Bernard
 

The A/C aux. fan does not turn on when A/C is turned on, electrical test confirms that the fan is operational and the coolant reservoir is full.

How should I proceed to troubleshoot the problem?

1987 260E, no conversion from original. operation manual not much help

First things first... What model car are we talking about?

Second... Has the A/C been converted to 134Afrom r12?

The best way to proceed is to study an operations manual to determine exactly how your system should function, then troubleshoot from the simplest to the most complex.

The auxilliary fan may not automatically come on with the A/C. This fan is controlled primarily by temperature, and functions to draw air though the A/C condensor and engine radiator when the engine temp is high. This means that the aux fan will come on generally when the vehicle is hot and not moving, such as idle after running on a hot day, or in stop/go driving on hot days. When the car is going down the road at 65 mph, fans are not needed for condensor and radiator cooling. You can best test after a good run on a hot 80 degee day with the A/C on, let the car run at idle, the fan should come on.

Bernard
 

My MB is a 1987 260E, no conversion from original A/C, operation manual not much help.

what about the fan relay?

The aux fan should come on when the A/C system builds pressure..The original aux fan switch(green in color) on the drier was designed to come on at 300psi (20bar). The replacement switch (red in color) is designed to come on at 235psi (16 bar). IF the fan doesn't come then the system pressure is too low, the pressure switch isn't closing, the low speed relay is defective, or the ballast resistor is defective....or even a bad aux fan motor.

Bernard
 

Thanks for the Information, I will check it out.
Thanks again.

AlKraut hit the nail on the head!! Temp controled!!:)

Smokie