The HVAC blower motor will not turn on. We tested the motor by applying 12v directly to it and it works fine. We then installed a new blower motor control regulator to no avail. Finally we installed a replacement dash unit but that also had no effect. The dash unit lights up all the buttons work fine. We ran the diagnostic function on the dash unit but there were no errors. We're stumped as to where to look next.

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire

If this has the 97 era HVAC display, go to the data screen ( key on / display showing that the HVAC is on and fan requested to run / Press and hold the REST button until you see a alternating " 01 " and a " value " .


Using the fan + and - buttons, scroll until you find the fan control voltage ( or similar ) display. ( I don't have chart handy. )

At the fan controller, measure voltage between the small signal wire and ground, it should match the voltage on the display. If not, find this wire on the HVAC control and measure voltage there. If you have voltage at the HVAC control, the signal wire is broken.

Turns out the trouble was in the fuse box. The red/yellow wire going to the blower regulator had a fusible link in it which was open and the wire on both sides of it, while not burned, were well cooked. We cut out the link, along with the cooked wire and patched in a fuse holder with a 40 amp fuse. It now works perfectly.

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire