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 ruptured oil filter. The Loss of Prime fault 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:
Every 350 ms
Set Condition:
If the transmission begins to slip in any forward gear, and the pressure switch or switches that should be closed for a given gear are open, a loss of prime test begins. All available elements (in 1st gear LR, 2/4 and OD, in 2nd, 3rd, and 4th gear 2/4 and OD) are turned on by the PCM to see if pump prime exists. The code is set if none of the pressure switches respond. The PCM will continue to run the loss of prime test until pump pressure returns. The vehicle will not move or the transmission will slip. Normal operation will continue if pump prime returns.

Possible Causes

LOW TRANSMISSION FLUID LEVEL

SHIFT LEVER POSITION

PLUGGED TRANSMISSION FILTER

TRANSMISSION OIL PUMP

Always perform the 42RLE Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - 42RLE - STANDARD PROCEDURE)

Diagnostic Test

1. CHECK TO SEE IF DTC P0944 IS CURRENT

Place the gear selector in Park.
Start the engine.

NOTE: The Transmission Temperature must be at least 43 degrees C (110 degrees F) before performing the following steps.

The Transmission must be at operating temperature prior to checking pressure. Cold transmission fluid will result in higher pressure readings.
Place the Transmission in Reverse.
With the scan tool, observe the Transmission Pressure Switch states.

Are any of the Pressure Switches closed?

Yes

Go To 2

Go To 4

2. INTERMITTENT OPERATION

The conditions necessary to set this DTC are not present at this time.
Test drive the vehicle. Allow the Transmission to shift through all gears and ranges.

Was a delayed engagement and/or a no drive condition present during the test drive?

Yes

Go To 4

Go To 3

3. INTERMITTENT WIRING AND CONNECTORS

The conditions necessary to set this DTC are not present at this time.
Using the schematics as a guide, inspect the wiring and connectors specific to this circuit.
Wiggle the wiring while checking for shorted and open circuits.
With the scan tool, check the EVENT DATA to help identify the conditions in which the DTC was set.

Were there any problems found?

Yes

Repair as necessary.

Perform 42RLE TRANSMISSION VERIFICATION TEST. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - 42RLE - STANDARD PROCEDURE)

Test Complete.

4. SHIFT LEVER POSITION TEST

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

Did the Shift Lever Position Test pass?

Yes

Go To 5

Perform the diagnostic procedure for P0706-TRANSMISSION RANGE SENSOR RATIONALITY.
Perform 42RLE TRANSMISSION VERIFICATION TEST. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - 42RLE - STANDARD PROCEDURE)

5. CHECK FOR PLUGGED TRANSMISSION FILTER

Remove the Transmission Pan and inspect the Transmission Fluid and Transmission Filter per the Service Information.

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

Yes

Repair the cause of the plugged Transmission Filter. Refer to the Service Information for the proper repair procedure.

Perform 42RLE TRANSMISSION VERIFICATION TEST. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - 42RLE - STANDARD PROCEDURE)

Replace the Transmission Oil Pump per the Service Information.
Perform 42RLE TRANSMISSION VERIFICATION TEST.
Perform 42RLE TRANSMISSION VERIFICATION TEST. (Refer to 21 - TRANSMISSION/TRANSAXLE/AUTOMATIC - 42RLE - STANDARD PROCEDURE)