Front day running lights conundrum

[rtbcomp]

Originally Posted by Clifford Pope

I agree that would explain it if they are wired in series and the holder itself is not earthed. But they are not meant to be wired like that. Each bulb and each filament is earthed at one end, and a positive supply is switched to whichever bulb or filament is required.

...

"one probe on the 5W supply contact and the other on the wall of the socket shows a negative earth." I don't follow that. That simply shows a voltage between the two and the polarity. It doesn't prove that the wall of the socket is properly earthed. What you seem to have is a floating "earth" which is being polarised from one of the other lights.

It is odd. I can see a reason to wire two bulbs in series to dim one of them. A 21w bulb in series with a 5w bulb will dim the 5w bulb slightly, the 21w acts as a dropper resistor. I'm not saying Volvo do this, just working on basic principles.

A floating earth looks like a good solution, in the second diagram the tip of bulb B will be connected to the corresponding bulb on the other side of the car, providing a path to earth.

A floating earth and having the meter -ve terminal in the wrong place looks like the best answer.

Scan132.jpg

[Stephen Edwin]

Yes the connected in series theory works, except, that would mean wrongly wired so that both filaments lit up together. I don't think that is what David is saying.

Do we know the age and model of this 240?

Was it designed for the lamp units it has or have they been changed?

Has the wiring been altered or added to at any time?

Do the filaments shine at their usual anticipated brightness?

Stephen

[rtbcomp]

As said by Clifford Pope and my last post, it's more likely a floating earth causing the filaments to end up in series.

[Stephen Edwin]

I had not seen your broken or bad connection note on that diagram. Yes a definite possibility. And current would go via the other side of the car.

By the way, is the "reverse polarity" issue evident if one checks the other side of the car?

Unless checking for that broken or bad connection gives a quick fix, it still comes down to checking the basic facts. (And of course cleaning up ALL the connections.)

For example, model and year of the 240, whether any wiring has been altered or any lamp units changed, whether the symptoms include any non standard brightness of filament bulbs, can both filaments of the same bulb be seen shining at the same time....

Stephen

[rtbcomp]

Originally Posted by Stephen Edwin

...

By the way, is the "reverse polarity" issue evident if one checks the other side of the car?

...

Stephen

It shouldn't be if the earth on that side is intact, if it's broken too there will be no path to earth*** for the light on the first side.

***Assuming there aren't other lights such as the tail lights, which also illuminate with the DLRs I believe. It's probably quicker just to check and clean the earths as you suggest than to theorise too much, and the chances are you'll be cleaning them anyway!

[Stephen Edwin]

And I forgot to mention the old simple "Christmas Tree" test for the condition rtb describes. Remember the rear light displays when a 1950s 1960s car had flashing indicators and suffered a rusty rear light earth. All lit up like a Christmas Tree when braking and indicating at the same time. One nstill sees it occasionally afflicting trailer electrics. The suspected diagnosis here is the same but at the front of the car so mutatis mutandis.

So. On this 240. With all the bulbs in place, and the DLR need to be on which they will be if you are using the indicators. *And try this both with and without the sidelights on. Switch on each indicator in turn and observe the 21w DLR filaments. Do those filaments pulse/flash in time with the indicator? That would confirm rtb's diagnosis for you very quickly.

*With and without the sidelights because I can't be bovvered to work out the significance if any of the sidelights being switched on.

I remember in my early years an uncle nearly had his car completely re-wired EEK and all for one rusty earth connection causing the Christmas Tree effect. As a lad I visited him before his mate started the work. I cleaned up all the connections of the rear lamps including the earths to body. I fixed it basically by luck and dogged determination, thinking "it MUST be at least one of the connections....".

Stephen


.