Fairly easy diagnostics. First be sure your fuse is good. Some cars have had their fuse moved out of the box to a single fuse holder similar to diesel glow fuses.
If the fuse is good (don't assume verify voltage at both ends), then you should look for the fan speed signal. This signal goes from the pushbutton controller to the blower regulator. It is easily available at a three wire connector just inboard of the brake booster on the firewall. It will be the yellow wire. The voltage on the wire will be from about 1v to 6-7v as you go from slow to high speed. In auto the speed signal will change as you change the temp. If this signal is good, expose the blower and reverify the power (red wire) side of the blower. The regulator controlls current in relationship to input voltage by regulating the ground side. I always like to jumper the ground side just to prove the motor will work, if I am condemning the regulator (which is below the motor).
Bosch Master, ASE Master, L1
33 years MB technician
Last edited by stevebfl; 08-26-2001 at 08:32 AM.