What is circuit loading?

Hey, guys,

I'm struggling to understand circuit loading tests. Could anybody please explain to me, as if I was a 4 year old, what is the purpose of using a lightbulb and how that differs from a multimeter/test light? Actually, if anybody could give me a few examples of some tests on different outputs, I would be REALLY grateful. I visualise better on examples.

As far as I understand it... If you have an output and remove it, then just doing wiring checks doesn't prove a lot. It may prove there is no complete open. That's why you would stick in a bulb, eg headlight to fuel pump circuit , to see if the circuit can light it. 

However, how does that differ to a voltage drop test using a multimeter? The DMM cannot carry much current, so it's useless for loading the circuit, I understand that now. 

There's one post here showing how a door lock wiring had 12v, seems good, right? But then he "loads" (does that mean switches on?) The circuit, only to see the V drop to 0. Turns out there was a green crusty. 

So, I was at my fuel pump (it works). And I disconnected it trying to simulate a fault. I connected the DMM, and thought it would be best to actually load the circuit. So I cranked it. 12v dropped to 10.8 which is fine, because that's the starter dropping the system V. 

However, I didn't load the circuit by cranking it, did I? Loading means having the component eg bulb drawing current?

Was my test with the DMM valid in anyway? But for another reason?

Thank you, it's all a bit complicating. I can't really figure out how to use drop testing, bulbs in a real scenario. 


​​​​​​For outputs, SD uses an incandescent test light to see if the transistor works. But that's not loading the circuit, right? That's just to find out if there is a responsive driver.

A good few examples of scenarios would be really helpful. 

Actually, I have one. Let's say we have a cooling fan that isn't working and we wanted to check the ground (for whatever reason). I connect the bulb to the + and - and if it lights, am I safe in saying there are no crusties in those wires?

It seems that you, already, have a basic understanding.   Here is an awesome video that may help visualize things and make it all fit together, a little bit better.

That did clear up a lot, thank you. This would actually nicely tie in with any circuit that uses a constant 12v feed, like electric cooking fans. Just measuring for a 12v isn't enough to say the circuit is fine.

Also, I can imagine myself pulling out a headlight bulb to see if the ground was fine. Although... Come to think of it, it would be enough just to do a gnd to gnd drop test with the connector plugged in, no? If I saw voltage on the gnd, like SD teaches with sensor circuits, then I'd know the gnd is a problem, am I thinking correctly?

But when it comes to other circuits, I've noticed SD doesn't use bulbs to check output wires. Why is that? When would there be a situation in which you would need to use a bulb?Do you use that as a standard procedure when an output doesn't work and you want to just check the integrity of the wires?

Because before I would just measure for V... Which could get me in trouble. I suspect that if we had a bad output, we would have to bi-directionally switch it on and see if there are any V drops? But that would only show that the command is there from the ECU.

Say there's a problem with a valve. Doesn't work. I find out if it's GSS or PSS, say that's fine, then I command it to see if the ECU isn't at fault. But then what? How would I go about making sure the circuit could handle the load (if the output is dead)? Bulbs? Test light? Like I said, I haven't seen SD do this, I assume there's something else he does.

I think you can see my confusion!

Ok, so, the SD videos "Voltage Drop Testing With A Test Light?" Part 1 & 2 helped in understanding a lot as well.

SD says that he just doesn't feel the need to have to use bulbs (load the circuit with anything more than a test light) if he finds a dead output.

If it's GSS or PSS, then you have to remember what to expect to see on both wires. If it's GSS and he sees PWR on one but nothing on the other, he's done, makes a call for a faulty output.

Wiring problems AND faulty outputs do not happen, but, like he says, you can just reverse polarity of the test light to load the ground (on GSS with open output) just to see. He doesn;t feel the need to go beyond the test light and check with a bulb.

I'll trust that judgement.

I guess it's all a little confusing. Maybe I'll wait a few days and see if it settles nicely.

In Polish we have a saying that goes "dzwony dzwonia, ale nie wiadomo w ktorym kosciele" which is roughly translated to "you can hear the bells ringing but you don't know from which church".

I've noticed SD doesn't use bulbs to check output wires. Why is that? When would there be a situation in which you would need to use a bulb?"

An incandescent test-light IS a "bulb", in that it has a glowing filament.  The difference between using a test-light vs. a "bulb" is the resistance. A test-light has a higher resistance than most other bulbs. (not all test-lights have the same resistance). The higher resistance means less current flow. On low-current circuits, a test-light is all that is needed to load and voltage drop test the circuit. However, on higher current current circuits, such as headlights and motors, a test light might not pull enough current to sufficiently load the circuit.

It is best to test the circuit with a load similar to that which the circuit was designed to carry. This is why other bulbs are sometimes used. Larger bulbs have less resistance and pull more current. On a higher current circuit, they are more likely to reveal a problem that a test light may not. A head-light bulb can pull up to 4 amps, where most test-lights will pull, significantly, less than 1 amp.

Use caution not to pull more current than the circuit was designed to carry.