1992 300E w/137,000 miles. The A/C blower fan will not increase to maximum speed when the fixed max speed button is pushed on the climate control panel. The blower operates correctly when the fixed minimum speed or auto button is pushed. All other functions of the climate control panel work fine. Per the MBZ Automatic Climate Control W124 Service Manual, I checked the voltage output at the climate control panel and got the correct voltage outputs for all the different button positions. The min showed 0.9 volts when pushed, the auto button showed 4.6 volts when pushed, and the max button showed 9.2 volts when pushed, so it seems the right signal is going to the blower motor regulator for each setting including the high blower speed. So, how do I tell if the blower motor regulator is the problem since it seems to regulate the blower motor speed in the auto and min modes, or might the brushes in the motor be bad enough so that the motor is not able to produce maximum speed ? The approximate amps to the blower regulator should be 5(min), 7.5-22(auto) and 28(max). I have not pulled off the blower motor cover under the hood to check the amperage until I perhaps get some suggestions from someone. Any thoughts on further diagnosis?

Blower max speed can be chcked by grounding the blu wire at the motor. This test by-passes the reg circuit.
If blower still does not get max rpm, then the fan should be spun by hand to see if it is free..if not, the bearings are dry..if free, then check the brushes and check the armature collector slots under the brushes..they get full of worn brush carbon and will limit rpm. They can be cleaned/cleared with a thin blade and cleaner. Also see if brushes are free to float.
If high fan w/blu jumper , then reg is bad if you have variable trigger voltage from CP at yellow wire at regulator.