4 replies to this topic

[minivictor]

Hello Everyone! I recently purchased a 1972 Austin Cooper MK3 and I wanted to reach out to you all to get some clarification on an odd electrical issue that I was having.

 

In an attempt to trace some known shorts in the wiring harness, I found that the Red/Green (red wire, green tracer) wire circuit which controls the taillight bulbs (and the small bulbs in my headlights) has continuity to ground when the bulbs are installed as well. Since this Red/Green circuit is supposed to only contain hot 12 volts, I assumed there was a short in the wiring harness because this circuit should not be exposed to ground.

 

However, after checking every inch of the circuit, there was no exposed wire to ground. "That's weird..." I thought. After short while, I decided I would check the taillight bulb housing for a short. Though there was no exposed wiring or anything, I removed the bulb and found that there was no longer any ground continuity on that particular tail light! Curious, I tested the headlights and the remaining taillight by removing the bulbs and there was no longer any continuity on the whole circuit.

 

Overall, my questions is: Is this normal that ALL the bulbs (from the Red/Green circuit) are hitting ground when installed in the housing, even though they are working 100% as expected? Why isnt the circuit blowing a fuse? Or sparking for that matter? --I do not know too much about how the electrical bulbs work to know whether I should be worried about this.

 

I hope I articulated my confusion in a way that is understandable. Please let me know if there are any questions. Thanks!

 

[minivictor]

Hello Everyone! I recently purchased a 1972 Austin Cooper MK3 and I wanted to reach out to you all to get some clarification on an odd electrical issue that I was having.

 

In an attempt to trace some known shorts in the wiring harness, I found that the Red/Green (red wire, green tracer) wire circuit which controls the taillight bulbs (and the small bulbs in my headlights) has continuity to ground when the bulbs are installed as well. Since this Red/Green circuit is supposed to only contain hot 12 volts, I assumed there was a short in the wiring harness because this circuit should not be exposed to ground.

 

However, after checking every inch of the circuit, there was no exposed wire to ground. "That's weird..." I thought. After short while, I decided I would check the taillight bulb housing for a short. Though there was no exposed wiring or anything, I removed the bulb and found that there was no longer any ground continuity on that particular tail light! Curious, I tested the headlights and the remaining taillight by removing the bulbs and there was no longer any continuity on the whole circuit.

 

Overall, my questions is: Is this normal that ALL the bulbs (from the Red/Green circuit) are hitting ground when installed in the housing, even though they are working 100% as expected? Why isnt the circuit blowing a fuse? Or sparking for that matter? --I do not know too much about how the electrical bulbs work to know whether I should be worried about this.

 

I hope I articulated my confusion in a way that is understandable. Please let me know if there are any questions. Thanks!

 

[Spider]

The bulbs, especially when they are all fitted, have a very low resistance and so will appear to be a short to Earth. The only way to check properly is to remove them as you did.

[GraemeC]

Yes, effectively a bulb is a short circuit.

 

(Although I would describe them as high resistance - certainly higher than the wires either side)

[Homersimpson]

In simple terms the green and red wire is linked to earth through the bulb, if you remove the bulb as you have done then you break the circuit and won't measure continuity (this is what you have found)

 

You have to look at it as 12V on green and red and 0V on the earth to the body the resistance of the bulb creates the potential difference between the two points.   If you were to wire two bulbs in series then it would be 12v then 6v between and 0v on the earth and the bulbs would be dim (like old Christmas tree lights)

 

If you use a simple continuity meter it will look like a short to earth (but all you are measuring is that there is an electrical circuit) but if you use a MultiMeter on the correct resistance scale you are actually be measuring the resistance across the bulb.

 

The cars with a single indicator warning light actually use this to their advantage by wiring the bulb for this between the live on the LH indicators and the live on the RH indicators, when either side is on and the other side is off the bulb works because the side that is off is providing an earth to the bulb (albeit through the front and rear bulbs).  The amount of energy that flows is enough to light up the indicator warning light but not the bulbs on the outside of the car on the side which is off.  If you could put both indicator sides on together (as the hazard warning switch on the late 70's models does) then the indicator in the dash wouldn't work as it has plus 12v on both sides of the bulb, to overcome this they fitted a bulb in the hazard switch.

 

Electrical circuits are all about resistance and potential difference, the bigger the load the lower the resistance the more current that flows. 

 

Two useful formulas :

 

Ohms Law V=IxR where V is voltage in Volts, I is current in Amps and R is resistance in Ohms. 

 

P=IxV where P is power in Watts, I is current in Amps and V is voltage in Volts

 

We can turn these around and work out that a pair of 5w tail light bulbs (10watts in total) have a current of 0.83A

 

10W/12V=0.83A

 

We can the use Ohms law turned around to work out that

 

12V/0.83A=14.46 ohms

 

So using a multimeter measuring across the two tail light bulbs only you should get 14-15 ohms, this does change as the resistance of the cables and connections (especially to earth) can have an effect.