35 Loss of Prime

THEORY OF OPERATION:
The loss of prime test is used to prevent transmission defaults which can be caused by lack of pump prime.

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 L-R, 2-4 and OD, in 2nd, 3rd, and 4th gear 2-4 and OD) are turned on by the Transmission Control Module (TCM) to see if pump prime exists. The code is set if none of the pressure switches respond. The TCM will continue to run the loss of prime test until pump pressure returns.

LIMP-IN:
NO

WHEN MONITORED/SET CONDITIONS:
If the transmission is slipping in any forward gear and the pressure switches are not indicating pressure, a loss of prime test is run.

TRANSMISSION EFFECTS:
All model year and calibrations. Vehicle will not move or transmission slips. Normal operation will continue if pump prime returns.

POSSIBLE CAUSES:
- Low transmission fluid level
- PRNDL indicates a valid OD code in the hydraulic reverse position
- Transmission fluid filter clogged or damaged
- Transmission fluid filter improperly installed (bolts loose or O-ring missing)
- Oil pump

If a customer has a problem when the transmission is cold whereby he shifts to reverse, gets reverse, and then shifts to OD and does not get OD (gets a neutral condition), and then can not get reverse or OD for between 3-20 seconds, replace the oil pump. High side clearance in the oil pump will set a code 35. The pump will prime upon start-up, but as the torque converter purges air (drain down) the air will leak across the inner rotor into the pump suction port and cause a loss of prime right after the shift into OD. After 3-20 seconds, pump prime will return and normal operation will continue. The pump should be replaced only after all other possible causes above have been checked and verified.