If the fan remains on constant then there is nothing to worry about.
If lower speed has failed on the three speed fan it usually takes the middle speed with it, there are exceptions to the rule on this, i have seen myself where the slow speed has failed but the medium has remained operative, very few and far between though. If the slow speed fails on a two speed fan it is usually only the external resistor.
So, if the fan pulses on and off at around 20 second intervals there is a problem. It will either mean re-brushing the three speed fan or changing the resistor if it is a two speed one. the latter is far easier but both jobs require full front end strip down if you cannot see the resistor at the 2 o'clock position as previously described, not a quick job.
Worth doing the on board diagnostics temp check via the instrument cluster, this will give you a definitive if the low speed fan is running. on the 190 the fan should come on at 100deg to cool the engine and stay on until 95 deg.