PCM Output Controls
Description:
SERVICE TELLTALE LAMP
The SERVICE telltale lamp is a warning indicator lamp that the PCM illuminates for a non-emission related fault. This lamp is illuminated for all type C related diagnostic trouble codes by the QDM A located internally in the PCM.
The PCM will illuminate the SERVICE telltale lamp in an event the ABS telltale cannot illuminate (or has detected a fault in) its own warning system. The ABS will send a message to the PCM that a fault has occurred and the SERVICE light will illuminate.
The SERVICE telltale lamp will illuminate for 2-3 seconds upon key up to test the bulb and circuit.
If a test by the PCM "passes" after a fault was found, the service telltale lamp will turn Off after three seconds but the fault DTC will remain in history as well as the malfunction history data.
HOT/LOW COOLANT TELLTALE LAMP
The hot telltale lamp is used by the PCM to indicate to the operator of the vehicle a possible engine overheat condition. If the PCM determines from signals received from the engine coolant temperature sensor that the engine is running too hot, it will turn on the coolant temperature telltale lamp by supplying a ground. The hot telltale lamp is also used when there is a transaxle overheat condition. This telltale lamp will flash for a low coolant condition and will remain On steady for an overheat condition.
The low coolant telltale lamp (the hot lamp and low coolant lamp are one bulb) is used to indicate when the level in the coolant bottle has fallen below a specified amount. This lamp is also used to indicate if the engine coolant or transaxle fluid oil temperature is hot.
CRUISE CLUTCH
The ground to the cruise clutch is controlled by QDM A located internally in the PCM. The PCM will supply the ground to the clutch when certain conditions for safety have been met. The cruise On/Off switch and brake switch are monitored by the PCM and in effect will be disengaged if a fault in these circuits are found regardless of driver input.
MALFUNCTION INDICATOR (SERVICE ENGINE SOON TELLTALE LAMP)
The MIL (SERVICE ENGINE SOON telltale lamp) is used by the PCM to signal the operator of the vehicle of a possible problem. This telltale lamp will be commanded On steady for A and B type DTCs.
When the PCM requires the MIL to be turned On, it will supply a ground signal for the MIL. If the microprocessor is disabled for any reason, a ground for the MIL (SERVICE ENGINE SOON telltale lamp) will be automatically provided. This telltale lamp will flash if a catalyst damaging misfire is being detected.
UPSHIFT
The upshift telltale lamp is used, on vehicles equipped with a manual transaxle, to signal the driver to upshift the transaxle. Shifting each time the upshift telltale lamp comes on will obtain maximum fuel economy.
To turn On the telltale lamp, the PCM will supply a ground allowing current to flow through the lamp, turning it on.
FAN CONTROL RELAY
The fan control relay is used by the PCM to control engine cooling fan operation. When the PCM supplies a ground to the coil of the fan control relay, current is allowed to flow through the switch contacts of the relay to the engine cooling fan.
The engine cooling fan will be turned On or Off by the PCM dependent upon engine coolant temperature, vehicle speed or A/C On request. Under high ambient conditions, the fan may run up to four minutes with the key Off. If the PCM should become disabled a ground for the fan control relay will be automatically provided.
A/C CLUTCH CONTROL RELAY
The A/C clutch control relay is used by the PCM to turn On or Off the A/C compressor clutch coil. When the PCM receives an A/C request signal, it will supply a ground to the A/C clutch control relay, energizing the relay and allowing the A/C compressor clutch coil to be energized. This allows the A/C compressor to be turned On. During certain driving conditions, the PCM will turn Off the A/C compressor by removing the ground for the A/C clutch control relay.
For more information on the air conditioning system, refer to the "HVAC Service Manual."
AIR CONDITIONER CLUTCH CONTROL
When the PCM receives an A/C request signal it will supply a ground to the A/C control relay allowing the A/C compressor to operate. During the following conditions the PCM will turn Off, or shut down A/C operation.
1. If the coolant temperature exceeds 118°C (244°F). A/C will be turned On again when coolant temperature decreases to 113°C (234°F).
2. During periods of wide-open throttle acceleration for up to 15 seconds. (Wide-open throttle is defined as throttle position sensor greater than 95% vehicle with an automatic transaxle, and 90% on vehicles with a manual transaxle.)
3. If vehicle speed is less than 16 km/h (10 mph), manifold air temperature is greater than 45°C (113°F) and throttle position sensor shows an increase greater than 8%. A/C will be shut-off for 0.3 seconds with an automatic transaxle or 1.3 seconds for a manual transaxle.
4. If engine rpm exceeds 6500 rpm. A/C will be turned back On when rpm is less than 4500 rpm for two seconds.
5. If A/C high side exceeds 2758±138 kPa (400±20 psi). A/C will be turned On when A/C high side pressure drops below 1724±3450 kPa (250±50 psi).
6. If ambient temperature, measured at the air temperature sensor, is between 4°C and 20°C (40°F and 60°F) and vehicle speed is greater than zero, the PCM will cycle the A/C compressor once every 90 seconds depending on manifold air temperature and vehicle speed.
IMPORTANT: The compressor contains a high pressure switch that can turn the clutch Off and the PCM will think the compressor is still On. This will affect the IAC learning process.
A/C REQUEST
The A/C request input is received from the A/C control head. When the A/C is turned On, a B+ signal from the A/C switch, located in the A/C control head, is sent to the PCM. If the A/C low-and high-pressure switches are closed then the B+ signal is received by the PCM. This signal notifies the PCM that the operator of the vehicle has requested that the A/C system be turned On. If engine conditions allow, the PCM will energize the A/C coil relay by supplying a ground.
COOLANT GAGE
The PCM controls the coolant gage by sending pulse width modulated (PWM) signal to the instrument panel cluster. The PCM uses the ECT to determine the gage reading in the cluster
TACHOMETER
The PCM controls the tachometer by sending pulse width modulated (PWM) signal to the instrument panel cluster. The PCM uses the crankshaft position sensor (CKP) to calculate engine rpm and relay that information to the tachometer.
SPEEDOMETER
The PCM controls the speedometer by sending 5000 pulses per mile. The frequency of the signal is read by the speedometer and converted to mph/km/h.
LOW FUEL LAMP
The LOW FUEL telltale is controlled by the PCM through a quad driver module (QDM). During the bulb check, the QDM commands the bulb On for three seconds by supplying a ground to the bulb. The telltale will also be commanded On if the fuel tank level is below one gallon. If the PCM detects a fault in the fuel level sender or fuel gauge, the LOW FUEL telltale will be commanded On.