Idle Speed Control Closed Throttle Determination (applications without Electronic Throttle Control)
One of the fundamental criteria for entering rpm control is an indication of closed throttle. Throttle mode is always calculated to the lowest learned throttle position (TP) voltage seen since engine start. This lowest learned value is called "ratch," since the software acts like a one-way ratch. The ratch value (voltage) is displayed as the TPREL PID. The ratch value is relearned after every engine start. Ratch will learn the lowest, steady TP voltage seen after the engine starts. In some cases, ratch can learn higher values of TP. The time to learn the higher values is significantly longer than the time to learn the lower values. The brakes must also be applied to learn the higher values.
All PCM functions are done using this ratch voltage, including idle speed control. The PCM goes into closed throttle mode when the TP voltage is at the ratch (TPREL PID) value. Increase in TP voltage, normally less than 0.05 volts, will put the PCM in part throttle mode. Throttle mode can be viewed by looking at the TP MODE PID. With the throttle closed, the PID must read C/T (closed throttle). Slightly corrupt values of ratch can prevent the PCM from entering closed throttle mode. An incorrect part throttle indication at idle will prevent entry into closed throttle rpm control, and could result in a high idle. Ratch can be corrupted by a throttle position sensor or circuit that "drops out" or is noisy, or by loose/worn throttle plates that close tight during a decel and spring back at a normal engine vacuum.
Fail-Safe Cooling Strategy
The fail-safe cooling strategy is activated by the PCM only in the event that an overheating condition has been identified. This strategy provides engine temperature control when the cylinder head temperature exceeds certain limits. The cylinder head temperature is measured by the Cylinder Head Temperature (CHT) sensor. For additional information about the CHT sensor, refer to PCM INPUTS for a description of the CHT sensor. Note: Not all vehicles equipped with a CHT sensor will have the fail-safe cooling strategy.
A cooling system failure such as low coolant or coolant loss could cause an overheating condition. As a result, damage to major engine components could occur. Along with a CHT sensor, the fail-safe cooling strategy is used to prevent damage by allowing air cooling of the engine. This strategy allows the vehicle to be driven safely for a short time with some loss of performance when a overheat condition exist.
Engine temperature is controlled by varying and alternating the number of disabled fuel injectors. This allows all cylinders to cool. When the fuel injectors are disabled, their respective cylinders work as air pumps, and this air is used to cool the cylinders. The more fuel injectors that are disabled, the cooler the engine runs, but the engine has less power.
Note. A wide open throttle (WOT) delay is incorporated if the CHT temperature is exceeded during WOT operation. At WOT, the injectors will function for a limited amount of time allowing the customer to complete a passing maneuver.
Before injectors are disabled, the fail-safe cooling strategy alerts the customer to a cooling system problem by moving the instrument cluster temperature gauge to the hot zone and a PCM DTC P1285 is set. Depending on the vehicle, other indicators, such as an audible chime or warning lamp, can be used to alert the customer of fail-safe cooling. If overheating continues, the strategy begins to disable the fuel injectors, a DTC P1299 is stored in the PCM memory, and a malfunction indicator light (MIL) (either CHECK ENGINE or SERVICE ENGINE SOON), comes on. If the overheating condition continues and a critical temperature is reached, all fuel injectors are turned off and the engine is disabled.
Applications Using a Fuel Pump Relay for Fuel Pump On/Off Control
The Fuel Pump Monitor (FPM) circuit is spliced into the fuel pump power (FP PWR) circuit and is used by the PCM for diagnostic purposes. The PCM sources a low current voltage down the FPM circuit. With the fuel pump off, this voltage is pulled low by the path to ground through the fuel pump. With the fuel pump off and the FPM circuit low, the PCM can verify that the FPM circuit and the FP PWR circuit are complete from the FPM splice through the fuel pump to ground. This also confirms that the FP PWR or FPM circuits are not shorted to power. With the fuel pump on, voltage is now being supplied from the fuel pump relay to the FP PWR and FPM circuits. With the fuel pump on and the FPM circuit high, the PCM can verify that the FP PWR circuit from the fuel pump relay to the FPM splice is complete. It can also verify that the fuel pump relay contacts are closed and there is a B+ supply to the fuel pump relay.
Fuel Pump Driver Module Applications
The fuel pump driver module (FPDM) communicates diagnostic information to the powertrain control module (PCM) through the Fuel Pump Monitor (FPM) circuit. This information is sent by the FPDM as a duty cycle signal. The three duty cycle signals that may be sent are listed in the following table.
| Duty Cycle (1) | On Time (mSec) | Comments | FP_M PID (2) |
|---|---|---|---|
| 50% | 500 | "All OK" output from FPDM. With this input, the PCM can verify that the FPDM is powered and able to communicate on the FPM circuit. | 80-125% |
| 25% | 250 | FPDM did not receive a Fuel Pump (FP) duty cycle command from the PCM, or the duty cycle that was received was invalid (refer to PCM OUTPUTS , FUEL PUMP ). | 15-60% |
| 75% | 750 | The FPDM has detected a fault in the circuits between the fuel pump and FPDM. | 250-400% |
| (1) If a duty cycle meter and breakout box is used, be aware that these values may be reversed depending on the trigger setting of the specific meter (for example, 25% from FPDM may read as 75% on duty cycle meter depending on trigger setting). (2) Some scan tools will display the FP_M PID as the duty cycle in column 1. Other scan tools will display the FP_M PID as a value shown in the FP_M PID column. This value will fluctuate randomly. It is OK for the value to briefly go outside this range, then return. | |||
| (1) | If a duty cycle meter and breakout box is used, be aware that these values may be reversed depending on the trigger setting of the specific meter (for example, 25% from FPDM may read as 75% on duty cycle meter depending on trigger setting). |
| (2) | Some scan tools will display the FP_M PID as the duty cycle in column 1. Other scan tools will display the FP_M PID as a value shown in the FP_M PID column. This value will fluctuate randomly. It is OK for the value to briefly go outside this range, then return. |
FUEL PUMP DRIVER MODULE DUTY CYCLE SIGNALS
The Fuel Pump (FP) is a PCM output signal that is used to control the electric fuel pump. With the electronic EC power relay contacts closed, vehicle power (VPWR) is sent to the coil of the fuel pump relay. For electric fuel pump operation, the PCM grounds the FP circuit, which is connected to the coil of the fuel pump relay. This energizes the coil and closes the contacts of the relay, sending B+ through the FP PWR circuit to the electric fuel pump. When the ignition key is turned on, the electric fuel pump runs for about one second, but is then turned off by the PCM if engine rotation is not detected.
For applications with two speed fuel pumps, a normally closed low speed fuel pump relay (Scheme 29) is wired into the fuel pump ground circuit. With the low speed fuel pump relay contacts in the normally closed position, there is no extra resistance in the ground circuit for high speed operation. For low speed fuel pump operation, the PCM will ground the Low Fuel Pump (LFP) circuit, which opens the relay contacts. With the relay contacts open, the fuel pump ground circuit now passes through a resistor that is wired into the circuit.
Scheme 29
Fuel Pump Driver Module Applications (and Applications with Fuel Pump Functions Incorporated in Rear Electronic Module)
Note. For the Thunderbird and LS6/LS8, the FPDM functions are incorporated in the Rear Electronic Module (REM). Fuel pump operation is the same as applications using the stand-alone FPDM. The REM will, however, communicate diagnostic information through the BUS +/- circuits (SCP) instead of using a fuel pump monitor (FPM) circuit.
The Fuel Pump (FP) signal is a duty cycle command sent from the powertrain control module (PCM) to the fuel pump driver module (FPDM) ( FUEL PUMP DUTY CYCLE OUTPUT FROM PCM ). The FPDM uses the FP command to operate the fuel pump at the speed requested by the PCM or to turn the pump off.
| FP Duty Cycle Command | PCM Status | FPDM Actions |
|---|---|---|
| 0-5% | PCM will not output this duty cycle. | Invalid FP duty cycle. FPDM will send 25% duty cycle signal on the fuel pump monitor (FPM) circuit. The fuel pump will be off. |
| 5-51% | Normal operation. | FPDM will operate the fuel pump at the speed requested. "FP duty cycle" x 2 = pump speed % of full on. (for example FP duty cycle = 42%. 42x2=84. Pump is run at 84% of full on). FPDM will send 50% duty cycle signal on FPM circuit. |
| 51-67.5% | PCM will not output this duty cycle. | Invalid FP duty cycle. FPDM will send 25% duty cycle signal on the fuel pump monitor (FPM) circuit. The fuel pump will be off. |
| 67.5 -82.5% | To request the fuel pump off, the PCM will output a 75% duty cycle. | Valid fuel pump off command from PCM. FPDM will not operate the fuel pump. FPDM will send a 50% duty cycle signal on the FPM circuit. |
| 82.5-100% | PCM will not output this duty cycle. | Invalid FP duty cycle. FPDM will send 25% duty cycle signal on the FPM circuit. The fuel pump will be off. |
FUEL PUMP DUTY CYCLE OUTPUT FROM PCM
Note. Also refer to PCM INPUTS , FUEL PUMP MONITOR and POWERTRAIN CONTROL HARDWARE , FUEL PUMP DRIVER MODULE .