How to Test a 6L90 Valve Body Before You Replace It

Your 6L90 is banging into gear, flaring on the 2-3, throwing a solenoid performance code, or shuddering at highway speed. Somebody told you it is the valve body. Maybe it is. But a 6L90 valve body is not a cheap guess, and a meaningful share of the complaints that get blamed on the valve body are actually coming from the pump, the TEHCM, a separator plate mismatch, or a checkball that fell out of its pocket during a previous rebuild.

The part for this fix

6L90 Control Valve Body with TCM 2006-2017 OEM 24275873

$352.99 · In stock

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This is how to test the valve body first, so you are buying a part because you proved you need it, not because you ran out of ideas.

What valve body wear actually looks like on a 6L90

The 6L90 shares its valve body family with the 6L45, 6L50 and 6L80, so the wear pattern is well documented. Two bores account for most of the complaints.

Actuator feed limit (AFL) valve. Sonnax states that GM 6L45, 6L50, 6L80 and 6L90 units often suffer from shift abnormalities caused by excess wear in the actuator feed limit bore, and that this wear prevents proper regulation of the oil pressure supplied to the control solenoids. The result, per Sonnax, is harsh or soft shifts, solenoid codes, wrong gear starts, and clutch or band failure. If your 6L90 shifts differently every time you drive it, this bore is the first suspect.

TCC regulator valve. Sonnax lists the symptoms of wear in this bore as overheated fluid, failsafe mode, harsh shifts, harsh TCC apply, loss of fuel economy, high TCC slip RPM, and codes P0741 and P0742. Sonnax also explains the split: if the valve sticks in the apply position you get harsh TCC apply, and if it sticks in the release position you get no TCC apply or TCC slip. That distinction matters, because a shudder and a bang can be the same failed bore behaving two different ways.

Why do these bores wear at all? Because the tolerances are microscopic. In its valve body inspection guide, Sonnax gives the specification for total diametrical valve spool-to-bore clearance as .0005 in. minimum to .0016 in. maximum, and puts that in perspective: a human hair is typically .0034 in. and a piece of paper is .0045 in. A contamination particle larger than that clearance can embed itself in the aluminum and score the bore. Once the bore is oval, the valve leaks, and no amount of fresh fluid fixes it.

Step 1: prove it is not electronic or software

Before anything comes apart, rule out the cheap stuff. Sonnax notes that during OE programming or a relearn, a temporary no-move condition can occur, and the fix is simply to remove the scanner, take the key out of the ignition, open and close the driver door, wait 15 to 20 minutes and return. Sonnax also warns that an incorrect part number or an incorrectly programmed TEHCM can cause a no-movement or no-upshift condition, and that when replacing a TEHCM you should always verify its part number and program file match the VIN.

The TEHCM is also a common source of shift-quality complaints that get pinned on the valve body. Sonnax notes that a common fault with the TEHCM is failed pressure switches, and that these can be checked and repaired with a pressure switch rebuild kit rather than replacing the entire TEHCM.

Step 2: read line pressure, because the pump is not the valve body

This is the single most important test on the list, and the one people skip.

Sonnax is explicit about this on its 6L90 remanufactured valve body listing: the main pressure regulator valve lives in the pump on these transmissions, not the valve body. Shift, clutch and performance conditions caused by wear at the pressure regulator valve bore or its associated boost valve cannot be corrected by replacing the valve body. The TCC control valve and converter limit valve also live in the pump, and wear there can result in converter failure. Sonnax recommends checking those pump bores for wear while the transmission is being serviced.

Two line-pressure fingerprints from Sonnax that point away from the valve body:

• No line pressure rise at all, pressure sitting around 60 to 68 psi with no rise as RPM comes up. Sonnax attributes this to a TEHCM line pressure solenoid stuck in exhaust mode, and notes that unplugging it makes no difference.
• Pressure below roughly 30 psi at idle. Sonnax attributes this to a missing pressure regulator boost valve roll pin in the pump. There may be some movement, but it will not be normal.

Sonnax also gives a useful in-vehicle check: the pump TCC control valve can be air checked with the valve body removed. A valve stuck against its spring blocking apply oil is usually a no-movement condition, and depending on where it sticks it can also produce a shudder during TCC apply or when coasting to a stop.

Step 3: check for assembly and identification errors

If the transmission has been apart before, the valve body may be innocent and simply installed wrong. Sonnax lists several conditions where line pressure reads normal but the vehicle still misbehaves:

• A Type 2 separator plate on a Type 1 valve body causes a no-Forward condition. Early Type 1 valve bodies cannot be updated to the later eight-checkball plate; the correct plate, #24245720, uses seven checkballs.
• That same Type 1 plate #24245720 installed on a Type 2 valve body causes no 3rd and no Reverse.
• Clutch select valves or springs installed backwards cause no movement. Sonnax specifies valves go in first with springs outboard.
• A shrunken or displaced No. 1 checkball causes no Forward; a displaced No. 5 checkball causes no Reverse, sometimes intermittently.
• A stuck 1-2-3-4 clutch regulator valve causes no Forward, and the 3-5-R clutch regulator valve is known to stick and cause no Reverse plus shift complaints.
• A 6L90 valve body installed on a 6L80 is a no-Reverse condition.
• A TEHCM filter gasket left out during assembly causes no movement, typically with no or low line pressure.

Identification matters here. Sonnax warns that GM and BMW 6L45 through 6L90 early- and late-style units cannot be interchanged but look very similar. Its identification guide breaks the family on a 2010 change, and uses the machined-down boss on the upper casting to identify the unit, with the D boss indicating MYD/6L90. The 6L80 and 6L90 are further separated by the center support feed holes: offset front for the 6L80, offset rear for the 6L90. Sonnax notes the boss system is not foolproof, since some valve bodies have no bosses machined down at all.

Step 4: vacuum test the bores

This is the test that turns opinion into a number. Sonnax describes vacuum testing as the way to properly diagnose bore wear, and calls it quick, quantitative, reliable and economical: tests return a specific value, so you can set your own pass/fail standard for each bore, and the results are repeatable when the routine is followed.

For this family, Sonnax makes vacuum test plate kit 104740-VTP, which covers the 6L45, 6L50, 6L80 and 6L90. The plate and seal fit over the entire valve body casting to seal all critical passages at once, with test ports open at the recommended locations so you can move a vacuum tip from bore to bore. It runs on a vacuum test stand, and Sonnax publishes free vacuum test guides that identify the test locations for the family.

Sonnax is clear that there is no universal pass number: you record readings, calculate an average for each bore, then set a minimum standard that fits your warranty and your customer. For a truck owner, the practical version is simpler. If the AFL and TCC regulator bores read materially worse than the rest of the same casting, you have found your problem.

If you do not have a vacuum stand, this is the step to hand to a shop. It is a short bench operation once the valve body is off, and it is the difference between a diagnosis and a guess.

Step 5: one quick shortcut on the AFL circuit

Sonnax offers a fast sanity check for the actuator feed limit valve: determine whether the transmission shifts through the gears on the rack. If it shifts, that is generally an indication it is not an AFL valve issue.

What the fix looks like once you have an answer

If the vacuum test says one or two bores are worn and the rest of the casting is sound, a targeted repair is on the table. Sonnax makes drop-in kits for exactly these bores: TCC regulator valve kit 104740-46K, whose valve lands have been lengthened so the valve operates in unworn portions of the casting, and actuator feed limit valve kit 104740-47K, a sleeve-and-valve assembly that provides a new unworn bore while the sleeve seals off existing moderate wear. There is also an oversized AFL valve, 104740-12, for bores that get reconditioned.

If multiple bores are worn, the casting has been apart repeatedly, or you simply do not want to chase it valve by valve, a replacement valve body walks away from every bore at once. Sonnax describes its 6L90 remanufactured unit as designed to address burnt clutches, codes P0751, P0741 and P0742, harsh TCC apply, bump, harsh or flare shifts, bind-ups, solenoid performance codes and wrong gear starts, and says each unit is tested to verify that actuator feed limit pressure, clutch pressures, torque converter pressures and compensator feed pressures match OE specification. Note that Sonnax sells that particular unit without the TEHCM, so confirm what any replacement valve body does and does not include before you order.

Replace while you are in there

The valve body is behind the pan, so the incremental cost of these items is close to nothing once the job is started:

• Valve body to case gasket and the TEHCM filter gasket. Sonnax specifically calls out a missing TEHCM filter gasket as a no-movement cause, so do not reuse a questionable one.
• Transmission filter and pan gasket.
• Separator plate, verified against the correct type for your casting.
• Checkballs, inspected for shrinkage and correct position.
• TEHCM pressure switches, if the shift-quality complaint pointed there.
• Pump pressure regulator and boost valve inspection, since Sonnax notes wear at those bores cannot be corrected by a valve body swap.

Effort and difficulty, honestly

Dropping the pan and pulling a 6L90 valve body is an in-vehicle job on most applications, but it is not a beginner job. The TEHCM comes out with the valve body, fluid goes everywhere, bolt torque and sequence matter, and the identification traps above mean a wrong part turns a one-day repair into a two-week one. Add the relearn window on the back end: Sonnax notes a temporary no-move condition can follow programming, cured by keying off, cycling the driver door and waiting 15 to 20 minutes before you panic.

The real cost lever here is not labor. It is buying the right part once. A vacuum test on the bench is a fraction of the cost of a valve body, and it is the only step that tells you whether your money belongs in a drop-in valve kit, a replacement valve body, or the pump you were about to ignore.

FAQ

Can I test a 6L90 valve body without removing it?
Partly. Line pressure readings, code checks and the on-rack shift test can all be done with the unit in the vehicle, and Sonnax notes the pump TCC control valve can be air checked in-vehicle once the valve body is out of the way. But vacuum testing the individual bores requires the valve body off and on a bench with a test plate.

My scan tool shows a solenoid performance code. Does that mean the solenoid is bad?
Not necessarily. Sonnax lists solenoid performance codes as a symptom of actuator feed limit bore wear, because the worn bore delivers erratic supply pressure to the control solenoids. The solenoid itself can be fine and still report a fault because the pressure feeding it is unstable.

I replaced the valve body and the problem came back. What did I miss?
Check the pump. Sonnax states plainly that conditions caused by wear at the pressure regulator valve bore or the associated boost valve cannot be corrected by replacing the valve body, and that the TCC control and converter limit valves also live in the pump. Also verify you installed the correct early or late style casting for your unit, since Sonnax notes a 6L90 valve body on a 6L80 is a no-Reverse condition.

Sources

• Sonnax - Vacuum Testing: Start a Rebuild Right
• Sonnax - Vacuum Test Plate Kit 104740-VTP (6L45, 6L50, 6L80, 6L90)
• Sonnax - Valve Body Inspection and Reaming for Repair
• Sonnax - GM 6L45/50, 6L80/90 No Movement Conditions: Analysis and Repair Tips
• Sonnax - TCC Regulator Valve Kit 104740-46K
• Sonnax - Actuator Feed Limit Valve Kit 104740-47K
• Sonnax - Remanufactured Valve Body GM6L90E