This was an interesting failure that came in the other day that most techs have probably not seen. This Malibu Hybrid came in with a complaint of auto-stop inoperative and Service Hybrid System message on. The only code set was a P2797. It was set as a hard code and would not clear. This code is set when the auxiliary transmission fluid pump is commanded on during auto-stop and the TFP switches do not indicate the fluid pressure increase from the running pump. Since the pump running is necessary for auto-stop, it is disabled when this code is set.

If you ever look at the S.I. diagnostics for this code, they are a total joke and unusable. There are two tests that can be performed with the Tech 2, you can command the relay on or you can command the pump on. The relay on test works, but the pump on test does not as the Tech 2 cannot command the pump on although this test is offered.

This pump does not operate as most relay controlled devices do as neither power or ground are provided to the pump at all times while the relay switches either on or off. The power circuit to the pump is switched on and off by the relay which is controlled by the SGCM and the ground circuit is controlled by the Hybrid Pump Driver Module which supplies a PWM ground to the pump.

The easiest way to test this circuit is to first unplug the HPDM and supply power and ground to the pump through ckts 7435 and 7436. If the pump runs and the TFP A parameter switches from low to high, the hydraulic portion of the circuit is OK. Next, command the relay on with the Tech 2 and see if 12 volts is supplied to the pump through ckt 7435. If so, with the HPDM plugged back in, unplug the X1 connector at the SGCM. Remove the relay and jump across the connectors with a fused jumper to supply power to the pump. Supply power to the HPDM with another fused jumper at terminal 1, ckt 7439. Hook a test light to battery positive and rapidly tap terminal 15, ckt 7438 with the test ligh to simulate a PWM command from the SGCM to the HPDM. The pump should run. If not, the HPDM is at fault. If it does run, the SGCM or either connector or wiring is at fault.

With my vehicle, the pump did not run during this test and the HPDM needed to be replaced. This resolved the complaint.

James from Florida