p0979: 4-5 shift solenoid control circuit low voltage
conditions for setting the DTC:
The TCM detects an open in the 4-5 shift solenoid valve circuit when HSD2 is commanded ON
The TCM detects a short to ground in the 4-5 shift solenoid valve circuit when HSD2 is commanded OFF
Description of Circuit:
The 4-5 shift solenoid valve is located in the control valve body of the transmission. The TCM selects the required gear by turning the appropriate valve ON or OFF which enables the transmission to shift. The TCM provides voltage through high side driver 2 (HSD2). The TCM uses a second driver to control the solenoid ground circuit. The controlled ground driver reports feedback voltage to the TCM. When the TCM commands the 4-5 shift solenoid valve ON, the voltage of the control circuit should be approximately 0 volts. When the TCM commands the 4-5 shift solenoid valve OFF, the voltage of the control circuit should be approximately system voltage.
the "driver" term used above is the power supply used to control the circuit. There are 2 drivers (power supplies) involved. One that handles putting out a positive voltage and the second one puts out a ground. The one that puts out the positive voltage is an "always ON" and the ground state being turned on and off is what turns the valve ON and OFF. The ground circuit is also used to test if the solenoid is good or bad. When the ground driver is not outputting a ground signal the side of the solenoid it is connected to should read close to the same voltage as what is being put into it from the positive driver. If it is not that means there is an open or a broken connection.
This is what I believe you have going on. Your 4-5 shift solenoid is no good. It appears to be random or intermittent but more then likely it is not. There is a bad connection in the solenoid that when cold it works fine and once the transmission heats up expansion occurs the connection opens up and the DTC gets set. You can test this theory by getting the car into 5th gear while it is cold and monitoring the engine temperature and when it does it record what the engine temp is. Do this several times and see if there is any consistency to the temperature readings. The temperature of the transmission is going to lag behind what the engine temperature is and it may only occur once the engine has been up to temperature (195 to 205) for a little bit before it occurs. The fact that it only occurs once the vehicle is up to temperature is the symptom and it points to a bad control valve in the transmission.
