2CV dim-dip system

The 2CV dim-sip system that was fitted as standard for a short time is a nasty hack and can cause potentially dangerous overheating of electrical components.

Credit for solving this problem goes to John Wood.

Original lighting system

In its original form, the 2CV’s headlight circuit diagram is nice and simple – each switch activates a single pair of lights.

Standard 2CV headlight circuit diagram

[I’ve simplified this diagram somewhat: the side light circuit also has the rear lights on it, the dip beam circuit has the fog-light system hanging off the side and there are some dashboard indicator lights – however, none of these are relevant in this context.]

Dim-dip system

In 1997 and 1998 2CV’s were fitted with a dim-dip system. Due to the complexities of the requirement this was achieved by the addition of two relays, a diode, a resistor and lots of extra wires to connect them all.

2CV dim-dip system circuit diagram

[The ignition system is  much more complicated than just this switch but, again, none of that is relevant in this context.]

With four switches there are theoretically 16 (24) possible states for this system but there are only three that we need to concern ourselves with:

  1. Side light switch on, ignition off:
    • relay 1 de-energised (3-4 connected)
    • relay 2 de-energised
    • side lights illuminated
    • dip beam off
  2. Side light switch on, ignition on:
    • relay 1 de-energised (3-4 connected)
    • relay 2 energised (3-5 connected)
    • side lights illuminated
    • dip beam illuminated via resistor (dim-dip)
  3. Dip beam light switch on:
    • relay 1 energised (3-5 connected)
    • relay 2 isolated
    • dip beam illuminated directly

The other states are either unreachable due to the sequential nature of the light switch (side light -> side light + dip beam)  or are rendered irrelevant by the isolation of relay 2 when the dip beam switch is on.

Whilst this arrangement does achieve the desired effect there are two significant flaws:

  1. The power for the dip beam (and fog light) are now passing through the side light switch:  this adds somewhere in the region of 100W to a circuit designed for around 30W.  Meanwhile the dip beam switch is only energising a 6W relay. This results in the light switch heating up1.
  2. The return-to-ground for relay 2 is via the main beam filament: when the ignition is on and the main beam is off this is putting 6W thorough them. This reduces the life of the main beam lights2.

In addition to these design flaws, there is the possibility of an additional flaw if the diode from the ignition fails:  In this case, when the main beam switch is on, it will allow current from the main beam circuit to power the ignition even when the main ignition switch is off.

Dim-dip removal

Given these issues the removal of the dim-dip system is advisable from a safety point of view.  From a legal (IANAL) point of view the dim-dip system was only ever required at the time of sale, it has never been an MoT requirement – even if originally fitted.

Whilst a complete removal would basically require a new wiring loom it’s relatively easy to functionally remove the system.  The majority of the original system is still in place, it’s just been re-routed through the relays.

The minimum required is to remove the two relays and connect pins 1 and 5 of relay 1.  This routes the dip beam current back through the correct switch and removes the chance of the main beam current flowing back through the ignition circuit.

2CV headlight circuit diagram with dip-dip removed

The resistor and the diode can be left in-situ since the removal of the relays isolates them.  However, for the sake of completeness it’s a good idea to remove them.

Acknowledgements

The circuit diagram is take from the Haynes manual for 1967-1990 A-series cars which, given the colour abbreviations are in French, I’m guessing has come from Citroen originals.

The instruction to connect pins 1 and 5 is taken from a 2002 guide by Mike Phelan based on an article in the 2CVGB magazine by John Wood. This is verified by my own reading of the circuit diagrams and experience with performing the operation on Judith.

Caveat lector

The colours of the wires are taken from the diagrams in the Haynes manual for 1967-1990 A-series cars and from my examination of Judith – your experience may vary.

Footnotes

  1. When I drove Judith back from York it rained most of the way so I had the lights on and I noticed the light switch was hot to the touch. ^
  2. Before leaving York we checked all the lights and they were in working order but, when I checked again after the drive back I found the near side main beam had gone. ^

Other posts in this series

  1. 2CV dim-dip system overview
  2. Removal (quick version)
  3. Removal (medium version)
  4. Full removal