My 91 300TE, new blower motor 5 years ago (brand unknown but not MB) to replace squealing original, the resistor assembly and pushbutton CCU are original, and vacuum pods & aspirator blower replaced 6 years ago with MB parts. The 30A strip fuse was replaced and contact surface cleaned recently at the start of the intermittent blower issue.

Sometimes the blower works, and sometimes not. When the blower does not run, it does not respond to any setting of the CCU including the default defrost setting. I disconnected the 3 wire plug (of the red/black/yellow line that runs from the resistor) at the connector near the brake booster. I measured over 12volts at the red wire pin out, and measured about 0.5 volts on the yellow wire pin out at all settings of the pushbutton unit. That indicates pushbutton CCU not working?

However, yesterday the blower worked as it should, and responded correctly for all pushbutton settings. I measured a range from about 1.5 to 9.5 volts on the yellow wire pin out, depending on which button was pushed, and of course 12v at the red pin out. That says pushbutton unit is working?

Today the blower does not work again.

Does this sound like a pushbutton CCU that has solder cracks that maybe expand or contract with the weather and therefore works or doesn’t work? That I now need to buy a rebuilt CCU?

Thanks for your input, ds.