Yes, they only work with the windshield washers while the headlights are on.

ALSO... I've learned that there is a (now closed?) recall campaign for 90-94 129 washer PUMPS.

A copy of the document is displayed in another post somewhere on this site. It states that if the washers are activated when the wiper arm is mis-aligned or STUCK, such as down in the groove below the headlight, the pump will run continuously, expelling all of the fluid and will overheat. This could possibly lead to a vehicle fire.

There are two of these pumps per car. They are mounted down low on the washer fluid reservoir.
The improved PUMP is labeled 129 869 00 21 and costs around $30 I recall. REPLACE THEM BOTH along with the rubber grommets that hold them in place.

My SL made a BIG puddle by pumping the fluid all over the floor. BAD PUPPY!!! I should have done a better job of training it to USE THE PAPERS!!! I forgave it and still love it!

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02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)