It seems my fan is not working properly as the other day I had the car up to temp. and it wanted to start but would not.It kept doing this but never did engage this was with the A.C. off or on idleing.It's like it wants to start but does'nt.The fan and clutch were recently replaced with a used one.Where should I start to look and check for this problem.

the auxiliary fan only comes on when the A/C system becomes hot enough.

To test the electric fan, hook it up directly to a 12v battery.

...you mentioned the 'clutch'... why?

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The electric fan only comes on with hot engine temperature on the 78-80 300SD.

Could my relay be bad?If so how can I test it? In the meantime I might try to put 12 volts directly to it tosee if it spins.