Disappearing current

**vnx205** · 7th September 2007, 07:32 PM

Just had an interesting time trying to find out why the reversing light on my 300Tdi traytop didn't work.

Armed with just a multimeter and a layman's understanding of how electricity works, I started with the simple things.
1. Checked the bulb. Looked OK. Multimeter suggested filament was intact.
2. Checked the bulb appeared to be making contact in the holder. Seemed OK.
3. It shouldn't have been the fuse because the same one is used for the stoplights and they were OK. I checked it anyway and of course it was OK
4. Removed the reversing light switch and checked with multimeter. OK. Fixed a couple of fairly dodgy crimped on bullet connectors while I had things apart.
5. Started checking to see how far down the system the fault was located. Tried the multimeter on the contacts in the reversing light. It showed 12 volts (with no load).
6. Put the bulb back. Still no light.
7. Checked all accessible wires and connections. Nothing obvious.
8. Checked for power reaching switch. Multimeter said 12v. Tried the bulb at the switch connection. Light at last. So problem had to be between there and back of the vehicle.
9. Tried checking for poor connections or broken wires near the back of the vehicle, but nothing obvious in the section before it disappears inside the chassis.
10. For some reason which I no longer remember, I used a short length of wire to create my own earth at the bulb holder. IT WORKED.

I assume the fault was associated with the earth wire from the light which disappears inside the chassis. I guess the earth connection is so poor that it allows just enough current to indicate 12v on the multimeter, but would not allow enough current to make the bulb even begin to glow.

I realise this has been a long drawn out story, but it was after all a long drawn out saga trying to find the fault.

I suppose some electrician out there will now tell me that you should always check first that you have a good earth. Maybe I would have done that if the multimeter hadn't shown 12v across the contacts at the light.

I don't suppose anyone else has had a similar problem and can tell me exactly where to look for the faulty earth.

My plan at the moment is just to ignore the existing earth wire and fit a new one of my own.

And no, I don't have any pictures of the bulb not working, or of the bulb working or even of the wire that caused me so much fiddling around.

**incisor** · 7th September 2007, 07:39 PM

Originally Posted by vnx205

And no, I don't have any pictures of the bulb not working, or of the bulb working or even of the wire that caused me so much fiddling around.

and why not ?
we live for our pics around here....

**ladas** · 7th September 2007, 07:57 PM

Poor earths account for 90% of auto electrical probs.

Run a new earth - it should be sweet.

**Blknight.aus** · 7th September 2007, 08:52 PM

I bet when you were measuring your voltages you had the multi meter connected to a different earth and not the bulb holder which would give you an alternative earth for the multimeter to work from....

annnyy how... even if that wasnt the case volts work like water pressure and current is like water flow.

if you turn of everything in the house that lets water out hook up a hose take the end of it out to where the meter is and turn it on the water flows and the meter spins. if you put your thumb on the end of the hose the meter stops spinning.

Now lets pretend that on the supply side of your water meter you have a pressure gauge when you have the water flowing there would be a slight pressure drop and when you put your thumb on the end the pressure would rise, at the same time the usage meter would flow and then stop. cool yeah.... think of your thumb as the high resistance point that stops the water from flowing..

its exactly the same with the vehicles electrics just substitute PSI for volts and liters for amps.

try this....

measure the volts on the battery and then turn the headlights on. voltage will drop, same as the water pressure would drop when you open the hose.

ok Its not a perfect anology but its pretty close.

**vnx205** · 7th September 2007, 09:14 PM

Originally Posted by Blknight.aus

I bet when you were measuring your voltages you had the multi meter connected to a different earth and not the bulb holder which would give you an alternative earth for the multimeter to work from.....

Actually I didn't this time because I got caught that way years ago and I still haven't forgotten how much extra work I created for myself by bypassing the earth wire attached to the device I was checking. I made sure I measured the contacts that the bulb would touch.

If I understand your analogy correctly, does that mean that because the resistance is just very high, but not infinite (ie an open circuit), the multimeter is still able to measure the voltage (pressure), even though there is no current (flow)?

**rovercare** · 7th September 2007, 09:17 PM

Originally Posted by vnx205

Actually I didn't this time because I got caught that way years ago and I still haven't forgotten how much extra work I created for myself by bypassing the earth wire attached to the device I was checking. I made sure I measured the contacts that the bulb would touch.

If I understand your analogy correctly, does that mean that because the resistance is just very high, but not infinite (ie an open circuit), the multimeter is still able to measure the voltage (pressure), even though there is no current (flow)?

Basically yes, its very resistive, and the meter will pick it up but breaks down under load

**Blknight.aus** · 7th September 2007, 09:24 PM

wot he said...

I Like to use a test lamp that has a 5w bulb in it when im trying to fault find electrical stuff, especially lighting stuff in rovers... (that said DONT use that type of test lamp in the engine bay on your EFI type stuff or electronic igntion modules, the results can be either shocking, expensive or both)

**rovercare** · 7th September 2007, 09:28 PM

^^^^test light is the best way, as it creates a draw on the cct, very seldom yet it does happen, can even be caught out at work with a similar scenario.....I'm a heavy industrial lektrishan

**vnx205** · 7th September 2007, 09:30 PM

Originally Posted by Blknight.aus

wot he said...

I Like to use a test lamp that has a 5w bulb in it when im trying to fault find electrical stuff, especially lighting stuff in rovers... )

I can see now why that would have been more useful than the multimeter.

**p38arover** · 7th September 2007, 10:19 PM

Originally Posted by vnx205

I guess the earth connection is so poor that it allows just enough current to indicate 12v on the multimeter, but would not allow enough current to make the bulb even begin to glow.

That's the reason I don't use digital multimeters on cars. I use my 42 year-old analogue meter of 20 Kohm/volt (I built it as an apprentice). If I had an even older 1000 ohm/volt meter I'd use that.

It's all to do with the input impedance of the meter. A digital multimeter has an intentionally high input impedance (in the megohms range) so it can read a voltage without loading the circuit under test. In fact, if you are checking a switch, if there is even the slightest leakage across the switch, it will read OK even if it's not.

A 1000 ohm/volt meter loads the circuit ever so slightly and will show up bad connections.

For checking lights, a test lamp is often better as it provides an even higher load.

Ron

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