Thread: Disappearing current

Originally Posted by p38arover

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.
Ron

I think I see the problem now. My multimeter was only about 20 or 30 yrs old, bought at Dick Smth for $9.

Originally Posted by Bradtot

This does not just happen on cars. Last week got caught out on a 150 grand woodworking CNC router
Brad

I think I feel better and a little less foolish knowing that machines more expensive than my Defender have similar problems and that people who are much more knowledgeable and experienced than me don't always get it right first go.

Originally Posted by vnx205

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

You can confirm the fault on the earth by measuring the resistance between the earth in the bulb socket and a good chasis earth with your multimeter. Once confirmed, you just have to find the cause of the resistance. It could be in the socket or at the chassis attachment point. Unless the reversing lights have a dedicated earth to the chassis (possible but unlikely), I would guess the socket end.

Originally Posted by Bradtot

This does not just happen on cars. Last week got caught out on a 150 grand woodworking CNC router That would not come on, everything measured ok with multimeter but when current was trying to go through relay contacts it would go high resistance, open cct. <snip>
Brad

had this happen quite a few times over the years on 3 phase motor start contactors.
Motor tripping the O/L due to phase down yet contactor appears to meter OK. Invariably burnt contacts which wont carry the current yet meter OK with the digi multimeter.

Originally Posted by awabbit6

You can confirm the fault on the earth by measuring the resistance between the earth in the bulb socket and a good chasis earth with your multimeter. Once confirmed, you just have to find the cause of the resistance. It could be in the socket or at the chassis attachment point. Unless the reversing lights have a dedicated earth to the chassis (possible but unlikely), I would guess the socket end.

Actually, another, easy, way to check for an earth fault with a light such as this is to measure the voltage between the bulb holder and a good chassis earth, with the bulb in place - it will typically be almost the full voltage if the earth is faulty, but will be zero if it is a bad connection in the live wire. Just been through this getting my trailer OK for a pink slip - everything worked except one stop light - and this turned out to be the connector on the earth lead, six inches from the fitting.

John

Allan,
Like you I shudder when it comes to fixing the sparky bits, but you have done exceedingly well using logic to find the problem.

Buy a test lamp from any auto store with the globe in the handle and a pointy probe on the other end; very useful for people like me and you

cheers Chazza

Another simple analogy for the phenomenom that is electricity, which supports all of the previous posts and is close to the actual physics of it:

Think of voltage (V) as the amount of stored energy in the form of electrical charge (either +ve or -ve, take your pick). This easily explains why a multimenter of any type set on V will measure the amount of electrical charge (energy) available to be used, but not the amount used thus far nor will it indicate whether it can be used (no circuit or conductor path - think dodgy earth or blown fuse).

Think of current as the (resultant) flow of the electrical charge (stored energy) once released - after all, the stored charge (energy) will try to equalise (flow) with other bits of the circuit that have a lesser amount of charge (energy). Again, this supports all the previous posts that electricity must have a path (circuit or conduction path) for it to flow along. For a multimeter to measure this flow (current) the multimeter must be part of the path and will only measure the current that flows through it.

Ok, for those still reading, that ends the hard explanations.

Think of resistance as the resistance of the circuit to the flow of the released charge (energy), the less resistance, the more (higher) flow. The more resistance, the less (lower) flow.
A multimeter when measuring resistance, puts a little but known charge on one end of the device being measured and measures the flow through it thus calculating a resistance. Logically, if there is low or zero resistance (think short) to a circuit the flow is very high and to much charge (energy) is released in too short a time ....see Power. If the resistance of the circuit is too high (think open or bad earth or other connection) too little charge (energy) is released and not much happens!

Think of power as the 'work' done by the electrical charge (energy) or the rate at which the charge (energy) is converted into other forms eg heat or light. Logically, it is a product of the amount of charge (V) and the flow of the charge (I or current). As mentioned above, if too little energy is converted due to an open cct (bad earth etc) there is little or no result eg no light or heat. If too much energy is converted, or more accurately, if not enough resistance is available to slow the flow, more is converted than the resistance (globe, heater coil etc) can use and the device (globe etc) over heats until something physically melts or removes itself from the cct causing the smoke to escape!

Simple isn't it!
For those who have stayed, this is a simple explanation that works in nearly all cases bt is nether definitive nor the only one. I try, due to years of teaching apprentices and further study, try to keep analogies as close as possible to the physics.
But I also like the water analogy and pressure (hydraulic) analogies, I just don't use em!


Ralph

Originally Posted by awabbit6

Unless the reversing lights have a dedicated earth to the chassis (possible but unlikely), I would guess the socket end.

I expected a poor contact in or near the bulb socket too, but all the connections and contacts were quickly eliminated as the cause of the problem.
If there had been a dedicate earth to the chassis near the light, things would have been much easier. What made it difficult was that the earth wire from the light follows the live wire into the large bundle of wires that supply all the rear lights. It is encased in that corrugated split sleeve and the whole lot is mostly inside the chassis.
Even though I am sure that, as others have suggested, a separate earth will solve the problem quite simply, I am still curious about the way the wiring is set up.
Is there an earth at the other end of the wiring loom at the front of the chassis near the firewall? If so does it earth just the reversing light? All other lights work perfectly, so it can't be a common earth. What possible reason could there be to run an earth wire all the way to the front of the vehicle to earth a light at the back of the vehicle?
By the way, thanks for all the helpful suggestions such as using test lamps and for all the explanations which have helped me get my head around a few of the issues involving electricity.

So if I have understood the explanations so far, the title of this thread is incorrect.
I never had any current.
All I had was voltage.