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. An 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 a circuit that drops out or is noisy, or by loose/worn throttle plates that close tight during a deceleration 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 POWERTRAIN CONTROL MODULE (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) illuminates. 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 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 (FPDM) Applications
Note. The Ford GT uses 2 FPDMs to control fuel for the dual-injection fuel delivery system. The PCM will individually monitor both FPDMs through their FPM circuits.
The FPDM communicates diagnostic information to the PCM through the FPM circuit. This information is sent by the FPDM as a duty cycle signal. The 3 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 diagnostic tools will display the FP_M PID as the duty cycle in column 1. Other diagnostic 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 diagnostic tools will display the FP_M PID as the duty cycle in column 1. Other diagnostic 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 used to control the electric fuel pump. With the electronic engine control (EEC) 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 and will be turned off by the PCM if engine rotation is not detected.
Fuel Pump Driver Module (FPDM) Applications (and Applications with Fuel Pump Functions Incorporated in Rear Electronic Module)
Note. For the Thunderbird and LS, the FPDM functions are incorporated in the rear electronic module (REM). The fuel pump operation is the same as in applications using the stand-alone FPDM. However, the REM will transmit diagnostic information over the communications network instead of using a fuel pump monitor (FPM) circuit.
Note. The Ford GT uses 2 FPDMs to control fuel for the dual-injection fuel delivery system. The PCM will send one FP duty cycle which is used by both pumps.
The fuel pump (FP) signal is a duty cycle command sent from the PCM to the 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 sends 25% duty cycle signal on the fuel pump monitor (FPM) circuit. The fuel pump is off. |
| 5-51% | Normal operation. | FPDM operates 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 sends 50% duty cycle signal on FPM circuit. |
| 51-69% | PCM will not output this duty cycle. | Invalid FP duty cycle. FPDM sends 25% duty cycle signal on the fuel pump monitor (FPM) circuit. The fuel pump is off. |
| 70-81% | To request the fuel pump off, the PCM will output a 75% duty cycle. | Valid fuel pump off command from the PCM. FPDM will not operate the fuel pump. FPDM sends a 50% duty cycle signal on the FPM circuit. |
| 82-100% | PCM will not output this duty cycle. | Invalid FP duty cycle. FPDM sends 25% duty cycle signal on the FPM circuit. The fuel pump is off. |
FUEL PUMP DUTY CYCLE OUTPUT FROM PCM
Note. Also refer to FUEL PUMP MONITOR (FPM) and FUEL PUMP DRIVER MODULE (FPDM) .
Idle Air Control (IAC) Valve (Applications Without ETC)
The IAC valve assembly controls the engine idle speed and provides a dashpot function. The IAC valve assembly meters intake air around the throttle plate through a bypass within the IAC valve assembly and throttle body. The PCM determines the desired idle speed or bypass air and signals the IAC valve assembly through a specified duty cycle. The IAC valve responds by positioning the IAC valve to control the amount of bypassed air. The PCM monitors engine RPM and increases or decreases the IAC duty cycle in order to achieve the desired RPM.
Note. The IAC valve assembly is NOT ADJUSTABLE and CANNOT BE CLEANED, also some IAC valves are normally open and others are normally closed. Some IAC valves require engine vacuum to operate.
The PCM uses the IAC valve assembly to control
- No touch start.
- Cold engine fast idle for rapid warm-up.
- Idle (corrects for engine load).
- Stumble or stalling on deceleration (provides a dashpot function).
- Over-temperature idle boost.