Tough question. First they give almost no problems. This makes deciding extra hard. The force exerted probably can be qualitatively determined by an experienced tech, but even then its a lot like deciding about shocks.

The only feature I know to test is the thermostatic engagement. If you hold the engine at 2000rpm from about 160deg temp till it gets to stable temp you will notice a distinct change in the noise level of the fan when the t-stat closes (in the fan clutch). For this to mean anything you have to know the temp of the radiator. Most shops now use infrared thermometers to make these judgements.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician