My fan bearing is on its way out at 186,000 miles and I plan to take care of it soon after reading threads on this subject. My question is does the fan speed increase proportionally to the engine speed or is there a limit to the speed? This car is my daily driver for me and my two kids and I don't want a major breakdown or get stuck some where with the kids in the car.

Actually you have three fans...the main fan behind the shroud and two auxiliary fans.

The main fan is on a clutch, and when engaged, it does spin in proportion to the engine speed.

The auxiliary fans get a signal from the temperature sensor to indicate that additional cooling is needed, and they spin a either a fixed low or high speed, depending on cooling demands.

Actually the W124 cooling system process is a bit complex, with all sorts of backup plans to handle overheating issues. One such plan is that the A/C compressor gets shut down under dire circumstances.

If the weather is mild (not hot) right now, you should be okay.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

I thought the main fan was on a viscous coupling. When engaged it runs roughly in proportion to engine speed up to a maximum of about 3000rpm when it doesn't go any faster.

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Mick J
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)

If you're talking about the engine driven main fan and the respective bearing..... the viscous coupling will/can/should keep the fan itself from spinning too fast, BUT the pulley and bearing hub are still driven by the belt and will continue to turn at engine speed. With a mechanics stethascope (or long solid rod ie; extension ) you should be able to listen to all bearings and track down the noise, just beware of moving parts!!! Hope this helps and good luck!!
MV