Full-size Truck Two-Mode Hybrid Auxiliary Pump Diagnosis

Owners of a 2009-2010 Escalade, Silverado, Sierra, or 2008-2010 Tahoe or Yukon, with the Two-Mode Hybrid option (RPO HP2) and 2ML70 transmission (RPO M99) may notice a driveability condition where the vehicle jerks or surges when the engine is in Auto Stop mode, as observed on the tachometer. (Fig. 8)

The condition may include:

·       A lull (clutch slipping) followed by a jerk (clutch grabbing) during acceleration from a stop with light throttle that occurs just before an engine restart, or

·       A surge during deceleration below 15 mph with light to moderate braking that occurs after the tachometer drops to Auto Stop.

Either of these conditions may be caused by an auxiliary transmission fluid pump or a slipping clutch within the transmission.

Auxiliary pump part numbers 19168291 (2008-2009) and 19207983 (2010) have been placed on restriction. If diagnostics lead to auxiliary transmission fluid pump replacement, contact TAC. PIP4751A in SI includes additional information for auxiliary pump diagnosis.

Diagnosis

The main hydraulic system of the Hybrid Two-Mode 2ML70 transmission primarily consists of a vane-type pump, two control valve body assemblies, converter housing and case. The pump maintains the working pressures needed to stroke the clutch pistons that apply or release the friction components. The hydraulic system also uses a secondary system consisting of an auxiliary pump attached to the bottom of the transmission. This is a gerotor-type pump that maintains working pressures when the engine is off in Auto Stop mode. (Fig. 9)

TIP: A low State of Charge (SOC) of the 300V battery may cause a similar jerk or surge condition. An updated minimum SOC of 60% has been established since a recent Emerging Issues seminar discussing the auxiliary transmission pump was released. Let the vehicle sit for one hour and make sure the SOC is above 60% before beginning diagnosis of the auxiliary transmission fluid pump.

During diagnosis, check for DTC P2797, Auxiliary Transmission Fluid Pump Performance, in the Hybrid Powertrain Control Module. If DTC P2797 is present, it is necessary to follow SI diagnostics for this code before proceeding with the repairs outlined in PIP4751A.

TIP: If any DTC other than P2797 is present, follow the normal flow charts for the DTC in SI to identify the issue.

After completing repairs for any set DTCs, take a snap shot while in Auto Stop mode with the vehicle in Drive and both the parking brake and service brakes applied to maintain zero vehicle speed. Next, slowly increase the throttle position to 25% until the ICE restarts (the Motor 2 torque command will exceed 100 Nm).

Review the snap shot and graph Calc. Throttle Position, Motor 2 Torque Actual, Motor 2 Speed Actual, Aux. Trans. Fluid Pump Actual, and Engine RPM. (Fig. 10)

In the accompanying snap shot plot, the auxiliary pump percentage is above zero, which means it's working electrically. Also, since the clutch has sufficient hydraulic force to keep Motor 2 from rotating, the auxiliary pump is operating mechanically. However, during operation of the pump, its percentage drops to near zero and the Motor 2 Speed increases.

If the Motor 2 Speed exceeds 20 RPM and, at the same time, auxiliary pump percentage drops to near zero prior to the engine restart, the auxiliary pump has stalled. As a result of the drop in pressure to the clutch, Motor 2 is allowed to rotate.

TIP: There may be audible indications of electric motor speed increases and clicking noises associated with the auxiliary pump stall.

If the Motor 2 Speed stays below 20 RPM and the auxiliary pump percentage stays relatively steady until the engine restart, the pump is operating within normal parameters.

When diagnosis reveals a stalling auxiliary pump, contact TAC for information on part replacement.