Ford Pick-Ups/Bronco 1976-1986 Repair Guide

Glow Plug System


See Figures 1 and 2

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Fig. Fig. 1: Diesel glow plug system

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Fig. Fig. 2: Diesel glow plug wiring schematic

The diesel engine utilizes an electric glow plug system to aid in the start of the engine. The function of this system is to pre-heat the combustion chamber to aid ignition of the fuel.

The system consists of eight glow plugs (one for each cylinder), control switch, power relay, after glow relay, wait lamp latching relay, wait lamp and the eight fusible links located between the harness and the glow plug terminal.

On initial start with cold engine, the glow plug system operates as follows: The glow plug control switch energizes the power relay (which is a magnetic switch) and the power relay contacts close. Battery current energizes the glow plugs. Current to the glow plugs and a wait lamp will be shut off when the glow plugs are hot enough. This takes from 2 to 10 second after the key is first turned on. When the wait lamp goes off, the engine is ready to start. After the engine is started the glow plugs begin an on-off cycle for about 40 to 90 seconds. This cycle helps to clear start-up smoke. The control switch is threaded into the left cylinder head coolant jacket. the control unit senses engine coolant temperature. Since the control unit senses temperature, the glow plug system will not be activated unless needed. On a restart (warm engine) the glow plug system will not be activated unless the coolant temperature drops below 165°F (91°C).

Since the fast start system utilizes 6 volt glow plugs in a 12 volt system to achieve rapid heating of the glow plug, a cycling device is required in the circuit.

Never bypass the power relay of the glow plug system. Constant battery current (12 volts) to glow plugs will cause them to overheat and fail.