6T70 P0894: Transmission Component Slipping Causes & Fix

P0894 is the code that tells you the least and points you the furthest. "Transmission Component Slipping" names no solenoid, no circuit and no part. All it says is that the control module watched input shaft speed against output shaft speed, did the division, and got a number that does not match the gear it commanded.

That is actually good news, because on a 6T70 or 6T75 the ratio math is reversible. If you know which gears the slip shows up in, and you know which clutches are applied in each gear, the code hands you the failed component. This unit has five clutches and a one-way, and each of them is applied in a specific set of gears. Two data points and a chart get you to a part.

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What the module is actually measuring

The 6T70 has an input speed sensor and an output speed sensor, both magnetic pickups. The TCM divides ISS by OSS and compares the result against the ratio it expects for the commanded gear. Hold too far off that number for too long and you get a slip or ratio code.

Two practical notes before you touch anything. First, this is a transaxle: the output speed sensor reads shaft speed before the 2.77 or 3.16 final drive, so your calculated number should land on the gear ratio, not on an overall ratio that includes the ring and pinion. Second, look up the enabling criteria for the code in service information before you diagnose it. As Jerry Stewart put it in Gears, the enabling criteria tell you the temperature range the test runs in, the speed range required, how long the test is, and whether another code blocks the test from running. A code you cannot reproduce is usually a code whose test conditions you have not met.

While you are in there, pull Mode 6 data on the generic side of the scan tool. Mode 6 gives you the minimum, maximum and actual test values for the monitor. That tells you whether this failed by a hair or by a mile, and it leaves you a number to re-check after the repair instead of a feeling.

The ratios and the clutch chart

ATRA published the 6T70/6T75 numbers when the unit came out. Ratios are 4.48 first, 2.87 second, 1.84 third, 1.41 fourth, 1.00 fifth, 0.74 sixth and 2.88 reverse, with a 2.77:1 or 3.16:1 final drive. Input torque capacity is 280 lb-ft for the 6T70 and 299 lb-ft for the 6T75.

The apply chart from ATRA's 6T70 range reference is the other half:

Read it as a set of overlaps. A clutch that is slipping shows up in every gear that uses it and in no gear that does not. Lose third, fifth and reverse together and you have not got three problems, you have one 3-5-R clutch. That single pattern is the most common answer to P0894 on this unit, and there is a specific reason for it.

Stewart's rule comes at the same thing from the other direction: a slipping clutch only sets ratio errors in the gears where that clutch is engaged, while a bad speed sensor or its wiring sets scattered ratio codes with nothing in common. If your codes have no pattern, stop looking at clutches and start looking at sensors and harness.

The 3-5-R waved plate: the pattern failure

GM covered this one under Special Coverage bulletin 14404B for 10 years or 120,000 miles from in-service date. Affected units are 6T70 and 6T75 transaxles with RPO MH2, MH4, MH6 or MY9 in 2007-2009 Buick Enclave, GMC Acadia, Pontiac G6, Saturn Aura and Outlook, 2008-2009 Chevrolet Equinox and Malibu, Pontiac Torrent, Saturn Vue, and 2009 Chevrolet Traverse. GM's own description of the condition: a cracked or fractured wave plate, resulting in loss of 3rd, 5th and reverse.

The mechanism explains the code set. Transmission Digest describes it as the broken 3-5-reverse waved plate allowing the clutch piston to over-travel, which cuts the lip seal and results in no application of the 3-5-reverse clutch. The plate is a cushion; when it lets go, the piston runs past its designed stroke, the seal gets sliced, and apply pressure bleeds off instead of clamping the pack. So you get a clutch that is nominally commanded, nominally fed, and physically slipping.

Codes that ride along with it, per Transmission Digest: pressure control solenoid performance codes P0776, P0777, P2723, P2714 and P2715. And because hard-part damage puts metal in the fluid, debris sticks to the magnetic input speed sensor and sets P0716 and P0717. ATRA's 6T70 product issues material lists the same cluster and adds the complaint you will actually hear at the counter: check engine light, no reverse, and slip, flare or harsh shifts in third or fifth.

Sonnax studied why the plates break and the answer is the teeth. Every broken OE plate they examined separated exactly at a tooth corner, a classic stress point, and plates that look fine can carry micro fractures there. GM's response was to shot-peen the plate to relieve stress, and installing that updated plate became standard practice, but Sonnax notes that units running the newer plate — including Gen. 2 — are still failing in the field. Their answer is a waved plate with zero teeth, positioned by a billet cupped steel plate that replaces the bottom steel in the pack and holds clutch clearance.

The GM part number for the 3-5-R waved plate is 24254103. ATRA lists the updated plates for the other clutches too and recommends installing all of them any time you are in there: 1-2-3-4 is 24259063, 2-6 is 24259816, low-reverse is 24259817. The 3-5-R is the one that breaks.

The other things that set it

Fluid level, checked the way GM specifies

ATRA's procedure is not optional on this unit, because level here is unusually sensitive. Vehicle on a level surface, parking brake set, engine started, gear selector moved through the ranges, idle one minute, and read the dipstick with transmission fluid temperature between 180°F and 200°F. Cold or scalding both lie to you. The fluid is DEXRON-VI, which ACDelco specifies for GM transmissions from the 2006 model year forward. Capacities: 4.2 to 6.3 quarts for a fluid change, 5.3 to 7.4 quarts after valve body cover removal, 7.4 to 9.5 quarts on a rebuild.

The filter is not a maintenance item on this transaxle. It is sandwiched between the case halves, so it only gets replaced after an internal repair. Fluid-only service intervals are 100,000 miles normal and 50,000 miles severe.

Case cover hub sleeve ring groove wear

This one masquerades as a clutch problem and is not. ATRA documents a bind on the 3-4 or 4-3 shift, sometimes with P0797, caused by a worn ring groove on the case cover hub sleeve leaking between the 3-5-R and 4-5-6 clutch circuits. Pull the hub from the rear case cover and measure between the edge of the groove and the ring with a feeler gauge. Maximum clearance is .007 inch. Past that, the case cover gets replaced. Two clutch circuits cross-feeding will absolutely produce slip and ratio complaints.

4-5-6 accumulator piston bore wear

On Gen. 2 units, 2013 and up, wear in the 4-5-6 accumulator piston bore causes a loss of 4-5-6 clutch apply pressure. Transmission Digest reports that the resulting slippage produces harsh or delayed shifts, shift flares, ratio codes and clutch failure — which is the exact complaint list that brings a P0894 through the door. TransGo's re-engineered piston adds two O-rings to stop further piston-to-bore contact and salvage the valve body rather than replacing it.

Valve body wear feeding the clutches

Sonnax builds the Gen. 1 6T70 valve body as a reman specifically because of what wears in it, and their list of what the reman addresses reads like a P0894 differential: P0741 and P0742, harsh TCC apply, TCC slip, solenoid performance codes, wrong gear starts, slips and flares, delayed engagements, burnt clutches and erratic line pressure. Worth knowing before you order: the early and late Gen. 1 units are not interchangeable and look nearly identical — '07-'08 units use a 1.35 ratio 3-5-R clutch regulator valve, '09-'13 units use 1.83.

Speed sensors reading garbage

Both speed sensors are magnetic, and per GM bulletin 14404B they will attach and hold any ferrous material in the fluid. Debris on an ISS or OSS pickup gives you a false ratio, which gives you a slip code with no slipping clutch. GM's instruction after a wave plate repair is explicit: clean both sensors, or you will set DTCs on a transmission you just fixed.

Working it in order

1. Record everything before clearing. All codes, stored and history, plus freeze frame and Mode 6. The code with the most authority gets diagnosed first — a line pressure solenoid fault can cause a ratio error, so the pressure code comes before the ratio code.
2. Fluid level at 180-200°F, and read the pan. Note whether the magnet is furry, whether there is metal you can feel, and what the fluid looks like.
3. Graph ISS and OSS. Both in RPM, in graphing mode, on a drive. Divide input by output with the cursor in each gear and compare against 4.48 / 2.87 / 1.84 / 1.41 / 1.00 / 0.74. Write down which gears are off.
4. Map the bad gears onto the clutch chart. Third, fifth and reverse means 3-5-R. Fourth, fifth and sixth means 4-5-6. Scattered gears with nothing in common means sensor or wiring, not clutches.
5. Sanity-check the logic, not just the data. Stewart's point is worth repeating: watch what the module commands, not only what the sensors report. An absurd commanded line pressure, or an overdrive command at 25 mph, is a calibration problem, and no clutch you install fixes it.
6. Verify the ratio codes are real before teardown. Run it on the hoist with the wheels off the ground and the codes cleared. Ratio errors that persist with almost no load on the clutch point at a sensor or its wiring.

The repair, done so it stays fixed

When a broken wave plate is the answer, the plate is the cheap part of the job. GM 14404B walks the rest of it, and the debris rules are the part shops skip:

• Separate the case halves and clean everything. The bulletin states plainly that failing to separate the halves, disassemble, clean the debris and replace the pump fluid filter and the control solenoid filter plate could lead to repeat damage.
• Let the filter plate screens make the pump and converter decision. No debris on the screens means the pump filter did its job and the pump, converter and valve body do not need replacing. Debris on the screens means inspecting the pump and converter further.
• Know what does not condemn a converter. Fine metal particles you cannot feel — flakes, or a gray tint to the fluid — are not a reason to replace it. Metal chips you can feel, or debris found in the converter or when flushing the cooler and lines, are.
• Do not wash the control solenoid valve assembly in a parts cleaner, and do not replace it for a DTC unless service information diagnostics tell you to. Deformed pressure switch O-rings that are still attached do not affect switch performance and the module gets reused; damaged switch membranes mean it gets replaced.
• Clean both speed sensors. Covered above, and it is the one people forget.
• Reinstall the TCM heat sink bracket. The spring-loaded bracket forces the module against the heat sink on the valve body. Leave it out and the module overheats and shuts down.
• Run Service Fast Learn Adapts. The procedure runs a series of tests so the TCM learns individual clutch characteristics and writes them into the adaptive cells. Then shut the engine off and power down the module — you will lose scan tool communication, which is how you know it took — and restart to complete it.

One more thing worth knowing about adaptives here: the clutch pressure learn runs under steady conditions in specific gears. The 4-5-6 learn is enabled in third, the 1-2-3-4 and 2-6 learns in fifth, and the 3-5-R learn in sixth. A verification drive that never holds steady in those gears is not verifying anything.

Keeping it from happening again

• Install the updated waved plates in every clutch while you are in there, not just the 3-5-R. That is ATRA's recommendation, and it costs almost nothing next to a second teardown.
• Measure the case cover hub ring grooves whenever the unit is apart. .007 inch maximum, and a cross-leak there undoes good clutch work.
• Do not skip fluid service just because the filter is not serviceable. 100,000 miles normal, 50,000 severe, DEXRON-VI only.
• Verify hot. This family behaves on a short cold drive and misbehaves at full operating temperature. Get the fluid up to temperature before you call it done.

FAQ

What does P0894 mean on a 6T70?

It means the control module compared input shaft speed to output shaft speed, calculated the ratio, and found it did not match the gear it commanded for long enough to fail the test. It does not name a part. On this transaxle the pattern of which gears are affected identifies the clutch, because each clutch is applied in a fixed set of gears.

Can I drive a 6T70 with P0894 set?

Briefly and gently, to get it off the road. A slipping clutch generates heat and debris, and on this unit debris travels to the magnetic speed sensors, the pump filter and the converter. Every mile driven on a slipping pack widens the repair, and a broken 3-5-R waved plate can go on to damage the drum splines and the reaction carrier.

Is the 6T70 wave plate covered by GM?

GM Special Coverage bulletin 14404B covers a cracked or fractured wave plate causing loss of 3rd, 5th and reverse for 10 years or 120,000 miles from the in-service date, on listed 2007-2009 vehicles with RPO MH2, MH4, MH6 or MY9. Most of that window has closed on age by now, but it is worth checking a specific VIN before quoting a rebuild.

Will a fluid change fix P0894?

Only if low or degraded fluid was the cause, and you will know that from the level check at 180 to 200 degrees F and from what is on the pan magnet. If a clutch has already been slipping long enough to set the code repeatedly, fresh fluid will not restore the friction material. Check level first because it is fast and free, not because it is likely to be the fix.

Why do I have solenoid codes with a mechanical failure?

Because the module infers solenoid performance from what the clutch does. When a broken waved plate lets the 3-5-R piston over-travel and cut its seal, the clutch does not apply even though the solenoid is doing its job, so the module records P0776, P0777, P2723, P2714 or P2715. Metal in the fluid on the input speed sensor then adds P0716 and P0717.

Related reading: our 6T70 slipping under load, 6T70 P0748 pressure control solenoid, and reading transmission live data.