Intake Manifold Air Heater Relays

Fig. 6 Intake Manifold Air Heater Relays:

LOCATION
The 2 relays are located in the engine compartment below the left battery.

DESCRIPTION
The powertrain control module (PCM) operates the 2 heating elements within the air heater assembly through the 2 intake manifold air heater relays. The air heater elements are used to heat incoming air flowing into the intake manifold. This will help engine starting and improve driveability with cool or cold outside temperatures.

OPERATION
The relays may be energized by the PCM before and after cranking. This will depend on inputs the PCM receives from: the intake manifold air temperature sensor, the engine speed sensor and the vehicle speed sensor.

Fig. 7 Air Heater Cycle Chart:

With a cool or cold engine, the air heater relays and the air heater elements may be activated for a maximum time of approximately 3 1/2 minutes. Refer to the Air Heater Cycle Chart for a temperature/time comparison of relay engagement.

In this chart, Pre-Heat and Post-Heat times are mentioned. Pre-heat is the amount of time the relay circuits are activated when the ignition (key) switch is ON, but the engine has yet to be started. Post-heat is the amount of time the relay circuits are activated after the engine is operating.

WAIT TO START LAMP
The wait-to-start warning lamp is tied to this circuit. Lamp operation is also controlled by the PCM. The wait-to-start warning lamp will not be illuminated during the post-heat cycle.

The relays are not energized during engine cranking. When initially energized, they will make a clicking noise.

PREHEAT CYCLE
The PCM will supply a signal to the 2 relays when the ignition (key) switch is initially turned to the ON position. When this signal is supplied, electrical current is passed through the relays for operation of the 2 heating elements.

If the intake manifold air temperature is 15°C (59°F) or below, the air heater elements are energized and the wait-to-start warning lamp is illuminated. The heater is energized for a specific amount of time. Refer to the Air Heater Cycle Chart for a temperature/time comparison of relay engagement.

Once the heater has cycled, the wait-to-start warning lamp goes out. While the engine is cranked, the heater relays are not energized.

POST-HEAT CYCLE
After the pre-heat cycle is completed, the PCM must receive an engine crank signal (engine speed between 32 and 475 rpm) followed by an engine run signal (engine speed above 475 rpm). Intake manifold air temperature must also be below 15°C (59°F). All of these signals must be seen by the PCM before initiating the post-heat cycle.

Depending upon intake manifold air temperature, engine rpm and predetermined PCM values, one or both of the relays and one or both of the heating elements may be activated. This may be observed as a large needle swing on the vehicle voltmeter and is due to the high-amperage draw of the heating elements. Each heating element will draw approximately 95 amps at 12 volts. This voltmeter movement is a normal condition during the post-beat cycle.

BATTERY SAVE FEATURES
The PCM is also programmed with battery saving features. It will shut down the air heater relays if:
- The engine starter is operated during the preheat cycle.
- The engine stalls during the post-heat cycle.
- The engine starter is operated for more than 10 seconds during the post-heat cycle.
- The vehicle speed is above 10 mph during the post-heat cycle.
The post-heat cycle will continue for up to 3 1/2 minutes unless the PCM determines one or more of these preceding features interrupts the cycle strategy.