A very common problem with MB aux fans is the low-speed resistor fails. You can find this resistor behind the left headlight.

Low-speed fans are designed to run only when refrigerant pressure exceeds a certain limit, so in contrast to many other makes, your aux fans will NOT necessarily come on every time the AC is on. If you do wire them to run whenever AC is on, there is some concern (from other lists) that they won't last as long, and they're not cheap to replace.

High-speed fans come on when coolant temp exceeds a certain limit (don't recall exactly how high).

FYI, if your 400E is like mine (and I think it is), you don't have a Klima relay or over-voltage protection relay -- these functions have been incorporated into the 4 circuit boards in the protective metal case on the right side of the firewall (i.e., where the battery would normally be in most other W124s).

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Steve
'93 400E