P0944-LOSS OF HYDRAULIC PUMP PRIME

For a complete wiring diagram Refer to Section 8W

Theory of Operation

The Loss of prime test is used to prevent transmission defaults and erroneous fault codes during temporary loss of pump prime that may occur with low transmission fluid under severe braking conditions, start-up, etc. and to point towards more subtle problems such as a plugged or cracked oil filter.

The Loss of Prime DTC is set by a loss of hydraulic pressure in the transmission system. This condition, if sustained, will result in the vehicle being unable to move.

When Monitored:
If the transmission is slipping in any forward gear and all the pressure switches are not indicating pressure, a loss of prime test is run.
Set Condition:
If the transmission begins to slip in a forward gear and all the pressure switch(s) that should be closed are open a loss of prime test begins. Available elements are turned on by the PCM to see if pump prime exists. The DTC sets if no pressure switch(s) respond.

Possible Causes

SHIFT LEVER OUT OF ADJUSTMENT

IMPROPER FLUID LEVEL

CRACKED OR IMPROPERLY INSTALLED PRIMARY OIL FILTER OR SEAL

LOOSE COOLER RETURN FILTER

STUCK OR STICKING MAIN REGULATOR VALVE

TRANSMISSION OIL PUMP

Always perform the 45RFE/545RFE Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - 45RFE/545RFE - STANDARD PROCEDURE)

Diagnostic Test

1. SHIFT LEVER POSITION TEST

Using the scan tool, perform a Shift Lever Position test. Follow the instructions on the scan tool.

Did the Shift Lever Position Test pass?

Yes

Go To 2

Refer to the Transmission category and perform the appropriate symptom.
Perform 45RFE/545RFE TRANSMISSION VERIFICATION TEST. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - 45RFE/545RFE - STANDARD PROCEDURE)

2. CHECK TO SEE IF DTC P0944 IS CURRENT

Place the gear selector in Park.
Start the engine.
The transmission must be at operating temperature prior to checking pressure. A cold transmission will give higher readings.
Run the engine at 1500 rpm.
With the scan tool, monitor the Transmission Line Pressure.

Does the Line Pressure match the Desired Line Pressure within +/- 5 psi?

No

Go To 6

Yes

Go To 3

3. CHECK LINE PRESSURE IN DRIVE AND REVERSE

Firmly apply the brake and repeat the previous test in both Drive and the Reverse gear positions.
With the scan tool, monitor the Transmission Line Pressure.

Does the Line Pressure match the Desired Line Pressure within +/- 5 psi in all gear ranges?

Yes

Go To 4

Go To 5

4. REVIEW CUSTOMER COMMENTS

The conditions necessary to set this DTC are not present at this time.
Verify with the customer if a delayed engagement and/or an intermittent "No Drive" condition has occurred.
If the customers answer is "No" erase the DTC and return the vehicle to the customer.
Make sure to check for any TSBs or controller flash updates that my apply.
With the scan tool, check the DTC EVENT DATA to help identify the conditions in which the DTC was set.

Has the customer experienced any delayed engagement and/or "No Drive" conditions?

Yes

Go To 6

Test Complete.

5. DTC VERIFICATION

Read and record the DTC and the DTC EVENT DATA information.
Test drive the vehicle and attempt to operate the vehicle within the parameters in which the DTC set.
With the scan tool, read DTCs.

Did any following DTCs set, P0868, P0944, P0841, P0846, P0871, P0876, or P0988?

Yes

Go To 6

Test Complete.

6. CHECK TRANSMISSION OIL FILTER

Remove and inspect the Transmission Oil Pan per the Service Information.
Remove and inspect the Primary Oil Filter per the Service Information.
Inspect the oil filter seal for damage and proper installation.

Does the Oil Pan contain excessive debris and/or is the Oil Filter plugged or seal damaged?

Yes

Repair the cause of the plugged transmission oil filter or excessive debris. Check the Transmission Oil Filter seal for improper installation onto filter neck instead of into the pump bore, not fully seated against pump housing, filter neck not engaged into pump. Check that the spin on oil filter (if equipped) is not loose. Repair as necessary.

Perform 45RFE/545RFE TRANSMISSION VERIFICATION TEST. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - 45RFE/545RFE - STANDARD PROCEDURE)

Repair the Transmission Oil Pump as necessary. Check for a stuck main regulator valve and clean if necessary. Refer to the Service Information for the proper repair procedure.
Perform 45RFE/545RFE TRANSMISSION VERIFICATION TEST. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - 45RFE/545RFE - STANDARD PROCEDURE)