6L50 Sprag Clutch 2007

If your 6L50 pulls away in reverse and in manual first but sits in neutral or flares hard when you take off in Drive, the low sprag is one of the first parts a rebuilder suspects. This is a new Borg Warner low sprag assembly for the GM 6L50, Borg Warner number 140639BW, listed against GM part 24240431 for 2007 and later units. It is the 30-element version. That detail matters: the lighter 6L45 uses a 24-element low sprag, so count the elements on the one you pull out before you order. If yours has 24, you want our 6L45 low sprag instead.

What the low sprag does in a 6L50

The 6L50 builds six speeds from a simple front planetary, a compound rear planetary and five clutches. First gear in Drive needs one member of the rear gearset held still while the 1-2-3-4 clutch drives it. GM did that job with a one-way clutch rather than a band or another piston. The sprag elements wedge between the inner and outer races when torque tries to turn the member backward, and they tip over and let it spin when it turns the other way.

Sonnax's 6L application chart shows exactly where this part works. In Drive first, only two things are listed: the 1-2-3-4 clutch applied and the low sprag holding. On the 1-2 upshift the 2-6 clutch comes on and the sprag simply freewheels, so the computer never has to time the release of a first-gear clutch. That is why first-to-second on a healthy 6L50 is so smooth. When you select manual first, or in Reverse, the Low and Reverse clutch applies as well, which gives engine braking and carries the reverse load. That split is the key to diagnosing this part.

Symptoms of a failed 6L50 low sprag

• No movement in Drive from a stop, or the engine revs before the vehicle creeps away, while Reverse feels normal.
• The vehicle moves in manual first (L1) but not in Drive, because the Low and Reverse clutch is doing the holding the sprag can no longer do.
• A neutral feeling on takeoff that disappears once the transmission reaches second.
• Slip or gear ratio codes logged in first gear only.
• A sprag that locks in both directions instead shows up as a bind or tie-up on the 1-2 shift.

How to confirm it before you buy

Do the drive test above first. If it moves in L1 and Reverse but not in Drive first, the sprag goes on the list, but it is not the only thing that causes that pattern. Gears Magazine points out that a worn number 1 check ball in the upper valve body can give the same no-forward complaint. The ball shrinks from .250 in. to around .180 in. or smaller and leaks 1-2-3-4 clutch feed oil. Check that before you pull the transmission, along with a line pressure test. Sonnax notes that a line pressure solenoid stuck in exhaust leaves line pressure flat at roughly 60 to 68 psi with no rise as RPM goes up. Our 6L45 and 6L50 limp mode diagnosis guide walks through the scan data and codes to read first.

Fitment

This sprag is listed for the 6L50 from 2007 on. The 6L50 carries GM RPO code MYB on the vehicle's service parts label; the 6L45 is MYA. GM revised 6L internals more than once, so compare the outer diameter, element count and race splines against your old sprag, or send us your year, model and VIN and we will check it before it ships. Browse everything for this family on our 6L45 and 6L50 parts page.

Install notes

• Before final assembly, hold the outer race and turn the inner by hand. It must lock one way and spin freely the other. A sprag installed backward gives no first gear on the first test drive.
• Inspect both races where the elements ride. Pitting, flat spots or a polished ring mean the race needs replacing too, or the new sprag will slip on the same damage.
• A sprag that failed often left metal in the case. Replace the filter, clean the cooler lines, and look hard at the Low and Reverse clutch pack that sits with it.
• Most shops install it during a full overhaul with a 6L50 master rebuild kit, or with our 6L50 overhaul kit if the frictions are still serviceable.
• Fill with Dexron VI and check the level hot at the check plug with the engine running, then let the TCM relearn shift adaptives on a test drive.

How do I know if I have a 6L50 or a 6L45?

Look for the RPO code on the service parts label, usually in the glovebox or trunk. MYB is the 6L50 and MYA is the 6L45. With the unit apart, the element count is a quick check: this 6L50 sprag has 30 elements and the 6L45 version has 24.

My truck moves in L1 and Reverse but not in Drive. Is it the sprag?

That pattern points at the low sprag, because the Low and Reverse clutch holds in L1 and Reverse while the sprag holds alone in Drive first. A worn number 1 check ball or a pressure fault can mimic it, so check the valve body and line pressure before pulling the transmission.

Is this a new part?

Yes. It is a new Borg Warner assembly, part 140639BW, listed against GM 24240431.

What should I replace with the sprag?

Replace the filter and any thrust washers you disturb. Inspect the races and the Low and Reverse clutch pack. If the unit has many miles or burnt fluid, do it with a master rebuild kit rather than a sprag alone.

What fluid does the 6L50 take?

GM 6L50 applications use Dexron VI. Set the level hot with the engine running at the check plug, the way GM specifies for this transmission.

• 6L45 Limp Mode: Causes and How to Diagnose It