No dipped beam - though main beam working fine

[andycadabra]

During a period in which coolant was regularly squirted around the engine bay I lost my dip beam on both front lights. Main beam is working fine, as are the dashboard lights. I've tried new bulbs and tested for current, and there does seem to be some when switched to dipped beam. The seemingly odd thing I noticed is that the terminal that supplied positive current when switched to main beam switches to negative when dipped beam is selected, with positive current being supplied from the other terminal. Does anyone know if this normal?

I've heard of a DimDip relay - could this have failed and taken the dipped beam with it?

Any advice would be appreciated

[rmattila]

How about rear lights?

[andycadabra]

working normally...

[Kitch]

Was it shorting out?

[andycadabra]

Is there a short circuit? I don't know - I tested for current at the connector while switched to dip beam and there is some, but puzzlingly no light.

[mat_fenwick]

This might sound pedantic but what did you actually connect each probe of the meter to, and what was it set to? I suspect you were checking for voltage, not current.
I would suggest setting the meter to voltage, and connecting the black lead to battery negative. Then connect the red lead to the dipped beam terminal on the headlight bulb socket (minus bulb) and repeat with bulb in place. What voltage readings do you get in both situations?

[andycadabra]

You're right Mat, I was testing for voltage, and misusing the word current! ':oops:'
I'll try what you suggest.
As far as the terminals I tested on the triangular connector were concerned, - let's call them 9 PM 12 PM and 3 PM. On main beam 9 PM supplies positive and 3 PM negative. On dipped beam 9 PM supplies negative and 12 PM supplies positive.

[mat_fenwick]

OK, all that's saying is that one terminal is at a higher voltage than the other, and with the tiny current that a meter draws when measuring voltage, connecting the negative lead of the meter to something other than a good earth can give a misleading reading. To explain further, when you are checking main beam you get a positive voltage between 9 and 3 because 9 is on and 3 is earth. When you are checking dip you are getting a positive voltage between 12 and 9 because 12 is on and 9 is off (but there will be a path to earth via the other bulb!)

Looking at the bulb socket from the front, the 3 o' clock position is (or should be!) earth/negative, but if you use the method I suggested above you will purely be testing the postive side of the circuit. If that tests OK then you can look at the earth side, but as main beam works that shouldn't be a problem.

If I had to bet, I would say that on main beam there will be ~12V between 9 and battery negative (or 3) both with bulbs out and bulbs in. On dipped beam there will be ~12V between 12 and battery negative (or 3) with no bulbs present, but drop to bugger all when you put the bulbs back in...

Dim dip relay will have nothing to do with it.

[Gibbo2286]

I've found that more often than not when this happens it's the wire at the stalk switch come adrift though fatigue, if so a dab of solder in the right place will fix it.

[andycadabra]

Thanks for all that info, Mat and apologies for not responding for ages. The lights have been the least of my problems lately and nobody seemed to mind me driving around on main beam, which probably means their angle badly need adjusting..

I did as you said, (incidentally, if my meter was working correctly the voltage supply to the light seems to be about 30V?).
It was only when I ran a bit of wire from the dipped beam terminals on the socket to the main beam terminals on the bulb, and it lit, did I deduce that the dipped beam filament on both of my lights had failed, and the spare I had thought was good must have had the same problem. A bit of a coincidence, I thought at first, but bearing in mind water from the cooling system was being sprayed around at the time the lights went, I presume a short had knocked out both bulbs at the same time, which would have been on dipped beam at the time.
Gibbo, thanks for that tip, I got as far as taking the plastic cover half the steering column to expose the base of the stalk but couldn't figure out how to get at the switching mechanism/terminals - I suppose that would have involved taking the cover of the whole steering column, but luckily I don't have to!.
So thanks for your help, guys, and I promise to be a little more thorough and use a 'really known' bulb to test rather than an 'assumed known' and not to jump to conclusions about capricious Citroen electrics...

[Mike E (uk)]

I had an intermittant dipped beam filament once, it took ages to find that one.

Mike