[Piwoslaw]

Maybe the fuse you tapped into controls more than just the glow plugs? Have you been able to get an electrical wiring diagram of your car/engine? Have you asked around at a Renault forum about how the glow plug subsystem works??

My advice is to find your glow plug relay and check which wires do what: Usually you will have a pair of wires which activate the relay, plus one thick wire going in with the supply voltage for the plugs and either 4 or 1 (in older models) going out to the plugs. Some cars (PSA diesels, for example) also have an 8th 'control' wire which goes back to the ECU to let it know whether the relay activated when it was supposed to. You can tap into either the wire which activates the relay (may cause the ECU to throw a code) or the thick supply (switch/relay must handle higher power).

You could automate your set-up so that the glow plugs are deactivated whenever coolant temperature is above 60°C, for example. This can be done with a simple circuit of the type found in household thermostats for controlling central heating.