cause | Fix | Parts Cost |
|---|---|---|---|
| Flare on hard stops or grades, comes and goes | Low or aerated fluid | Check level at plug, top off with Dexron VI | $37 adapter + fluid |
| Flare after battery, TCM or fluid work | Adapts lost or wrong | Service Fast Learn | $0 |
| P0751/P0756, flare worse hot | TEHCM pressure switches | Pressure switch repair kit | $72 |
| P0776/P0777, erratic shifts | Control solenoid valve body | Valve body with TCM, program, fast learn | $353 |
| Flares worse under heavy throttle | Pressure regulator / boost valve wear | Pressure regulator kit or shift kit | $101-$169 |
| 2-3 and 4-5 flare, soft reverse | 3-5-R clutch or wave plate | Rebuild | $480 kit |
| 3-4 flare, P0735 or P0729 | 4-5-6 clutch worn | Rebuild | $480 kit |
Repair vs Rebuild
Clean fluid, correct level, no ratio codes, and a flare that a fast learn improves: fix it in the vehicle. Pressure switches, the regulator kit, or the TEHCM, then a fast learn.
Burnt fluid, friction material or wave plate pieces in the pan, or a ratio code that returns after a fast learn: the transmission is coming out. On the bench, a 6L80E/6L90E master rebuild kit with steel clutch plates ($479.99) covers frictions, steels, seals and gaskets. Replace the steels too; heat-checked steels glaze new frictions. Once you add labor to pull a 6L90 out of an HD truck or a van, a shop rebuild typically lands in the several-thousand-dollar range.
6L90 Parts, Ships Same-Day
Valve bodies, pressure switch kits, pressure regulator kits, shift kits and master rebuild kits for Silverado HD, Express/Savana and the rest of the 6L90 lineup. For complete rebuild kits and hard parts, our sister store Core Transmission Parts carries the full line.
Shop 6L90 Parts →FAQ
Will a fast learn fix my 6L90 shift flare?
Only if the flare started after a battery disconnect, a TCM or valve body replacement, a fluid service, or a tune change. Fast learn clears the stored clutch adapts and lets the TCM relearn fill times. It cannot make a worn 3-5-R or 4-5-6 clutch pack hold. If the flare comes back within a few hundred miles, the hardware is the problem.
Which clutch causes a 6L90 flare on the 3-4 shift?
On the 3-4 shift the 3-5-R clutch releases and the 4-5-6 clutch applies, so a 3-4 flare points at the 4-5-6 clutch or its feed circuit. If you also get slip codes in 5th and 6th, such as P0735 or P0729, the 4-5-6 pack is the likely culprit, because it is applied in 4th, 5th and 6th.
How do I check 6L90 fluid level without a dipstick?
The 6L90 has no dipstick. Bring the fluid into the temperature window listed in GM service information for your vehicle (read it on a scan tool), park level, engine running, cycle the selector through the gears, return to Park, then remove the check plug on the transmission case. Fluid should dribble out in a thin stream. If nothing comes out, add Dexron VI through the fill port until it does.
What codes come with a 6L90 flare?
Often none at first. As the slip gets worse the TCM sets ratio codes: P0731 through P0735 for 1st through 5th, P0729 for 6th and P0736 for reverse. P0751, P0756, P0776 and P0777 are solenoid and pressure control performance codes that point at the control solenoid valve body first. P0700 only means the TCM has a code stored, so read the TCM directly.
How much does it cost to fix a 6L90 flare?
Parts run from about $37 for a fill adapter, $72 for a pressure switch repair kit, $101 for a pressure regulator valve kit and $169 for a shift kit, up to $353 for a control valve body with TCM. If the clutches are burned, a master rebuild kit is about $480 in parts, and a shop rebuild on an HD truck or van typically runs several thousand dollars with labor.