Tales From the Bench - June - 2017

Understanding GM 6T40 Solenoids

The GM 6T40 is becoming very familiar in transmission shops; some are calling it the new breadwinner. The 6T40 came out in 2008 in the Chevy Malibu, but you’ll see it in the Chevy Aveo, Cruze, Equinox, and Sonic; the Buick LaCrosse, Encore, and Regal; the Saturn Aura; and the GMC Terrain. There are two generations of 6T40: generations one and two.

In this article, we’re going to look at the solenoids and pressure switches, and discuss how they operate. And we’ll look at how you can replace the solenoids so you don’t have to replace the TEHCM.

There are two generations of the 6T40, they are not interchangeable! So the first thing you’ll need to do is verify which one you have.

GENERATION 1

To identify what generation you’re working on, look at the eighth digit on the TEHCM (figure 1). On generation 1 units, the eighth digit will be a 1, 2, or 3. Generation 1 also has pressure switches, which we’ll look at later.

Generation 1 units have six pressure control solenoid and one on/off shift solenoid (figure 2).

The line pressure control solenoid is a normally high solenoid. With low current applied, the solenoid creates high pressure; in effect, the transmission has high line pressure. The line pressure control solenoid sends oil to the pressure regulator valve to control line pressure.

Pressure control solenoids 2 and 3 (PCS2 and PCS3) are normally high, just like the line pressure control solenoid.

Pressure control solenoid 2 (PCS2) controls oil to the 3-5-R clutch regulator valve. With low current applied, it sends oil to the 3-5-R clutch regulator valve, which applies the 3-5-R clutch. Applying high current releases the clutch.

Pressure control solenoid 3 (PCS3) controls oil to the R-1/4-5-6 clutch regulator valve. Applying low current sends oil to the R-1/4-5-6 clutch regulator valve, which applies the R-1/4-5-6 clutch. Applying high current releases the clutch.

The TCC pressure control solenoid is a normally low solenoid. With low current applied, the solenoid sends little to no oil to the TCC control valve and TCC regulator valve. Increasing the current allows a controlled flow of oil to the TCC control and TCC regulator valves, to control converter clutch apply.

Pressure control solenoids 4 and 5 (PCS4 and PCS5) are normally low, just like the TCC pressure control solenoid.

Pressure control solenoid 4 (PCS4) controls oil to the 2-6 clutch regulator valve, which sends oil to the 2-6 clutch. With high current applied, the solenoid sends oil to the 2-6 regulator valve; with low current, it sends little to no oil to the valve, releasing the clutch.

Pressure control solenoid 5 (PCS5) controls oil to the 1-2-3-4 clutch regulator valve and 1-2-3-4 boost valve that send oil to the 1-2-3-4 clutch. With high current applied, the solenoid sends oil to the 1-2-3-4 regulator valve and boost valve; with low current applied, no oil gets to the valve, which releases the clutch.

The shift solenoid is an on/off, normally closed solenoid that controls oil to the clutch select valve.

All of the pressure control solenoids should measure 3-5 ohms; the shift solenoid should be 16-20 ohms. When testing solenoids’ resistance, always remove the solenoid from the TEHCM to prevent damage.

The transmission fluid pressure switches are normally closed, allowing current to flow with no pressure applied (figure 3). When there’s pressure at the switch, it opens the circuit.

GENERATION 2

The eighth digit on the generation 2 TEHCM is going to be a B, C, or D. These units have six pressure control solenoids, one shift solenoid, and no pressure switches. The PCS2 and PCS5 solenoids are in different locations and operate differently from those in generation 1 units (figure 4).

These four solenoids are identical to the generation 1 solenoids in operation and location:

  • Line pressure control solenoid
  • Pressure control solenoid 3
  • Pressure control solenoid 4
  • TCC pressure control solenoid

The solenoids that have changed in generation 2 units are pressure control solenoids 2 and 5 (PCS2 and PCS5).

Pressure control solenoid 2 (PCS2) is now normally low, just like the TCC solenoid, and it’s in a new location on generation 2 units.

Pressure control solenoid 2 (PCS2) controls oil to the 3-5-R clutch regulator valve. With high current applied, it sends oil to the 3-5-R clutch regulator valve, which applies the 3-5-R clutch. Low current releases the clutch.

Pressure control solenoid 5 (PCS5) is now normally high, just like the line pressure control solenoid, and it’s in a new location on generation 2 units.

Pressure control solenoid 5 (PCS5) controls oil to the 1-2-3-4 clutch regulator valve and 1-2-3-4 boost valve that sends oil to the 1-2-3-4 clutch. With low current applied, the solenoid sends oil to the 1-2-3-4 regulator valve and boost valve; applying high current shuts off oil to the valves, releasing the clutch.

Just as with generation 1 units, all of the pressure control solenoids should measure 3-5 ohms; the shift solenoid should be 16-20 ohms. When testing solenoids’ resistance, always remove the solenoid from the TEHCM to prevent damage.

There are no pressure switches in the generation 2 TEHCM.

DIAGNOSING PROBLEMS

Among the more common 6T40 transmission problems to reach the ATRA HotLine are units with no forward or no reverse. Here are a few tests that may help identify whether the problem is being caused by a solenoid problem:

CAUTION: Never apply voltage to the solenoids or solenoid block; doing so could damage them.

If you remove the solenoid block from the TEHCM, you can control the solenoids with a solenoid driver or test machine. You can test the shift solenoid by applying 12 volts but never apply 12 volts to the pressure control solenoids.

For this test, you’re going to apply 45 PSI of air pressure, without energizing the solenoids (figure 5). Use a plastic test plate to cover the fluid entry port, and check for air coming out of the exit ports.

Remember, these are variable bleed solenoids so they’ll all leak a little air; even the normally low solenoids. The normally high solenoid will allow more air. With a little testing and playing around you’ll quickly learn how they should function.

If you find a bad solenoid, you can replace the TEHCM, confident that you found the problem. Or you may be able to replace just the solenoid in question.

You may be able to find broken TEHCMs with bad connectors cheaply from your hard part suppliers. Sure, the connector’s broken, but the solenoids may be okay to reuse. That way you can replace the individual solenoids instead of the TEHCM. And, since you’re reusing the same TEHCM, you won’t have to reprogram afterward.

These six-speed transmissions are quickly becoming one of the more common units to reach your shop. Learning how they operate and how to test them is the best way to make sure the future you’re looking at will be a bright one.