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Front day running lights conundrum

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Old Jan 4th, 2018, 19:30   #1
davidcheshire
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Default Front day running lights conundrum

My 1984 GLT is negative earth, with one exception - at least the only one I've found so far.

The contacts in the front 21/5W bulb sockets register negative earth feed for the sidelight filament, but positive earth feed for the DRL filament.

Any ideas why this should be, and how to change both current supply contacts to negative earth?

Filament bulbs work as they should, but LEDs do not because they are not made with dual polarity, as far as I have learnt so far.

I'm hoping to replace the front DRL bulbs with LEDs for extra brightness, but no luck so far.

The rear stop and tail bulbs have been changed to LEDs without problem.
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Old Jan 4th, 2018, 21:41   #2
Nicholas Lewin
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David - a word of warning with LEDs - you may find that the bulb failure sensor will register where an LED has been installed. I changed the rear indicators to LED and not only did they flash faster, the failure sensor registered them as failed...

Good luck
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Old Jan 5th, 2018, 15:03   #3
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Just put a shunt in to bring the circuit resistance back to where it should be, it will solve the flash rate and bulb failure warning problems.
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Old Jan 17th, 2018, 18:04   #4
davidcheshire
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Bump.

Anyone have an idea why the front P21/5W DRL circuit should register positive earth on the 21W supply, but negative earth on the 5W supply? In a negative earth car.

Thanks.
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Old Jan 17th, 2018, 21:58   #5
Stephen Edwin
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I don't know the wiring of those circuits anyway so I can't help. Sorry. But also I am puzzled just how are your side lamps and running lamps wired.

The filaments themselves don't have polarity. How have they been wired with different polarity from each other? Both filaments should surely be in the same bulb with a shared body cap connection?

As a further starting point:
  1. What does a wiring diagram for that type of Daylight Running Lights specify and
  2. When were DRL phased out? Are those lamps original for that year or perhaps a backwards modification?

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Old Jan 19th, 2018, 09:10   #6
Clifford Pope
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All I can think of is that there is some kind of mismatch between bulbs and holders. As Stephen says, the standard bulb has two filaments with a common earth to the cap. So either filament will illuminate depending on which contact on the bulb is connected to the power.
A bulb doesn't have a polarity. It just needs a positive supply to one end of the filament and negative to the other. The earth end is set at negative by design, so the supply to the contacts has to be positive. Obviously earthing both ends won't work.

Decades ago there were bulbs that looked outwardly like the larger 5/21 watt bulbs and had two contacts, but only had one filament and no earth. So one contact had the supply voltage and the other was independently earthed. If you plugged one of those into a dual-filament socket it wouldn't work because there would be two positive connections and no earth.
Could it be that some very modern LED bulbs work a bit like that and need a special socket?
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Old Jan 20th, 2018, 23:40   #7
davidcheshire
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To be clear:

the dual filament 21/5W bulb is removed;

the ignition key is turned to position 2, so that the dashboard lights are on and the DRL bulb on the other side is illuminated;

the two supply feeds in the bulb socket are tested on a multimeter;

one probe on the 21W supply contact and the other on the wall of the socket shows a positive earth;

the lighting is switched to sidelights, and one probe on the 5W supply contact and the other on the wall of the socket shows a negative earth.

A very helpful online retailer of LEDs is also puzzled. He sacrificed a pair of LEDs in an attempt to solve the problem, and finally commented that, while dual filament bulbs could cope with a dual polarity supply, there were no such LEDs on the market.

The DRL system is original to the car, a 1984 GLT. What has changed, and may be a factor, is that the headlights cum DRL/sidelight/indicator have been replaced, at least once, over the years with units almost certainly from later cars. My limited electrical knowledge doesn't allow me to work out how any difference in the wiring in the replacement light units would cause the problem.
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Old Jan 21st, 2018, 08:38   #8
rtbcomp
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Quote:
Originally Posted by davidcheshire View Post
Bump.

Anyone have an idea why the front P21/5W DRL circuit should register positive earth on the 21W supply, but negative earth on the 5W supply? In a negative earth car.

Thanks.
Are you sure you're measuring with respect to the chassis of the car?

I'm not sure of the circuit, but if two bulbs are wired in series and you take the common connection as earth and put the other probe on bulb A you'll get a +ve, but put it on B you'll get a -ve. The voltage readings will be different due to the different wattages (resistances) of the bulbs.

Make sure you have the -ve lead of the meter connected to the body of the car and try again.

The frame of the bulb holder may not be connected to the chassis.

Scan129.jpg

Last edited by rtbcomp; Jan 21st, 2018 at 09:42.
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Old Jan 21st, 2018, 09:39   #9
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This diagram shows why the polarity across the bulbs differs.

Scan131.jpg

and why an LED lamp at B won't light, it's reverse biased. This shouldn't damage an LED, but if there are any electronics in the unit they could be damaged if the circuit doesn't provide reverse polarity protection.

Last edited by rtbcomp; Jan 21st, 2018 at 09:42.
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Old Jan 21st, 2018, 10:04   #10
Clifford Pope
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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.

There are some components that have a permanent positive supply and switching is achieved by earthing them (horn actually is all I can think of off hand), but the lights are afaik all eathed and all receive a switched positive supply.

"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.
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