6T70 No Reverse: Diagnosis and Fix for the 3-5-R Wave Plate and Beyond

A 6T70 or 6T75 that will not back up is one of the most common calls on this transaxle, and it is one of the few 6T70 complaints where the failure pattern tells you almost exactly what broke before the pan comes off. The trick is asking the right question on the road test: does it still have 3rd and 5th gear? That single answer splits the diagnosis in two, and on the earlier units it points straight at a part GM has had a bulletin on since 2009.

This guide covers how the 6T70 makes reverse, the quick checks that separate a hydraulic or mechanical failure from an electrical one, the broken 3-5-R wave plate repair as GM spells it out, the valve body and seal causes, and one non-transmission fault that has fooled shops into rebuilding a good unit twice. The bulletin numbers, part numbers and procedures come from GM service bulletins filed with NHTSA, ATRA, Sonnax and Transmission Digest, listed at the end.

Recommended 6T70 parts

A no-reverse wave plate failure means a full teardown: a rebuild kit with new pistons and seals for the cut 3-5-R lip seal, and a clutch drum if the 3-5-R/4-5-6 housing took debris damage. Match Gen. I (2007-2012) or Gen. II (2013-up) by year and RPO.

6T70 Gen 1 Master Rebuild Kit with Pistons 2007-2012

$367.99View part

6T70 Clutch Drum 2007-12

$156.72View part

6T70 Transmission Master Rebuild Kit Gen 2 2013-Up

$447.99View part

How the 6T70 makes reverse

Per the clutch apply chart in Sonnax's 6T70/6T75 valve body tech sheet, reverse on this unit is two clutches: the 3-5-Reverse (3-5-R) clutch and the Low and Reverse clutch. The same 3-5-R clutch is also applied in 3rd gear (with the 1-2-3-4 clutch) and in 5th gear (with the 4-5-6 clutch). Outside of reverse, the Low and Reverse clutch is used for engine braking in 1st; normal 1st in Drive holds on the one-way clutch instead.

That layout is the whole diagnostic shortcut:

• No reverse, and also no 3rd and no 5th (or a slip/flare in 3rd and 5th): the 3-5-R clutch is not applying. Look at the 3-5-R wave plate, piston seals, housing and the 3-5-R hydraulic circuit.
• No reverse, but 3rd and 5th are solid: the 3-5-R clutch works, so the problem is on the Low and Reverse side, its hydraulic feed, or something outside the transmission holding the vehicle.

The 3-5-R clutch is controlled by pressure control solenoid 2. On Sonnax's solenoid chart, the 3-5 Reverse PC solenoid is listed as normally high, meaning it gives full pressure with no current. When the TCM sees that the clutch it commanded did not apply, it sets P0776 (Clutch Pressure Control Solenoid 2 Stuck Off). That code does not mean the solenoid is bad. In most no-reverse cases the solenoid is fine and the clutch it feeds has lost its seal.

Quick checks before you pull it

1. Fluid level and what is in the drain

ATRA's 6T70 seminar gives the fluid check: vehicle level, parking brake on, move the selector through the ranges, idle one minute, and read the dipstick with transmission fluid temperature between 180 and 200 degrees F on the scan tool. Low fluid can cause a delayed or missing reverse on its own, so rule it out first. Then look at the fluid. A broken wave plate sheds steel, and GM's bulletin warns that the debris can affect the speed sensors and the other clutches. Metal flakes in the drain on a unit with no reverse, 3rd or 5th tells you the unit is coming apart.

2. Road test 3rd and 5th

Use the manual shift mode to hold 3rd and then 5th. A clean 3rd and 5th sends you to the Low and Reverse side. A slip or flare in 3rd and 5th, or no 3rd and 5th at all, confirms the 3-5-R clutch.

3. Pull the codes, including history

GM lists P0776 as the main code, and says any of P0777, P2723, P0717, P0716, P2715 and P2714 may set on their own as a result of a broken wave plate. Sonnax adds P0842 (TFP switch 1 circuit low) to the no-reverse list. The speed sensor codes (P0716, P0717) and the P2714/P2715 performance codes make sense once you know the input and output speed sensors are magnetic and collect the steel that a broken plate throws off. Also check for chassis and brake codes in history. You will see why further down.

4. ATRA's unplug test

ATRA's seminar covers a no reverse, 3rd or 5th condition with P0776, P2723 or P2715 caused by a broken weld on a shell, mostly on transmissions built before the 2010 model year, with updated parts available and the new shell supplied with the #3 thrust bearing. ATRA's advice is to keep the diagnosis simple: unplug the transmission and see if reverse is being prevented electrically. If reverse engages with the unit unplugged, the shell is not the problem. That is a two-minute test that tells you whether the clutch can apply mechanically at all before you commit to a teardown.

Two more GM checks belong here before the unit comes out. First, GM's No First and Reverse Gears table lists the low and reverse clutch side of reverse, not just the 3-5-R: a disengaged low and reverse spring retainer, stripped plate splines, worn or debonding piston seals, a broken return spring, porosity in the case cover, and an improperly positioned or too-large bleed hole in the low and reverse piston. If manual low (1st braking) also slips or engages late, that points to low and reverse rather than 3-5-R. Second, if the complaint is a late or soft reverse right after a valve body, solenoid or TCM job, run GM's Transmission Adaptive Values Learn before condemning parts. GM requires it after those repairs, and its preconditions include fluid at 70-100°C (158-212°F) and cycling Park to Reverse 3 times to purge air from the reverse clutches. For a delay rather than a true no-reverse, see our 6T70 delayed engagement guide.

The main cause: a broken 3-5-R wave plate

GM bulletin 09-07-30-012F (No Reverse, 3rd or 5th, Check Engine Light Illuminated, DTC P0776) covers the 2007-2009 Acadia, Outlook, Aura and G6, the 2008-2009 Enclave, Equinox, Malibu, Torrent and Vue, and the 2009 Traverse with the 6T70/75 (RPOs MH2, MH4, MH6 or MY9). The complaint is an SES light and/or no reverse, often with a slip, flare or harsh shift in 3rd and/or 5th.

GM's stated cause is a broken 3-5-R clutch wave plate that lets the apply piston over-stroke. The piston leaks and the clutch loses apply. Transmission Digest describes the same mechanism in more detail: the piston over-travels and cuts its lip seal, so the 3-5-R clutch cannot apply at all. GM says it normally does not happen before about 20,000 miles (32,000 km). The fix is the updated waved plate, GM 24254103, and GM recommends installing it any time one of these transmissions is apart for any reason.

What GM says to inspect

The part is cheap. The labor is not, because GM requires a full teardown and clean. From the bulletin:

1. Remove the case cover and inspect it for damage. It is serviced separately.
2. Remove the 3-5-R/4-5-6 clutch housing and inspect for the broken wave plate.
3. Inspect the case. Peening or wear on the case lugs that drive the steel plates is normal and not a reason to replace the case. Some damage to the 2-6 backing plate shelf is acceptable if the plate lies flat; a raised burr can be dressed off. Confirm the snap ring is fully seated in its groove.
4. Remove the control solenoid valve body (TEHCM) and the filter plate and check the filter plate screens for metal. Clean screens mean the pump fluid filter did its job, and the pump, converter and valve body do not need replacing. Metal on the screens means the pump and torque converter need further inspection.
5. Fine metal you cannot feel, or a gray tint to the fluid, is not a reason to replace the converter. Chips you can feel, or debris found in the converter or when flushing the cooler lines, are.
6. Check the TEHCM pressure switches. Deformed O-rings that are still attached are fine and the TEHCM should be reused. Damaged switch membranes mean the TEHCM must be replaced.
7. Disassemble and clean the valve body per bulletin 08-07-30-005. Do not wash the TEHCM in a parts cleaner, and do not replace it for a code unless SI diagnostics say to.
8. Separate the case halves, clean every part, and clean both speed sensors. Install a new pump fluid filter and a new control solenoid filter plate. GM warns that skipping this can lead to repeat damage.
9. On 2007-2008 units, confirm the control valve channel upper plate has the slot GM shows. A plate without it can set codes.

Replace all four wave plates, and watch the planetary

ATRA's seminar notes that the 3-5-R wave plate is the most common to break, but recommends installing the new-design wave plates in all of the clutches to protect the gear set: 1-2-3-4 24259063, 2-6 24259816, 3-5-R 24254103 and Low-Reverse 24259817. ATRA also points out that the reaction carrier is often damaged by the debris from a broken plate. When you replace it, know that the reaction carrier, input ring gear and ring gear snap ring are serviced as separate pieces, even though many catalogs draw them as one. Leave the ring gear off and the unit will have no movement and may set P2723 after the repair.

Transmission Digest lists the other parts to check for collateral damage: the 3-5-R and 4-5-6 clutch housings, the 2-6 clutch piston return spring and the end cover. The same article notes that the 6T70/6T75 shares these components with the Ford 6F50 and that the Ford parts counter has often been the cheaper source.

Hydraulic causes: the 3-5-R regulator valve and dam seal

If the wave plate and piston are good, move to the circuit that feeds them. Sonnax states that oscillation of the 3-5-R clutch regulator valve wears its bore until the circuit loses pressure and the 3-5-R clutch cannot be applied properly. The symptoms Sonnax lists include delayed reverse, reverse slip, a burnt 3-5-R clutch, burnt reverse clutch, 2-3 and 4-5 flares, ratio codes and bind-up. The repair is reaming the bore and installing an oversized valve: kit 124740-21K for the 1.83 ratio valve with a .335 inch small spool, or 124740-26K for the 1.35 ratio valve with a .390 inch spool (Gen. 1 6T70/6T75 and Ford 6F50/6F55). Put a new clutch pack in without fixing a worn bore and it will burn again.

Sonnax's valve body tech sheet also flags a failure that air checks will not catch: a damaged 3-5-R clutch piston dam seal causes repeat 3-5-R clutch failure, harsh 2-3 and 4-5 shifts and harsh reverse engagement, and it will not show up in an air check of the 3-5-R clutch. If a unit comes back with the same burned 3-5-R clutch, suspect that seal.

With the valve body out, Sonnax recommends air checking each circuit with low, regulated air: Low/Reverse, compensator feed, 3-5 Reverse, 4-5-6, 2-6 and 1-2-3-4. On a no-reverse unit with good 3rd and 5th, a Low/Reverse circuit that will not hold air is your leak.

When it is not the transmission at all

Sonnax published a case that every shop should read before quoting a rebuild. A 2012 Acadia with a 6T70 and 85,000 miles came in with a complete slip in reverse; it would not back up even at 2,500 rpm. Forward gears were perfect, 3rd and 5th were strong, and there were no active codes. The fluid was black and metallic, and the unit had already been rebuilt twice for the same complaint. The cause was a defective yaw rate sensor. Hill Start Assist was holding the brakes each time reverse was selected, so the vehicle felt like it had no reverse. The history had yaw and lateral accelerometer codes that had been cleared before it arrived.

The lesson fits the split above: a no-reverse complaint with strong 3rd and 5th gears should send you to the chassis codes and a check for dragging or applied brakes before anything comes apart.

Order of attack

1. Check fluid level at 180-200 degrees F TFT and look for metal.
2. Road test 3rd and 5th in manual mode.
3. Pull transmission and chassis codes, including history: P0776, P0777, P0842, P2723, P2714, P2715, P0716, P0717, and any yaw or brake codes.
4. Good 3rd and 5th: rule out Hill Start Assist and brake drag, then diagnose the Low and Reverse clutch and its circuit.
5. Missing or slipping 3rd and 5th: do ATRA's unplug test, then plan on the 09-07-30-012F repair with all four updated wave plates.
6. During the teardown, inspect the 3-5-R/4-5-6 housings, end cover, reaction carrier and 2-6 return spring, clean both speed sensors, and replace the pump fluid filter and filter plate.
7. Repair the 3-5-R regulator bore and replace the 3-5-R piston dam seal before it goes back together.
8. After the repair, run Service Fast Learn Adapts, then power down the TCM and restart, per ATRA.

FAQ

Why does my 6T70 have no reverse, 3rd or 5th gear?

All three gears use the 3-5-R clutch. On 2007-2009 units, GM bulletin 09-07-30-012F traces the condition to a broken 3-5-R wave plate that lets the piston over-stroke and leak, usually with P0776 set. The fix is updated wave plate 24254103 plus a full teardown and clean.

Is P0776 on a 6T70 a bad solenoid?

Usually not. P0776 means the TCM commanded the 3-5-R clutch through pressure control solenoid 2 and it did not apply. GM says not to replace the TEHCM for a code unless SI diagnostics call for it, and in the wave plate failure the solenoid is fine while the clutch has lost its seal.

My 6T70 has no reverse but all forward gears work. What is wrong?

If 3rd and 5th are strong, the 3-5-R clutch is working, so look at the Low and Reverse clutch and its circuit. Also check for yaw sensor or brake codes: Sonnax documented a 2012 Acadia where Hill Start Assist held the brakes in reverse and the unit was rebuilt twice for nothing.

Can I just replace the broken wave plate?

GM requires separating the case halves, cleaning all debris, cleaning both speed sensors and replacing the pump fluid filter and control solenoid filter plate, or the damage can repeat. ATRA recommends installing the updated wave plates in all four clutches and inspecting the reaction carrier for debris damage.

Do later 6T70 transmissions break the wave plate?

GM's bulletin covers 2007-2009 models, and ATRA says the related no reverse, 3rd or 5th condition mainly affects units built before the 2010 model year. Later units can still lose reverse from a worn 3-5-R regulator valve bore or a damaged 3-5-R piston dam seal, per Sonnax.