Hi,

 

Perhaps someone here can help me solve what looks like a mystery to me, I am really a noob though so it could be a silly/easy thing to fix.

 

The light bulb warning started turning on, telling I should check the brake lights. Turns out all brake bulbs work. But one of the "normal" rear light bulbs doesn't. Obviously it's not just the bulb.

 

What I don't understand is why I measure tension on the circuit leading to this particular bulb when the bulb (any bulbs, actually) is not in place, but I see 0 volts when I attach the bulb.

 

In one of the attached images you can see the tester measuring 16 volts on one of the (working) circuits. The other two images show the problematic circuit with the bulb in (tester shows 0) and without the bulb (test shows 43 volts - isn't that too much? - anyway the third "working" circuit shows ~100...).

 

Ideas on what is going on?

 

Thanks for your hints!

 

G

If a poor connection/bad bulb is detected the power is cut to that circuit, you need to cycle the ignition to restore power (assuming a well connected good bulb is fitted).

 

I can't see where you have the black lead so I can't comment on the 0v reading, the other readings are 1.6v & 4.3v, you missed the decimal point,

11 minutes ago, MicMac said:

If a poor connection/bad bulb is detected the power is cut to that circuit, you need to cycle the ignition to restore power (assuming a well connected good bulb is fitted).

 

Thanks MicMac for chiming in.

 

You can see from the tester that there is power on the circuit and the bulbs are all good, when placed in a different socket.

 

 

Quote

I can't see where you have the black lead so I can't comment on the 0v reading, the other readings are 1.6v & 4.3v, you missed the decimal point,

 

The black lead is on the earth and the tester is set to show 10V units.

 

Is it normal that I get so different readings, though all bulbs are 12V (and make approximately the same light)?

 

 

Edited 20 April, 20215 yr by Giff

I think you need a new tester to start off. I can't see how you can get 43v on a 12v vehicle.

3 minutes ago, Tinbum said:

I think you need a new tester to start off. I can't see how you can get 43v on a 12v vehicle.

 

It is a cheap tester I have to admit.

If you enlarge the image you can see the decimal point, and it reads 4.34V.

One of the problems of taking readings with a meter is that the supply to some bulbs, such as the brake/side filament, is pulse width modulated so the same filament can be used for both normal and bright applications (brake and side for example).

 

Using a digital meter can give false readings reading due the way it treats a PWM waveform; using a cheap analogue may give a better reading but not fool proof.

The other problem is that when the supply is switched off (which it does when a failed bulb is detected) you are left with the leakage current from the driver circuit. A digital meter such as yours will have a very high input resistance and sees that leakage current as low level voltage - hence your 4.34V. [voltage read is IL x Rin ]

 

Best to check your bulbs using a 12V d.c source such as your 12V Aux socket.

Also check the bulb mount assembly connections by taking it off the car and using a continuity checker.

 

44 minutes ago, pikpilot said:

If you enlarge the image you can see the decimal point, and it reads 4.34V.

 

does that mean in the other image it shows 1.6V?

 

I’ll check the lights on the other side.

 

44 minutes ago, pikpilot said:

One of the problems of taking readings with a meter is that the supply to some bulbs, such as the brake/side filament, is pulse width modulated so the same filament can be used for both normal and bright applications (brake and side for example).

 

Using a digital meter can give false readings reading due the way it treats a PWM waveform; using a cheap analogue may give a better reading but not fool proof.

The other problem is that when the supply is switched off (which it does when a failed bulb is detected) you are left with the leakage current from the driver circuit. A digital meter such as yours will have a very high input resistance and sees that leakage current as low level voltage - hence your 4.34V. [voltage read is IL x Rin ]

 

Best to check your bulbs using a 12V d.c source such as your 12V Aux socket.

Also check the bulb mount assembly connections by taking it off the car and using a continuity checker.

 

thanks, this is interesting though I don’t grasp all the details

 

I still fail to understand why the tensions should be 0 when the (working!) bulb is in but >0 when it is not.

3 hours ago, Giff said:

I still fail to understand why the tensions should be 0 when the (working!) bulb is in but >0 when it is not.

 

It is just physics. It is like when you have an AAA battery that is almost flat. When you read the volts with a meter you notice it reads a lowish voltage but when you then put a load on the battery and read the volts, it shows zero volts. The car and the battery behave the same way. The car can only supply a very small current (the leakage current). When the lamp is switched off you can read something small on your meter but with the bulb in situe this small current is diverted through this the and effectively shorts the source to ground and reads zero volts on a meter.

So I opened the other side as well and found out that:

 

- the bulb in the same position is also not turning on

- the voltage readings are the same as at the left side

 

No idea what’s going on.

 

I also bought and fitted new bulbs, just in case. Nothing changed.

Edited 21 April, 20215 yr by Giff

Are you still getting a defective bulb warning?

 

Are you testing the fog or reverse bulbs?

2 hours ago, MicMac said:

Are you still getting a defective bulb warning?

 

Actually no! Well, it was gone for some 20 kms, but then it reappeared :( Now it does not show again when the car is parked.... go figure. I just need to do the Swiss equivalent of a MOT tomorrow so I hope it won't show then

 

2 hours ago, MicMac said:

 

Are you testing the fog or reverse bulbs?

Indeed, the bulbs that would not turn on were the reserve ones (facepalm)!