Street Smart - December - 2018

A Closer Look at GM’s AF50 Transmission

In this article, we’re going to take a look at some of the basic diagnostic and repair procedures for GM’s AF50 transmission. As always, having the right information makes things easier.

A clutch-and-solenoid chart is a good start (figure 1); with it, you’ll be able to see what clutch is coming on and off and what solenoid is being used to operate that clutch. That’ll help you focus on where the problem is and where to connect a gauge. That’s more important than ever on the AF50, because it has a tap for every clutch (figure 2). 

But before we get ahead of ourselves, first things first: Check for codes. Once you have the code or codes, you need to know what type of code you’re dealing with. For this example, we’ll use code P2808: Pressure control solenoid valve 7 stuck off (EMOP) electro-magnetic oil pump (if equipped).

This solenoid engages the C-1 clutch during an autostop event. It indicates a hydraulic or mechanical problem.

If we had codes P2814 (pressure control solenoid valve 7 (EMOP) control circuit low voltage), or P2815 (pressure control solenoid 7 (EMOP) control circuit high voltage), we’d be testing for a short to ground, short to power, or an open circuit.

Code P2808 is a little different than the other codes. Let’s look at how it works:

The electro-magnetic oil pump is mounted on the upper valve body (figure 3). The structure of the electro-magnetic element is the same as that of a linear solenoid.

The piston in the EMOP operates in response to on/off signals from the TCM, creating pressure.

Hydraulic pressure generated by the EMOP is only used to engage the C-1 clutch during an autostop state, smoothing the transition from a stopped engine to a running one (figure 4).

With this condition, you’d notice engine speed flare up from a stop as the system applies the C-1 clutch.

Checking line pressure is a little tricky (figure 5):

  1. Apply the parking brake and chock the wheels.
  2. Remove the C-1 test plug and install a line pressure adapter and gauge.
  3. Start the engine.
  4. Shift to drive and check the oil pressure with the engine at idle.

Don’t depress the brake pedal; the oil pressure will be detected low if you apply the brakes.

With code P2808, if you step on the brake pedal and put the unit in autostop, you should see line pressure drop to 0 PSI.

For now, the only way to get a solenoid for this valve body is to buy a new valve body assembly for $941.48. Then you’ll have to refill the transmission using AW-1 automatic transmission fluid at $36 a quart.

So before you just pull the unit out or throw a valve body at it, take the time to check the pressure on the tap for the clutch that’s causing the problem. If the pressure is good, the valve body is doing its job.

If pressure is low or slow coming up, you’ll need to air check the circuit with low air pressure (25-30 PSI; figure 6). If the air c++heck fails, it’s time to go inside, but if the air check is good, you’re looking at the valve body.

RELEARNING PROCEDURE

After performing repairs you must reset the learned values:

  1. Perform the scan tool configuration/reset function: Learned Values Reset
    1. Select: Module Diagnostics
    2. Select: Transmission Control Module
    3. Select: Configuration/Reset Functions
    4. Select: Learned Values Reset
  2. Follow the instructions on the scan tool.

Initial Learning

  1. Engine running
  2. Move the selector lever from N to D and wait 5 seconds in each range. Then move the selector lever back to N.
  3. Move the selector lever from N to R and wait 5 seconds in each range. Then move the selector lever back to N.
  4. Transmission in drive
    NOTE: Steps 5, 6, 7, and 10 require driving the vehicle on the road. Never use manual mode function to perform the following steps:

  5. From a stop, accelerate to 72 km/h (45 MPH) more than 5 times, with the throttle position between 15 and 25%.
  6. From a stop, accelerate to 72 km/h (45 MPH) more than 5 times, with the throttle position between 45 and 55%.
  7. From a stop, accelerate to 72 km/h (45 MPH) more than 5 times, with the throttle position greater than 70%.
  8. Turn the ignition off and leave it off for more than 2 minutes.
  9. Restart the engine. 10 From a stop, accelerate until the transmission reaches gear 8, then coast to a stop. Repeat this procedure more than 10 times.

So there you have it: the procedures necessary for component operation, including tap location with air testing procedures, how to perform the scan tool configuration/reset function, and road test to learn.

And that’s not just smart: That’s street smart!