THis is how it works:

YOu get in the car, turn the key, the instrument panel warning lights come on, you turn key to "START" but the starter does not engage.
You relax the key, let it go back to "ON".
You wait 30 seconds.
If the engine cooling fan speeds up to very high, noisy speed - this tells us that the Fan module is powered up but the ECU is not powered due to a bad RCM.
If the fan does not speed up, we would look at other items like starter interlock, pulse module, weak battery, etc.

The reason we are quick to blame the RCM is because the speeding up of the fan is due to the "engine control relay" output from the RCM not powering up, so the ECU is not powered up, but the Fan Module IS powered up. Anytime the Fan Module is powered up, it expects to see "tickle pulses" from the ECU. If the ECU is not powered up, there will be no 'tickle pulses' and the Fan Module assumes its lost contact with the ECU and runs the fan in emergency mode to keep the car from overheating - because the ECU is not there to tell the Fan module how fast to spin the fan.

This is always a bad RCM, either the solder junctions at the connector pins fractured, or the engine control relay is bad. This happens very often to our RCMs and to the RCMs in the SLK320 and SLK230, as the RCM is the same for all three cars.