Ford- Fairmount/Zephyr 1978-1983 Repair Guide

Feedback Carburetor Electronic Engine Control



This system actually consists of three subsystems: a two part catalytic converter, a Thermactor (air pump) system, and an electronically controlled feedback carburetor.

The converter consists of two catalytic converters in one shell. The front section is designed to control all three engine emissions (NOx, HC, and CO). The rear section acts only on HC and CO. There is a space between the two sections which serves as a mixing chamber. Air is pumped into this area by the Thermactor system to assist in the oxidation of HC and CO.

The Thermactor system is the same as that found on conventional Ford models, with the addition of a second air control valve and a second exhaust check valve.

An electronically controlled feedback carburetor (Motorcraft mode 6500 or 7200/2700) is used to precisely calibrate fuel metering. The air/fuel ratio is externally controlled and variable. There are two modes of operation: closed loop control and open loop control. Under closed loop operation, each component in the chain is sensitive to the signals sent by the other components. This means that the carburetor mixture is being controlled by the vacuum regulator/solenoid, which is adjusted by the control unit, which is receiving signals from the oxygen sensor in the exhaust manifold, which is measuring a mixture determined by the carburetor, and so on. In this case, the feedback loop is complete. Under open loop operation, the carburetor air/fuel mixture is controlled directly by the control unit according to a predetermined setting. Open loop operation takes place when the coolant temperature is below 125°F, or when the throttle is closed, during idle or deceleration.

The control unit (ECU or MCU) receives signals from the exhaust gas oxygen sensor, the throttle angle vacuum switch, and the cold temperature vacuum switch, analyzes them, and sends out commands to the vacuum solenoid/regulator, which in turn adjusts, by means of vacuum, the height of the carburetor fuel metering rod. In this way, the fuel mixture is adjusted according to conditions. The control unit also varies the transition time from rich to lean (and vice versa) according to engine rpm. The rpm signal is taken from the coil connector TACH terminal.

There are two differences between the ECU, used in 1978 and 1979, and the MCU, used in 1980 and later models. The MCU is programmable, enabling it to be used with many different engine calibrations. Additionally, the MCU controls the Thermactor solenoid valves, thus directing the air flow to the exhaust manifold, the catalytic converter mixing chamber, or the atmosphere when air flow is not needed or wanted.

Because of the complicated nature of the Ford system, special diagnostic tools are necessary for troubleshooting and repair. No attempt at testing or repair should be made unless both the Feedback Control Tester (Ford part no. T78L-50-FBC-1 or equivalent) and a digital volt/ohmmeter (Ford part no. T78L-50-DVOM or equivalent) are available. A tachometer, vacuum gauge, hand vacuum pump and gauge, and a special throttle rpm tool are also required for diagnosis. No troubleshooting procedures will be given here, since they are supplied with the testing equipment.