Exhaust Temperature Sensor: Description and Operation

Fig.24 - Coasting Richer & Over Temperature Control System:

Fig.33 - Over Temperature Control System:

Fig.34 - Over Temperature Control System:

This system, Figs. 24, 33 and 34, protects the catalytic converter from overheating when the vehicle is coasting. This overheating condition occurs when the catalytic converter tries to oxidize the increased amount of Unburned hydrocarbons during coasting.

When the vehicle is coasting, the Coasting Richer System and AIR System are both in operation to prevent overheating of the catalytic converter due to poor combustion. When the catalytic converter temperature reaches 1350° F. due to high temperature and in- creased load driving conditions, the AIR system air is diverted to atmosphere in order to reduce the chemical reaction. When catalytic converter temperature exceeds about 1S30° F. due to a malfunction such as an ignition failure, a warning light and buzzer indicate an overtemperature condition.

Fig.34 - Over Temperature Control System:

Fig.35 - Vacuum Switching Valve:

Fig.36 - Vacuum Switching Valve:

The vacuum switching valve, Figs. 35 and 36, used in this system, has an electrically controlled solenoid plunger. When the catalytic converter temperature is over about 1350° F., the solenoid plunger is energized and the vacuum switching valve connects the diaphragm chamber "B" of the air switching valve, Fig. 34, to intake manifold allowing manifold vacuum to be applied to chamber "B". When the solenoid plunger is de-energized, the port is closed, so diaphragm chamber "B" of air switching valve is connected to diaphragm chamber "A".

Fig.37 - Air Switching Valve:

The air switching valve, Fig. 37, is used to switch air flow from the air pump and is controlled by manifold vacuum and air pump pressure which are switched by the air switching valve. When air pump pressure is applied to diaphragm chamber "B" through the vacuum switching valve, the air from the air pump flows to the check valve. When manifold vacuum is applied to diaphragm chamber "B," the valve closes air flow to the check valve and diverts it to atmosphere.

A thermo sensor is installed on the front side of the converter and is covered with a stainless steel sheath. Sensing electrical resistance of the thermo sensor, a thermo controller sends ON and OFF signals to the vacuum switching valve, warning lamp and buzzer. When the catalyst temperature reaches about 1350° ., the controller sends an ON signal to the vacuum switching valve, and when temperature exceeds about 1830° F, it sends an ON signal to the warning lamp and buzzer.