I have a 1987 Bertone X/19, during driving the radiator fan comes on and continues to blow until the vehicle is turned off
That sounds like the temp switch is not turning off once it sets to on and I would also replace the fan relay, sometimes the points inside get pitted and they don't release properly.
Thanks so much tennisshoes, from you answer it seems like this is the solution. To be correct in diagnosing the problem the fan is only supposed to come on when the car reaches a certain temperature correct?
That is correct, the last X 19 I worked on was in 1991 so I don't remember the turn on and off temps.
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