What is the best way to test for a short with a DMM? Do you remove the + or - lead off of the battery? Then put your DMM in between them? What should it read without a short and what would it read with a short? I know the clock and interior dome light will cause some draw. Any info from you elect. experts would be appreciated.
We need a bit more information in order to help you. 1. Why do you suspect a short? 2. Where do you think it is? Did you isolate it to one circuit? A DVM connected across the +/- of the wires will tell you next to nothing in most cases. A short will also blow a fuse, that tells you what circuit it is, but not what component is shorting. Normally, a short will cause heat some place, that is one indication. I have once found a short in a car stereo amplifier that kept on blowing fuses by taking it out of the car, attaching it to a battery on the floor, bypass the fuse with a hard wire, connect it all together, stand back and watch which component gets really hot and fried. It turned out to be an FET power transistor. Changed that, and it has been working for ... 7 years now.
I don't have a short but I would like to know how as this topic has come up and I think this is a good post to have. I think a good symptom would be if your battery was dead after a few days of not using the car. I don't mean putting a DMM accross the terminals. I mean removing the - wire and putting a DMM in series with the battery, car off and checking it that way. I assume you put the DMM on Amps and the draw should be no more than ???? 30-60ma???? Am I right???? It is always fun answering your own ?'s. Where are the consultants? Basically is this the correct method? Any tips, etc.
It's called a short circuit because instead of the voltage taking the "long" way through the circuit to ground, it takes the "short" way to ground. These can be a little tough to troubleshoot. Since all the battery amperage wants to go through the wire, it will blow the fuse. This is kind of a Catch-22, you can't find the short unless there's power, and there's no power until you find the short. There's a way to remedy this situation, by putting a load in the circuit so the fuse will not blow. To put a load in the circuit,*you need an old fuse, a bulb and socket and two lengths of wire. They can be as long or short as you like. You can take off the plastic from around the end of the fuse and soldered a piece of wire to each end. Then solder the socket to the other ends of the wire. Then simply plug the modified fuse into the fuse box and there's your circuit load. Now you can troubleshoot the circuit and find the short. In looking for a short, the first thing you need to do is look at your wiring diagram and see what is on that circuit. Then, one by one, unplug every component and look at the light as you do so. When the light goes out, you found the part that's shorting out. If, after all the components are unplugged, the light is still on, then the short is in the wiring. Now what you need to do is cut up the circuit into blocks as you did in looking for a open circuit. Look at your diagram and locate the connectors, and working from back to front, unplug them one at a time, checking the light. When it goes out, you have isolated the area that is shorted. ***These two techniques will work to find most common electrical problems. More advanced problems require the use of a voltmeter and ohmmeter to locate.
"It is always fun answering your own ?'s" -- especially when you originally mis-state the question. I was thinking: "how do you detect a short?" -- if it's not blowing a fuse, just look for the smoke "Basically is this the correct method?" -- Yes, ammeter in series with the battery. "Do you remove the + or - lead off of the battery?" -- What year/model? If it has a battery cut-off switch, I'd disconnect the same one that is disconnected by the battery cut-off switch, but on anything pre-90s probably doesn't really matter much. "30-60ma" -- probably model dependent with later models higher (but shouldn't be 250mA)
This would be on a 308 like my name implies. Steve, I just don't see anything misleading about how to test for a short. Key work Test. Make sense to me. My question wasn't where is my short coming from or why did my fuse blow but "How to Test for a Short".
I'm just joking so don't take my previous complaint seriously, but the misleading aspect is that the term "short" is more commonly thought of as a very, very low resistance unwanted connection so it would have a massive current draw (not a small current draw) -- which, if isn't protected by blowing a fuse, eventually melts/burns something. If your problem is that a true short is drawing just a bit of current than I guess you could call that a "high-resistance short" -- but that's sort of an oxymoron. Of course, you could have a short causing something else downstream to have a small current draw, but you would not be directly testing for the "short". What you're testing for would more commonly be referred to as an "abnormal parasitic current draw" rather than a "short" but that's JMO -- good hunting! (and post the data please whether it's good or bad)
Remove battery + Wire Remove all fuses Insert DMM between battery + and wire; set to AMPs Repeat until all fuses checked { insert fuse, look at meter, remove fuse } Now that you have isolated which circuit has a short, leave the rest of the circuits without fuses. For all devices on the circuit with the short, until short is found { remove device + wire, look at DMM, replace wire } If no device is causing short, then the wiring is causing short. If the wireing has inline fuses or couplers, remove, look at DMM, replace until found. If still no luck, then you are looking for a wire that has frayed, abraded, melted,.. to another wire. Good luck.
Thanks Guys. I appreciate the info. That helps out alot. No harm Steve. All is fine. I just thought I worded it correctly. Thanks Again for answering my ?.
That isn't really "testing for a short" -- that's testing for excessive key-off load ... which might be caused by a short, or by a long list of other factors. "Testing for a short" is something you do when a device such as a bulb isn't working. If the voltage across the bulb is zero, you have either a short or an open. If the resistance across the bulb is zero, you have a short. If the resistance across the bulb is zero with the bulb removed, then you have a short in the wiring somewhere. Tracking down short or open circuits can be done with a DMM, a wiring diagram (updated for any mods or accessories added), and a methodical approach. Patience is also required. There isn't a quick "C" program that will diagnose auto electrical problems, as there are odd things in auto wiring. For example, I had a Plymouth that would lose its headlights at odd times. That turned out to be a dirty contact in the headlamp switch -- one that did not appear with a DMM test, as it was only a fraction of an ohm. But instead of a fuse, it had a circuit breaker (bipolar metal type) built into the headlamp switch. With the high beams on, the contacts generated enough heat to warm up the breaker and cause the circuit to open. To make matters worse, with silicon getting cheaper than copper, a number of modern cars are now replacing large wiring harnesses with serial busses, so turning on a device may only be a data packet across the serial line, rather than a voltage on a dedicated wire. Diagnosing auto electrical problems often requires a pointy hat adorned with stars and comets, a few magic words of power, and a whole lot of words you don't want your children to hear.
