767-200 with CF6-80A engines returned to gate with an engine that failed to start.
The pneumatic start valve was operational. The spar fuel valve was also working correctly. The engine fuel valve was not opening and valve indication light stayed on.
We hung a replacement Fuel Conditioning Unit, which was no help. The conditioning unit has an actuating piston which is attached to the fuel control (or MEC - Main Engine Control as it is usually called). Fuel is allowed to flow with the piston extended.
We noticed the installed unit and the replacement had "maybe" an inch worth of travel distance. To be sure this was enough, we opened up the other engine. We noticed twice (maybe three times as much) piston travel on the known good conditioner. We hung the replacement unit on the good engine, which also moved this greater distance.
A conditioner control relay (K168) was tried which was no help. Open command voltage at the conditioner plug was significantly lower than 28VDC.
The problem turned out to be high resistance on the fire control handle switch. After a few cycles of the fire handle, all valve operations were normal. A dirty fire handle switch contact was restricting current flow. The piston moved... just not enough.
Power for the conditioner goes through the fire handle switch first. Next in line is the fuel shut-off handle, followed by the conditioner. Two situations are represented by the engine valve light. With the shut-off handle up and the conditioner not fully extended, the light will be on. With the shut-off handle down and the conditioner not fully retracted, the light will be on.