I'm in the process of figuring out a battery drain when parked. It's an old fire truck with 4 separate wires connected to the positive batter post, going to various gizmos besides the main cable. By disconnecting them all and testing one by one with an ohmmeter, against the negative cable, I have identified the one wire with continuity. Now I just have to figure out what it goes to... One of the other wires however, and I have experienced this before testing other circuits, when touching the meter lead, results in the meter going from open (1) to blank and then back to 1. So it is sensing something, as the other wires just stay open at 1 when testing. It does this on all the various ohm settings. What causes this?
If it is a switched circuit and the switch is in the off position you should not have continuity. I'd be inclined to get a buss bar and run the leads to it and one lead to the battery if you find that all four circuits are working circuits. All those connections at the battery post along with creating clutter are more chances for corrosion at the post.
some things have a capacitor in them, that might behave this way. The ohm meter applies voltage to the circuit to measure resistance, this voltage will put a little charge in the capacitor, then it will bleed off. Using an analog ohm meter will give you a better feel for how this works, try it on several capacitors of different uF ratings and see what happens. Might want to short out the capacitor first, to make sure it has no charge on it.
I would measure current. Leave wires off. Meter set to DC Amps. One lead on wire, one on battery positive
No...for voltage you have to measure a drop across a device. I'm assuming you are talking about as I described the leads above for current.
Ah yes, that would explain it. I have played around trying to test capacitors before, never very successfully but that was one of the results. Thanks. I did that too. The one wire that showed continuity measured 10 on the 200m amp setting as I recall. I wasn't too concerned with the reading, was just trying to find which of the 4 wires was causing the drain. Good idea. Right now it has one of those bolt down replacement cable ends with all the wires bolted to it. This truck was recently retired and replaced, and it is now under my possession and care, to be used at parades and events. So nothing critical, it's no longer responding to calls
since it's not driven regularly, maybe it's a good candidate for a battery cutoff switch....so you don't have to worry about it, just turn it off until you're ready to use it again. But maybe put a battery charger on it the day before, if it's been sitting for a while.
This is the best solution for many reasons. The current draw on that one lead would take time to trace down. It may be a clock, a radio, etc. Using a meter the voltage scale is the "safest" one to use, safe for meter or other damage. Resistance or ohms you have to be 100% sure battery is disconnected. Then grounding the disconnected wire a few seconds will discharge any capacitor if it's in that circuit, although the meter's battery can charge it up again. Current scales again you have to be careful, starting highest amps first then going lower. Leads are in series with wire (the load), positive to battery +, negative to wire. Analog and digital both have their merits. Most quality digital meters have a high input impedance (10 meg ohms or so), so they're sensitive. Sent from my SM-S320VL using Tapatalk
Analog meters are good for "seeing what's going on", when you can't just get a nice solid number from a digital meter. If you want to combine this ability with high input sensitivity, you can get a VTVM, which is really going back in time
That's a pretty typical reaction when using some digital meters. Most take a second or two to 'decide' if the input meets the parameters of the scale it's set to before displaying anything. Basically what it's telling you is it's reading some continuity but the ohms value is beyond the capability of the meter to measure on whatever scale you're using. If you turn to a MegOhms scale, you may actually get a reading.
A VTVM like this? The B&K LED was a big deal when it came out...but the Non Linear Systems first ever digital meter with the etched tile edge lit display, vacuum tube (of course) you won't see one every day. It's as old as me and still works! Sent from my SM-S320VL using Tapatalk
Clamp on inductive DC amp meter is a good way to diagnose current draw. Another way is with a infrared temp gun. With the vehicle parked in the shade, and temperature stabilized to ambient temp, go around with the temp gun to anything with wires. If you find something even 10* higher there is your draw.
I would just hook a 12v test light in series between the battery post and the wire in question. Start unplugging the add-on accessories until the light goes out. When it does, there's where to look.
Yes I have noticed that too, but as I recall the meter will just go blank and not revert back to 1. I may be mistaken, I will play with it more in a couple of days. I was using one of those cheap red HF meters I keep out at the property, I will bring a better one, and an analog, when I go back out. There is no electricity anywhere near where it is kept. And no sun. Members of the fire department still have access to use it so a cutout switch, although feasible, means everyone needs to remember it has it, and half of them probably couldn't find the hood latch. A proper system should not drain the battery in the times between when I am able to run it, which is at least once a month. Whatever the once accessory that is causing the drain is, if I can't find it, is probably no longer required. There is a boatload of wiring on this thing, exterior lights, cabinet lights, sirens, inverter, pump and pump panel, radios. I'll just leave that one wire off and eventually figure out what no longer works.
I'd look at the radios, inverter, etc first. Anything with electronics in it. As opposed to lights, pump panels, sirens, etc.
Looking with eyes won't help. Look with a IR temp gun, if it's on, it will be warmer than ambient air temps.
Here's something easy to try without any equipment, although it would take an hour or two. Faster if you have an assistant. 1) connect all wires back 2) check everything electrical, lights, siren, radios,etc. Be sure everything works. 3) disconnect that one wire 4) now see what doesn't work. Write it down. Now you know what's on that circuit. If you wanted to, based on what's on there, wire in an appropriate amperage spst switch just for that. That's assuming you can't at that point or narrowing it down find what of those things is causing current draw. Whatever is on that circuit (let's say it's 10 things) by process of elimination you could find the one current draw. That ONE thing leave disconnected or switch on IT. Sent from my SM-S320VL using Tapatalk
Did a little more testing today. Now I'm getting readings on two wires. The red wire goes to the battery tender which is unplugged. The other one goes to the alternator and continues on someplace. And, the one in between those two is the one where when I touched the probe it flashed from 1 to blank then back to 1. It would not do it again which leads me to think like Squirrel mentioned above that by touching the probe it charges a capacitor.
I give up... Checking for a parasitic draw with an ohm meter is wasting your time. I guess you did not read my previous posts.
#24 I explained an easy test. Another is an ammeter (DCA scale) in series with each wire. You can measure current drain, starting with highest scale. A drain would be probably 100mA to 500mA (1/2A) my guess. Instead of ammeter you could series a small 12v bulb like a 194. It shouldn't light, if it does that's a drain. (#194 is about 250 mA or so). Easy peasy. Sent from my SM-S320VL using Tapatalk
Electricity is the most befuddling thing not only because it's invisible but before I retired years ago I was an electronic technician troubleshooting things for over 43 years. What I learned in trade school and college was different in real world many times. Something as simple as a single resistor could change resistance, some by either vibration, temperature, moisture, light, etc. The foundation of electricity is ohms law. E=IR, E (volts)= I (amps) multiplied by R (ohms). Power (Watts) = voltage X current Test equipment is useful but can be confusing. Analogy meters watching the needle move can tell more than numbers flashing on a digital display, although digital is best for accuracy. A test light is great because it's simple and slightly loads the circuit. In the OPs current draw case the meter shows 81.2, but that's 81.2K ohms since it's on 200k scale. That's 0.0001601 or 160 uA! (Negligible current draw). Other shows 2.8k ohms or 4.64 mA which is a small current draw, enough to drain battery but over a long time. What's screwy is put an ammeter in series with that wire and I bet it wouldn't read 4.64 mA. One picture I show meter across my finger and it's 7.5 megohm, wet finger it's 224 kohm. One picture shows a resistance of about 5 ohms (6 ohms on Fluke 11) on a 55 Chevy. A #194 bulb (250mA @ 12.8V) lights brightly across switch. Close switch for few seconds, open it...bulb doesn't light! (The draw was clock, clock points were closed then they opened when solenoid wound spring) so now no current draw. Bottom line parasitic current draws can be from lots of things, even some corrosion/moisture to ground. Sent from my SM-S320VL using Tapatalk
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