I remember an old mechanics trick about using a test light between the battery and either the positive or ground cables and start pulling fuses until the light went out. Which cable was used, positive or ground? Thx Dave ss/ea #1355

I use the positive but I don't think it makes a difference as it's a complete circuit.[]

That would only work if you are 100% certain there is no other load proper or not on the system.

Thx for the reply, i realize that, just want to narrow down which circuit the short might be within, then try to narrow down even further. Dave ss/ea #1355

You can hook the test light to either positive or negative.

On modern cars it is very common to have a constant draw so the light may not go out but will get dimmer when you pull a fuse with a electrical draw.

Also it is very common on modern cars to have timed lights and circuits that will draw lots of energy for a minute or more then shut off. Those will drive you nuts while looking for a voltage drain. Most common is the interior/corn lights that stay on for so long after the door is opened then shut with the ignition off.

Don't know if this is of any help?

The problem I see with that intended solution, I see no current limiting fuse in the headlight circuit other than the light itself, and if the light draws more than the circuit is designed for, the existing cars wiring at play can overload, overheat, and make whatever problem is occurring with the car, much worse and complicated. and the longer the light is grounded by the direct short, the higher the heat that might/will be reached and maybe damage caused. Being able to see the bright light anywhere when working on the problem is indeed useful but at increased risk, IMO.

It depends on what type of issue you are looking for. I prefer to use the ground cable. There are many hot paths on the positive side and that can lead to misdiagnosis. If the problem is a draw the test light is an excellent method. Though be aware that on late model stuff, modules take time to go to sleep. Also disconnecting and reconnecting may wake them up. To prevent this use a jumper wire and the test lamp in parallel between the battery and cable. After allowing a few minutes (sometimes as long as 20 minutes) remove the jumper without disturbing the test lamp. Then pull fuses or hot feeds to unfused items until the lamp goes out. Now a true short where a hot circuit is in contact with a second hot circuit which should be off? This method will also work. But you will have 2 pcircuits lighting the lamp. If you are looking for a grounded hot circuit the issue should be evident by either melted wiring or a blown fuse. in this case if the suspect circuit is fed thru a test lamp (output side of the fuse block) it may light the lamp if the circuit is still grounded. If this is the case the lamp can be in place and the harness wiggled until the lamp goes out indicating the problem area. Suspected weak circuits (such as week grounds) can be tested by grounding one end. Then feed the other end thru a lamp that requires some amperage, such as a headlamp bulb. This works better than using an ohmeter as it creats a load the circuit. I can't think of any circuit that would be over powered by this method. Used it often. Using these methods on an older car will help you make quick work as the wiring is simple. The last 10 years of my auto repair career was spent at FCA. The job was debugging electrical issues on prototype mule builds. Saw off the wall stuff every day. Can Bus issues, hidden bad connections, hidden rub throughs, 500volt spikes due to routing causing inductance, all 196 wires out of the PCM the same color, and more. Never had an issue that didn't get solved. But many were time consuming. It was a challenging and interesting job.
Doug

Short circuit ??? or a current draw ??? What is the vehicle you are working on ???

This ^^^

A short usually blows the fuse or causes another circuit to activate (ever seen a double filament bulb have both filaments touching?).


Tim

When dealing with direct current electrical problems remember a short circuit versus a grounded circuit can be very similar

just wait until the "magic smoke" escapes, then you will know for sure where the problem is............ [i just couldn't resist ! ]
intermittent problems have caused me to have hair fall out in big clumps, affecting my "good looks".
what is the exact problem you are experiencing, and on what vehicle ? [if i missed that, please forgive my old, "overloaded, short circuited", memory.

For the record a typical 12 V headlight draws 4.58 amps

Symptoms? Blown fuses? Battery being drained? How long does it take to be drained? Weird things happening? Year, make and model? Any novice electrician hack jobs? Do you have a wiring diagram for the car?

That's not for finding a short circuit, it's for finding a current draw, one that usually drains the battery. Years ago I had a 72 Swinger that would kill the battery overnight. Took me days to find it, turns out trunk weatherstripping I had recently replaced was thicker enough to keep the trunk light from turning off. Unfortunately, the FSM schematic for 72 Dart's didn't match my car and the trick you mentioned didn't help me. I found the fuse but the diagram told me it went elsewhere.

No short circuit involved.

For the record it's a 2012 Hyundai accent, Problem popped up last Saturday & drained the battery overnight. & has continued as of this writing. There are no obvious lights. hidden or open that appear lit after the ignition is turned off. He did mention that the fan motor stayed on after he turned the a/c off. I've looked at fuses and relays but can't find anything suspicious. The battery is new, and terminals are clean. I haven't had a chance to work on it much, but need to do so. I'm not an electrical guy, but wondered if the regulator is not up to par and would not fully charge the battery, would it also drain the battery. Keep the suggestions coming, I'm open to any & all advice. Thx Dave ss/ea #1355

Hook an ammeter between the neg post and neg cable. Let the car go to "sleep"(could take some time for all the modules to time out) Measure the draw.......if excessive...start pulling fuses. I've had a Toyota fan not turn off and kill batteries overnight. Test lights work OK on older cars...not so much on new stuff.

Thanks for the reply I have a multimeter what setting would I use? Dave ss/ea #1355

12 VDC

I would be using amps or milliamps(preferably) I would be looking for 100 milliamps or less.