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A shift flare is the engine RPM rising for a split second during a shift instead of dropping cleanly into the next gear. On an 8L90 it usually shows up on the light-throttle 1-2, 2-3 or 3-4 upshift, or on a part-throttle downshift when you roll back into the pedal. With eight gears packed close together, the TCM shifts constantly, so a small flare gets noticed fast.
The plain truth: a flare means the off-going clutch let go before the on-coming clutch was holding. That is either a hydraulic or control problem (fluid level, solenoid, valve body, sealing rings, software) or a friction problem (worn clutch plates that take more fill volume than the TCM has learned). The first group can mostly be fixed with the transmission in the vehicle. The second cannot. Sort out which one you have before buying parts.
How the 8L90 Shifts
The 8L90 and its lighter-duty sibling the 8L45 are clutch-to-clutch transmissions with no one-way clutches doing the timing for you. There are five clutches, labeled A through E. As generally described, A and B are holding clutches (brakes) and C, D and E are rotating clutches, with three of the five applied in each forward gear. Confirm the exact apply chart against the service manual for your model year.
What that means for a flare: every shift is a handoff the TCM times with pressure control solenoids. If any part of that chain is late, soft or leaking, you get a flare. If the same clutch is on-coming in every shift that flares, that clutch circuit is your first suspect.
Step One: Fluid Level at Temperature
The 8L90 has no factory dipstick, and that is why so many of them are run low or overfilled. A low unit aerates under hard braking and turns, and aerated fluid makes clutch apply inconsistent. Check it properly:
- Park on level ground, engine running, shift through every range, then back to Park.
- Watch transmission fluid temperature on a scan tool and wait until it is inside the window the service information specifies. Checking cold reads wrong.
- Remove the check plug on the case. Fluid should just dribble out. Nothing coming out means low.
- Add fluid through the fill port until it dribbles, then reinstall the plug. A GM ATF filler adapter ($36.99) makes this a one-person job.
On fluid: GM filled these with Dexron VI originally, then issued bulletins calling for Mobil 1 Synthetic LV ATF HP (now marketed as Dexron HP), mainly to cure torque converter shudder. Check the bulletins for your VIN before you buy fluid. If the fluid is overdue, a filter and steel pan gasket kit ($86.99) lets you see what is in the pan. Fine gray paste is normal. Black friction material and bright metal are not.
Step Two: Codes and Scan Data
Pull codes before anything else. The incorrect gear ratio codes start at P0731 (1st) and run through P0735 (5th); higher gears and reverse have their own ratio codes, so read the full description on your scan tool rather than guessing. Nearby codes in the P0736-P0738 range can be reverse ratio or TCM speed output circuit faults depending on the description. A ratio code with no electrical codes leans toward friction or sealing ring leakage. Pressure control solenoid circuit codes lean toward the TEHCM.
Next, look at the adaptive data. A GDS2-type scan tool shows clutch fill and pressure adapts. One clutch sitting at the end of its range while the others look normal is the circuit to chase.
The Usual Suspects
TEHCM and Pressure Control Solenoids
The TEHCM (transmission electro-hydraulic control module) is the TCM and solenoid assembly mounted to the valve body. Its pressure control solenoids meter apply pressure to each clutch, and a sticky or contaminated solenoid gives a late, soft apply that is often worse hot. A 7-piece 8L90 valve body solenoid kit ($238.99) covers the solenoids without buying a whole TEHCM. If the TCM itself is bad, a replacement unit needs programming with GM software, so plan for that.
Valve Body Wear
Bore wear leaks regulated pressure, and early 8L90 valve bodies are known for it. The STL-8L90 valve body shudder fix kit ($255.99) is aimed mainly at converter clutch and regulation wear; read the kit instructions to see which circuits it covers. An 8L90 shift kit ($230.99) is the other common fix. Both help healthy units and neither fixes burned frictions.
Sealing Rings on the Input Drum
This one gets missed. Apply oil for the rotating clutches crosses sealing rings on the input drum and shafts. Worn or nicked rings bleed off pressure, and the flare usually gets worse as the fluid heats and thins. You will only confirm it on the bench. Replace rings with the 8L90 sealing ring expander and resizer kit ($127.99), and if the drum ring grooves are worn, a new 8L90 input drum ($159.99) is the real fix.
Worn Clutches
Burnt fluid, friction material in the pan and adapts at their limit means the plates are done. Tuned 6.2L trucks and supercharged Camaro and Corvette builds wear clutches faster because the stock pressure tables were never sized for that torque.
Symptom to Fix: Quick Reference
| Symptom | Likely Cause | Fix | Parts Cost |
|---|---|---|---|
| Flare comes and goes, worse on hard stops or turns | Low or aerated fluid | Check level at temp via check plug, top off | $37-$87 |
| Flare started after battery, fluid or parts work | Adaptive values reset or wrong | Clear adapts and run fast learn | $0 |
| Intermittent flare, worse hot, solenoid codes | Pressure control solenoid / TEHCM | Solenoid kit, or TEHCM with programming | $239+ |
| Soft shifts plus converter shudder | Valve body wear, old fluid | Valve body fix kit or shift kit, fresh fluid | $231-$256 |
| Flare gets worse as fluid heats | Sealing ring or drum leakage | Bench repair: rings and drum | $128-$160 |
| Ratio code, burnt fluid, debris in pan | Worn clutches | Rebuild with clutch module or overhaul kit | $279-$833 |
Adaptive Reset and Fast Learn
With a GDS2-type scan tool, clear transmission adaptive values, then run the service fast learn. The tool walks you through the conditions. This is required after a TEHCM, solenoid or valve body job, and worth doing after a battery disconnect or fluid change.
Fast learn does not fix hardware. If the flare clears and comes back within a few hundred miles with the same clutch adapt climbing, you just confirmed a friction or sealing problem.
Repair vs Rebuild
Clean fluid, normal adapts and electrical codes: fix it in the vehicle. Fluid level, a solenoid kit and a fast learn is a weekend job.
Burnt fluid, debris in the pan, or one clutch at its adaptive limit: the transmission is coming out, and a solenoid kit will not save it. For the frictions, use the 8L90 clutch module ($278.99) or the BorgWarner 35445KIT friction plate kit ($143.99). For a full refresh, the 2015-2017 8L90 overhaul kit ($502.99) or 2015 master rebuild kit ($559.99). A shop rebuild with removal and install usually runs several thousand dollars, which is why checking fluid and adapts early pays off.
8L90 Parts, Ships Same-Day
Solenoid kits, valve body fix kits, filters, sealing ring tools, drums, clutch modules and overhaul kits for GM 8-speed trucks, SUVs and performance cars. For complete rebuild kits and hard parts, our sister store Core Transmission Parts carries the full line.
Shop 8L90 Parts →FAQ
Will a fast learn fix my 8L90 shift flare?
Only if the flare showed up after a battery disconnect, a fluid service, a TEHCM or solenoid replacement, or a software update. Clearing adaptives and running the fast learn lets the TCM relearn clutch fill. It cannot make a worn clutch or a leaking sealing ring hold. If the flare is back within a few hundred miles, the problem is hardware.
Does an 8L90 flare set a code?
Not right away. The TCM keeps adding fill time and pressure until it runs out of adaptive range. Once slip gets bad enough you will see incorrect gear ratio codes such as P0731 through P0735, and the transmission may go into a default gear.
How do you check 8L90 fluid level?
There is no factory dipstick. With the vehicle level and the engine running, bring the fluid into the temperature window the service information specifies, read on a scan tool, then remove the check plug on the case. Fluid should just dribble out of the hole. If nothing comes out, it is low. Add fluid through the fill port until it does.
Which fluid does the 8L90 use now?
GM originally filled the 8L90 and 8L45 with Dexron VI. GM later released bulletins calling for Mobil 1 Synthetic LV ATF HP, now sold as Dexron HP, mainly to address torque converter shudder. Check the bulletins and service information for your VIN before you buy fluid, and do not mix the two casually.
How much does it cost to fix an 8L90 flare?
Parts for the common fixes run from about $87 for a filter and pan gasket kit, $239 for a valve body solenoid kit, $256 for a valve body shudder fix kit, and $128 for a sealing ring tool kit, to $279 for a clutch module and $503 to $833 for an overhaul kit. A full shop rebuild with removal and installation usually lands in the several-thousand-dollar range.