6T40 Won't Shift Into Overdrive: Causes and Fix

"It won't go into overdrive" is one of the most common ways a Cruze, Malibu, Equinox, Terrain, Sonic, Regal or Trax owner describes a 6T40 or 6T45 problem. The engine is spinning too fast on the highway, the tach never drops into the cruise range, and fuel economy falls off. The complaint sounds simple. On this transmission it can mean four different things, and the fix for each one is different.

This guide covers what overdrive actually is on a 6T40, how to find the highest gear the unit is really reaching, the clutch and solenoid circuits behind each "stuck below top gear" pattern, the drum hardware that fails, and the normal strategies that hold the unit out of 6th on purpose. Bulletin numbers and specs are cited, with the full source list at the bottom.

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What "Overdrive" Means on a 6T40

ATRA's 6T40/6T45 introduction lists the gear ratios. Only one of the six forward gears is a true overdrive:

Two things fall out of that table. First, 5th is direct drive, so a car stuck in 5th will feel only a little busy at highway speed, while a car stuck in 4th at 1.44:1 will be screaming. The RPM the customer describes tells you roughly where it is stuck. Second, the 4-5-6 clutch is applied in all three of the top gears. If that clutch or its feed circuit fails, the unit loses 4th, 5th and 6th together and tops out in 3rd. That is the pattern to rule in or out first.

Step 1: Find the Highest Gear It Really Reaches

Do not trust the customer's description or the dash. Road test with a scan tool and watch commanded gear, input speed and output speed. Per GEARS, the input speed sensor reads the 3-5-R clutch housing and the output speed sensor reads the park gear, and the TCM uses the two to calculate gear ratio and slip. Divide input speed by output speed and compare it to the ratio table above. That tells you what the unit is actually doing, not just what the TCM asked for.

Then sort the complaint into one of these patterns:

Cause 1: No 4th, 5th or 6th With P0796

GM's Preliminary Information PI1131 covers 2011 LaCrosse, Regal, Cruze, Equinox, Malibu and Terrain with the 6T40 or 6T45 built before April 1, 2011. It lists four missing-gear conditions, and condition 3 is the one this article is about: no fourth, fifth or sixth gear, with a possible P0796. GM's stated cause is a non-functioning pressure control solenoid inside the control solenoid valve assembly, the TEHCM.

The solenoid behind it is PCS3. Jarad Warren's breakdown of the 6T40 solenoids in GEARS shows PCS3 feeding one regulator that serves both the low/reverse and 4-5-6 clutches, with the on/off shift solenoid and the clutch select valve deciding where that oil goes. PCS3 is normally high on both Gen 1 and Gen 2, meaning it puts out full pressure with no current and the TCM turns it down with current.

GM adds a rule that saves a lot of wasted parts: only one of the four conditions will show up from this cause. If the car is missing 4th, 5th and 6th and also has no reverse, you are not looking at one bad solenoid. Look at line pressure, the TEHCM connector and wiring, or a mechanical failure common to both.

PI1131 also carries a warning about the fluid. On these 2011 units the ATF tends to turn very dark because of the rust inhibitors used on internal parts at assembly. GM tells techs not to assume internal damage, and not to replace the transmission or the fluid on appearance alone. If SI diagnostics do point to the TEHCM, the service part in that bulletin is the complete control solenoid valve assembly with body and TCM, 24256524.

A caution from GEARS applies here too: any solenoid performance code on any transmission can indicate a pressure leak in the clutch circuit that solenoid controls. P0796 says the 4-5-6 circuit is not doing what the TCM expected. It does not prove the solenoid is the leak. Test the solenoid before you buy one. Our 6T40 shift solenoid guide covers the 3-5 ohm resistance spec and the in-car air test.

Cause 2: The 4-5-6 Drum and Ring Tower

If the solenoid tests good and the 4th, 5th and 6th gear complaint stays, the problem is downstream in the 4-5-6 clutch itself. This is where the two generations split hard, and GEARS laid out the differences in its 4-5-6 drum interchange article:

The 4-5-6 clutch is fed through sealing rings on the ring tower. Worn rings or worn ring lands bleed apply oil, and a clutch that cannot hold pressure slips or never fully applies. Because the same clutch is in 4th, 5th and 6th, the symptom is a flare on the 3-4 upshift, slip in the top three gears, or a TCM that gives up and holds a lower gear.

Bushings play into this. Sonnax sells a replacement front 4-5-6 clutch drum bushing, part 144030-02, for Gen 1 and Gen 2 6T40 units (and the 6F35), specifically to address bushing wear and failure from the OE bushing material. It is a steel-backed alloy bushing with a 1.759" housing bore, 1.632" shaft diameter and 0.356" width, and the rear drum bushing is a separate part, 144030-01. Sonnax says to install it with an arbor press, not a hydraulic press or a driver and hammer, because those can distort it. When you have the drum out for a 4-5-6 complaint, check both bushings for clearance, and check the ring tower lands for grooving before you put new rings on.

The Gen 2 drum in a Gen 1 case trap

A Gen 1 car that "was already rebuilt" deserves a look at which drum is in it. GEARS explains that the Gen 1 fluid dam circuit is fed by a regulator valve that needs the inner seal and fourth ring to hold pressure. Drop a Gen 2 drum into a Gen 1 case and that regulator "just sits there wide open." The result may pass a short road test and then show up as a fall-out-of-gear at a stop or another low line pressure complaint. Keep the drum matched to the case generation. You can confirm generation by the eighth digit of the TCM identifier on the TEHCM (1, 2 or 3 is Gen 1; B, C or D is Gen 2), per GEARS.

Cause 3: It Gets 5th but Never 6th

If the unit shifts through 5th and stops, look at the other clutch in 6th. 6th gear is the 2-6 clutch plus the 4-5-6 clutch. The 4-5-6 is already proven working if 4th and 5th are good, so the suspect is the 2-6 circuit.

PI1131 condition 2 is no second or sixth gear, with a possible P2714 (PCS4 performance) or P0873. P0873 is a Gen 1 pressure switch circuit code; GM's PI0604C notes those pressure switch codes do not apply to Gen 2, which dropped the switches. The key road test question is 2nd gear. If the car also skips 2nd (a 1-3 shift) or flares badly on the 1-2, the 2-6 circuit is your problem. If 2nd is solid and only 6th is missing, suspect the shift strategy covered below before you tear anything down.

Cause 4: Stuck in One Gear, No Shifts at All

A unit that stays in one forward gear no matter what is usually not a clutch failure. It is the TCM in a failsafe or default mode, and the customer describes it as "no overdrive" because the engine is revving high on the highway. On the 6T40 the TCM lives inside the transmission on the valve body as part of the TEHCM, per ATRA, so power, ground and connector faults show up as transmission problems.

Transmission Digest ran a case on a 2011 Malibu 6T40 that came in in failsafe with a hard P0601 (TCM memory fault). The first checks they call for are power and grounds to the TEHCM. The fix in that case was a new TCM, and an aftermarket unit would not program; a dealer OEM module did. Before you condemn the module, pull every code in every module, check battery voltage and grounds, and look for U-codes that show the TCM has dropped off the network.

Cause 5: It Is Holding a Lower Gear on Purpose

Not every "won't shift into overdrive" car is broken. ATRA's introduction lists the 6T40's control features: Performance Algorithm Shifting, Performance Algorithm Lift foot, grade braking, winter mode, and sport mode with tap shift. All of them can hold the unit out of 6th under the right conditions. A few to rule out before diagnosis:

• Selector position. A car left in L or manual mode with tap shift will hold whatever gear was selected. Check the range the scan tool reports from the internal mode switch, not what the customer says.
• Grade braking and aggressive driving. On hills or after hard throttle and lift-offs, the TCM holds lower gears on purpose.
• Temperature. Cold operation delays upshifts. Check that transmission fluid temperature reads plausibly and tracks warm-up. A temperature signal stuck at an extreme will make the TCM run the wrong strategy.
• Overfill. GEARS warns that as little as half a quart overfull can push fluid out the vent and set P0218, transmission fluid over temperature. Check the level hot, at 85-95°C (185-205°F), at the level plug. There is no dipstick.
• Adapts and software. GEARS notes frozen adaptive values and adapt resets from power loss or bad grounds cause shift complaints. GM's PIP5164 fixed a 5th-gear surge and flare-sag on 3-4, 4-5 and 5-6 upshifts on early 2014 Malibu 2.5L cars with a TCM calibration, with no codes set. Check for newer calibrations and run a Service Fast Learn before you pull a unit for a no-code complaint.

Diagnostic Order That Saves Time

1. Verify fluid level hot at the plug. Correct an under- or overfill first.
2. Pull codes in all modules. Note P0796, P2714, P0873, P0218 and any TCM or U-codes.
3. Road test with the scan tool. Calculate ratio from input and output speed and find the highest gear actually reached.
4. Match the missing gears to one clutch. 4th-5th-6th together is the 4-5-6. 2nd and 6th together is the 2-6. Missing gears from two groups means a shared cause.
5. Test the matching solenoid, in the car or on the bench, before replacing it.
6. Check for calibration updates and run a Fast Learn on a no-code complaint.
7. If the solenoid is good and the pattern stays, the 4-5-6 drum, rings, bushings or piston need to be inspected, which means the unit comes out.

The Fix

If it is the TEHCM circuit, replace the solenoid or assembly with the correct generation, clear adapts and run the Fast Learn. If it is the 4-5-6 clutch, the repair is an overhaul. While it is apart:

• Replace the 4-5-6 frictions and inspect the steels and, on Gen 1, the wave plate.
• Replace the ring tower sealing rings and check the lands. Use a sizing tool so the new rings are not stretched on install.
• Check both 4-5-6 drum bushings and replace worn ones with a press.
• Replace the bonded pistons. GEARS recommends doing all of them on every rebuild, since heat shrinks the rubber whether or not a piston has failed yet.
• Keep the drum matched to the case generation.
• Refill hot, check at the plug, and run the Service Fast Learn before the road test.

Related guides

• 6T40 Common Problems and Fixes
• 6T40 Slipping: Causes and Fix
• 6T40 Shift Solenoid Symptoms and Replacement
• GM 6T40/6T45 Wave Plate Failure
• 6T40 Fluid Change Cost

FAQ

What is overdrive on a 6T40 transmission?

On the 6T40 only 6th gear is a true overdrive, at 0.74:1 per ATRA. 5th is direct drive at 1:1 and 4th is 1.44:1. 6th uses the 2-6 and 4-5-6 clutches, so a problem in either circuit can keep the unit out of top gear.

Why won't my 6T40 shift into 4th, 5th or 6th?

All three gears use the 4-5-6 clutch. GM PI1131 ties no 4th, 5th or 6th with DTC P0796 to a non-functioning pressure control solenoid in the TEHCM on 2011 units. If the solenoid tests good, look at the 4-5-6 drum sealing rings, ring tower, bushings and piston.

Why does my 6T40 get 5th gear but not 6th?

6th adds the 2-6 clutch to the 4-5-6 clutch. GM PI1131 lists no 2nd or 6th gear with P2714 or P0873 as a PCS4 circuit condition. If 2nd gear is also missing or flares, suspect the 2-6 circuit; if 2nd is solid, check shift strategy, selector mode and fluid temperature first.

Can low or high fluid stop a 6T40 from upshifting?

Yes. Fluid level affects pressure and temperature. GEARS warns that as little as half a quart overfull can vent fluid and set P0218, transmission fluid over temperature. Check the level hot, at 85-95 C (185-205 F), at the level plug, since the 6T40 has no dipstick.

Why is my 6T40 stuck in one gear?

A 6T40 that will not shift at all is usually in a failsafe mode, not suffering a clutch failure. The TCM is built into the TEHCM inside the transmission, so check codes in all modules, battery voltage, powers and grounds to the TEHCM, and communication codes before replacing the module.