HEV / PHEV Cooling System Diagnostics

The cooling system in the Plug-in Hybrid has been designed to include two functional cooling loops. This system is designed to maximize cooling efficiency when the vehicle is running on either the gas engine or the electric motor. The system operates in one of two different modes.

The first mode is the main or "combined" cooling loop mode which provides coolant flow through both the gas engine and the cabin heater core. While in this loop, both the gas engine and an electric heater can be used to maximize the heat transfer to the coolant thus providing both an increase in the engine metal temperature and heat for the vehicle cabin.

The second cooling mode is the "isolated" loop mode where coolant flow through the cabin heater core is isolated from the engine block. This loop is intended to provide cabin heat when the gas engine is not running. Coolant flow is maintained in the "combined" loop by default (isolation valve de-energized), and by energizing the isolation valve coolant flow is maintained in the "isolated" loop.

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While in the "combined" cooling loop, coolant flow is maintained by a PCM-controlled Engine Coolant Pump (internally known as the electric Water Pump (eWP)). The Engine Coolant Pump is a pulse width modulated pump that can be used to control coolant flow rates independent of engine speed. This allows the PCM to maximize fuel economy by minimizing cooling system power consumption as compared to a traditional belt-driven water pump. Additionally, it can be used to improve engine metal temperature heating/cooling rates. Even though the Engine Coolant Pump can provide coolant flow while the engine is off, it's primary purpose is to provide coolant flow while the engine is running. A low power consumption auxiliary water pump in the "isolation" loop is the primary source of coolant flow for cabin heating when the vehicle is operating in electric mode.

The Engine Coolant Bypass Valve position determined whether the cooling system operates in a combined loop (engine-heated coolant flowing through the heater core) or in an isolated loop (engine is off, cabin heat provided by PTC heater). The ECBV is checked for circuit faults and for actual valve position after the valve is determined to be electrically failed.

The Engine Coolant Pump (eWP) is a smart pump with four pins. Two of the pins are connected directly to battery power and electrical ground. The other circuits are connected to the PCM. One is connected to the LIN bus (primary PCM control) and the other is called the Emergency Run Input (ERI) line that can be used to control the pump with a PWM signal if the LIN bus goes down. This ERI line has been wired directly to ignition power so that the pump will be commanded to run any time the LIN bus is failed and ignition is on.

Courtesy of FORD MOTOR CO.

Below is a summary of the diagnostics associated with the Engine Coolant Pump. Circuit faults for the LIN bus and ERI lines are detected by the PCM while the pump power and ground line faults are detected by the Engine Coolant Pump Control Module and communicated to the PCM through the LIN bus line. All mechanical faults are detected by the Engine Coolant Pump Control Module and also communicated to the PCM over the LIN bus.

The Engine Coolant Pump speed is controlled by the PCM and communicated to the Engine Coolant Pump Control Module over the LIN bus. A LIN bus communication fault (U019F) is set when the engine coolant pump speed echoed back from the Engine Coolant Pump Control Module doesn't match the desired speed sent from the PCM (requires no other electric water pump faults exist).

Failures on the Emergency Run Input (ERI) line are detected by checking the status of the ERI line communicated to the PCM over LIN. Since the ERI line is hardwired to the vehicle ignition key, the returned ERI line state should always match the vehicle ignition state. When there is a mismatch, a DTC P26CA is set. This monitor requires that the LIN communications bus has not failed.

The Engine Coolant Pump power and ground line status is not communicated to the PCM by Engine Coolant Pump Control Module. Therefore, the status of these lines is inferred by the PCM. If either of these lines is faulted, the Engine Coolant Pump will not run, and there will no LIN bus communications from the Engine Coolant Pump Control Module to the PCM. The PCM first checks to see if a fault on the communication line (U019F). If the communication line is faulted and the engine coolant temperature is increasing then it is assumed that the pump is not running and a P26D3 DTC is set. If the engine coolant temperature is not increasing then the PCM identifies the communications fault only.

The Engine Coolant Pump Control Module can use the rpm feedback and current feedback to detect mechanical faults for a blocked impeller and a "dry run" condition, i.e. loss of coolant. These conditions are communicated back to the PCM over the LIN bus.

The Engine Coolant Pump Control Module communicates the control module electronics voltage and temperature over the LIN bus to the PCM. The voltage is compared to the PCM system voltage to set DTCs P26D0 and P26D1. The temperature signal is checked for out-of-range high which sets a P26D2 DTC.