Citroën SM – Wiper Innovation

Citroënvie member George Klein sent in these these pictures of repair work he has done on his SM time delay pot for the windshield wiper motor. 

 The delay mechanism for the wiper motor.

  It is located in the bottom of the steering column housing.

  Note the white post toward the rear that rotates with the knob and engages a the contact to be in the “full-time” mode.

George says:  “Amazing.  Totally mechanical.  Clean it and it works for an other 40 years.  A bit of a brain surgery though.   I had to repair because the harness wires to it because they started to melt.”

  Access to the delay mechanism for the wiper motor requires disassembly of the the steering column shroud.  Had more disassembled as he also installed fresh wires to the delay mechanism.

While many of today’s cars with intermittent windshield wipers have sophisticated optical sensors to initiate operation, Citroën was way ahead in the early 1970’s to an automated means of wiper operation.  And for over 3 decades after the SM other cars with intermittent wipers relied purely on a time delay, in some cases variable as set by the driver. 

The Citroën SM system adjusted the intermittent wiper delay based on the electrical current draw of the wiper motor.  A wiper blade glides more smoothly over a wet windshield, using less energy, and a control circuit in the SM used that to indirectly determine if rain was present.  Consequently the current flow to the motor is what George had to deal with but with a wire fix and a clean up of the delay mechanism, he’s good to go again with less complication and arguably greater longevity than the new electrical sensing systems.




1 comment

  1. I know that SM have a timer Integrated circuit that is built in onto a printed circuit board, this is in a different part, I.e controlling a cut off to the fuel pump relay via a very cool method of implementing two independent signals, that are coming from the dual points in the distributor, since the SM with a carburetor have the two separate firing points, if one set of points or both would have been defective, there would not be any fuel sent to the carburetor.

    I had the opportunity to study this effect via helping a club member many years ago, that when I had learned this.

    I therefore wonder if the same method would have been implemented in the case of the timer circuit, I know that George Klein says that it’s mechanical, perhaps the switch test he cleaned out, but surely the current draw where the more wet = > resistance, hence higher current draw through the motor, there have to be a timer circuit , I.e a timer IC NE555 in its circuit, this latter would intermittently deal with the wiper action , because a detection pulse which would be in proportion to the current draw, the various external , current shaping circuit feeding the duration of the feed, thereby cutting and or slowing down the speed or other as necessary.

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