Tales From the Bench - April - 2019

Stop Wasting Money on Jatco CVT Stepper Motors

If you’ve had to diagnose a Jatco CVT transmission by now, chances are you’ve seen the familiar stepper motor code, P1778.

The stepper motor is one of the most over-replaced parts in these CVTs. A lot of time and parts have been wasted because of this mystery part called a stepper motor.

We’re going to take a look at how a stepper motor works, and we’re also going to cover basic testing procedures for the stepper motor.

Let’s start with how stepper motors operate, both electrically and mechanically.

THE STEPPER MOTOR

Look for Jatco CVTs in Nissan, Dodge, Jeep, and other vehicles. These CVTs aren’t going anywhere, but the stepper motor is going away on the newer CVTs. The majority of the CVTs in shops today still have stepper motors.

The stepper motor connects to a ratio control arm and ratio control valve (figure 1). The other end of the ratio control arm connects to the pulley follower.

The main purpose of the ratio control valve is just as its name implies: to change the ratio of the transmission. It does this by changing the pressure in the pulleys.

The transmission control module sends a signal to the stepper motor in a certain pattern. To keep it simple, the stepper motor can extend or retract, moving the position of the ratio control valve.

The transmission control module knows that the stepper motor controls the ratio. When it sees a ratio issue, it blames the stepper motor and sets stepper motor code P1778. This code is why the stepper motor gets replaced a lot. We’ll come back to this later; first, let’s look at how the stepper motor works.

The stepper motor consists of a threaded shaft, a threaded motor/magnet, a coil with a set of two windings, a housing, and a vent end cap (figure 2).

There are two windings: Each winding has a ground wire and two controlled power wires. The motor in the center has a magnet with a north and south pole and is threaded. When the motor spins, the threaded rod extends or retracts to operate the ratio control arm.

Let’s see how to get the stepper motor moving (figure 3). When the 1A wire provides power to the winding, it magnetizes and attracts the north side of the motor.

The next step would be for the control to power the 2B wire and release power from the 1A wire (figure 4). Now the motor turns a quarter turn clockwise.

To continue to go in the same direction, you’d power wire 1B and release power on 2B (figure 5). The motor would turn a quarter turn clockwise.

To complete one final turn of the motor, power wire 2A and release power on 1B (figure 6). That causes the motor to turn once more a quarter turn clockwise.

If the motor needs to go in the other direction, the pattern repeats in the opposite direction.

This is just a basic description of stepper motor operation. Hopefully this helps you understand the mysteries of the stepper motor.

Jatco controls the stepper motor differently but it uses the same basic principles. Jatco controls the stepper motor in 1/12 of a turn per step, unlike our example of 1/4 of a turn at a time. Jatco has two coils on at the same time, which helps add strength and provides more control of the stepper motor.

If you wanted to see the full operation of the control of the stepper motor, you’d need to scope out all four circuits.

TESTING THE STEPPER MOTOR WINDING

The main circuits are very easy to test. Look into the stepper motor connector (figure 7). Winding one pin numbers are one, two, and three. Winding two are pins four, five, and six. Using a simple ohmmeter, check the resistance between the following pins:

1 and 2 = 15 ohms
2 and 3 = 15 ohms
1 and 3 = 30 ohms
4 and 5 = 15 ohms
5 and 6 = 15 ohms
4 and 6 = 30 ohms

It’s that simple to test the stepper motor circuits on the bench. There are stepper motor controllers and testers on the market but they come at a high price tag.

If you’re diagnosing a stepper motor circuit code, make sure you understand what kind of code you’re dealing with.

STEPPER MOTOR CODES P1777 AND P1778

There’s a major difference between codes P1777 and P1778: Code P1777 is a circuit code. If you have code P1777, you need to check the connectors, wiring, and the stepper motor windings. The resistance check would be a good start, or you could also perform the same test at the TCM. Just pull up a wiring diagram to get the pin numbers.

Code P1778 is a very common code and very misleading. The factory manual describes it as, “P1778 Step Motor Circuit Intermittent.”

The set conditions listed in the factory manual are, “There is a great difference between the number of steps for the stepping motor and for the actual gear ratio.” And its lists of possible causes are stepper motor — that’s it; there’s nothing else on the list. This is why the stepper motor may be the most over-replaced part in a CVT.

Code P1778 means you have a ratio issue. The TCM isn’t seeing the ratio it’s commanding.

The TCM calculates this code based on the primary and secondary speed sensors. First make sure the speed sensor signals are good. If you have a speed sensor code, repair that problem first.

If the speed sensors are in good operating condition, you have an internal transmission issue. The most common issues that cause the P1778 code are flow control valve wear, pulley leaks, and a worn or sticky valve body.

The next Jatco CVT you get in with a stepper motor code, think twice before replacing the stepper motor. First figure out whether the code you have indicates a wiring issue or internal transmission problem.

Hopefully, this will clear up any misunderstandings you had with the stepper motor and its codes, and it’ll save you time and money when faced with one of these problems.