12V cigarette lighter reading AC mV, not DC ? - Page 1
Forum section: electronics / beginners

author: Causailty
author_rank: Newbie
timestamp: October 16, 2013, 10:54:25 pm
content: Hey all, in advance appreciate the advice.I have a 19V 4.74A cigarette lighter adapter to convert 12V for my laptop. It works in every other cigarette lighter receptacle I have tried it in, apart from my own car.I have a multimeter and have been attempting to test to see what is going wrong.The Corsa has a typical 12V car battery.The 12V cigarette lighter in the Corsa is reading 0.04V AC when the enginer is on, and 0.004V AC when the enginer is off. When testing DC, I get a number increasing to 0.4mV (I think - might have the number of zeros wrong - I can re-check) and then going down and increasing again, which seems consistant with the AC reading. (I only tried the AC reading because the DC was fluctuating).I don't know if this is standard, but from my limited knowledge of electronics, this seems bizarre from a 12V DC car battery.I thought it might be three phase power at one stage because the wires are black, brown and black/grey, but after reading in a corsa forum that in a corsa, black is live and brown is ground, this latter forum makes more logical sense. Because the cigarette lighter is powered by the black and brown wires. The grey/black wire goes to an led which lights the cigarette lighter up when sidelights are on.So this is why my adapter isn't working. But I don't know why the cables are giving this reading, nor do I know how to fix it.Also, I think the cigarette lighter/charger receptacle was actually reading 12V before I put the adapter in it. So the adapter may have sent it on this crazy fluctuation?Welcome any information or knowledge on the topic.

author: ve7xen
author_rank: Super Contributor
timestamp: October 16, 2013, 10:58:37 pm
content: With no DC voltage there, something is wrong with the power feed. Blown fuse, perhaps?Automotive power is notoriously noisy, I would not be surprised to see some mV of AC. This tiny AC value is probably below where your multimeter is likely within the error range of your meter anyway. You can safely ignore it. The real problem is the lack of +12V DC.

author: c4757p
author_rank: Super Contributor
timestamp: October 16, 2013, 10:59:28 pm
content: Quote from: Causailty on October 16, 2013, 10:54:25 pmWith the engine on, or off?If you're getting a low, fluctuating DC reading like that with the engine on, then for some reason the voltage isn't reaching your meter. Blown fuse in the car, outlet is switched off for some reason, bad probes, bad meter, incorrectly configured meter. For a sanity check, probe the battery directly. If you can't get a reading off that, the problem is either you or the meter. If youcan, then measure the outlet the exact same way.These tiny, fluctuating "phantom" readings are typical of probing a disconnected system. The wires (including the meter probes) act as antennas and pick up electromagnetic signals. And most cheap (and some expensive!) multimeters will never read all the way to zero AC, they're not meant for such small measurements.Your car doesn't have three-phase power.

author: Stonent
author_rank: Super Contributor
timestamp: October 16, 2013, 11:03:25 pm
content: Also a typical functioning car should read just above 13V with the engine and everything off and about 14 to 14.3V when running.A 12V battery reading 12V after the engine has been running usually indicates a weak or failed charging system.But like was said above, reading that low from the socket means there's a fuse or wiring problem.If you can reach connect - to the outside metal on the socket. Then turn your key on but do not start the car and probe both halves of your fuse.Then connect + to the pin in the socket and do the same. You should read a voltage on both sides of the fuse for one of the wires if it is functioning correctly.

author: Causailty
author_rank: Newbie
timestamp: October 16, 2013, 11:06:31 pm
content: Super fast amazing responses.Blown fuse makes sense. Obvious now I've read you write it, but the tiny fluctuating ac value was confusing me.Now I shall go try and find the particular fuse. I have no idea where the car fuse box is, and I will test them with the multimeter.

author: orbiter
author_rank: Frequent Contributor
timestamp: October 16, 2013, 11:57:45 pm
content: If all fuses are ok, try removing the lighter socket (you may need to remove the surrounding console/housing first) and check the lighter socket is actually wired to the harness.Depending on the vehicles specifications it may not even be wired-up, even though it has a lighter element. Some manufactures do this making it an option.If there's no wiring link to the lighter socket, there will be a suitable source you can splice into around the lighter/heater control panel area, and some quite chunky wiring if necessary, as the fag lighter loom & heater motor wiring are normally accessible from behind the centre console area.Please do use proper connectors and fully insulate any work carried out.Sent from my HTC Incredible S using Tapatalk Pro.

author: David_AVD
author_rank: Super Contributor
timestamp: October 17, 2013, 03:48:00 am
content: Quote from: orbiter on October 16, 2013, 11:57:45 pmWTF?? Are you saying some car manufacturers fit a socket but never wire it ? What would be the point in that ?

author: c4757p
author_rank: Super Contributor
timestamp: October 17, 2013, 03:49:14 am
content: Charging more to plug a wire in, would seem the obvious answer to me.

author: David_AVD
author_rank: Super Contributor
timestamp: October 17, 2013, 03:56:10 am
content: Quote from: c4757p on October 17, 2013, 03:49:14 amI think any manufacturer that did that over here would be on the receiving end of an ACCC complaint and have to wire them all up for free.

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 17, 2013, 04:18:46 am
content: Unlikely as it is, i think he was implying that the spade terminals may have come loose over time.

author: David_AVD
author_rank: Super Contributor
timestamp: October 17, 2013, 04:31:30 am
content: Quote from: peter.mitchell on October 17, 2013, 04:18:46 amThis made me think otherwise:Quote

author: peter.mitchell
author_rank: Super Contributor
timestamp: October 17, 2013, 04:39:11 am
content: Quote from: David_AVD on October 17, 2013, 04:31:30 amEhh *shrugs* only he can tell us what he means.That said, on my falcon, there were footwell lights that were partially installed (small wires neatly separated from the harness and placed into the housing of the light fixture, but not connected to the fixture and with no bulbs installed), which were an optional extra. (I didn't get it, i DIY'd it)

author: 6502nop
author_rank: Regular Contributor
timestamp: October 17, 2013, 05:20:09 am
content: Coming from an automotive background (certified), my money is on the cig/accessory fuse. Let's do a little math, shall we? Let's shall!QuoteActually, 12.6V is the average "resting" voltage of the common battery, so we'll use that for our equations. First, let's see what that thing is supposed to shove down the pipe:19V * 4.74A = 90.06W (we'll just use 90W).In a perfect world, there'll be no loss in converting, so let's see what this thing may be drawing on a good day:90W / 12.6V = 7.142857143A (we'll call it about 7.2A).I'll go out on a limb here and say that converter can do 90% efficiency, so let's add about 10% to the current drawn, so we're now at ~7.9A.Most accessory fuses I've run into are low-load (7.5A {red}), or high-load (15A {blue}). The magic number here is the 7.5A. If the fuse wired to your socket happens to be the 7.5A, it'll blow if the load is high enough. If the fuse is the 15A (mostly protecting the socket and something else), then you'd be just fine, unless the other thing(s) are drawing too much at the same time. This would also explain why it works just fine with other cars, as they may have the higher load fuse, or your adapter wasn't pulling enough current to blow it.Note:NEVERthrow a larger fuse in there unless you have certified that the wiring can handle the extra current! Some manufacturers will use the same gauge wire for everything, and just pop in whatever fuse value they need for that branch, but some don't. You *may* get by with, say, a 10A for testing, but replace it with the proper value afterwards. The battery/alternator can probably handle the load, but don't count on the wiring to hold up unless verified.Now, if you check the fuses, and all are okay, then I'd go looking at all the relays. On my late (lamented) Toyota, there was a relay for darn near everything in the car, including the accessory power feeds. This allows the ignition switch to automatically switch off any battery-draining item left on (like a laptop charger plugged into the socket). A bad relay is the first thing to check, sometimes you can just swap it with another to check if the problem moved to another circuit. The next thing to check is the power/ground feeds to the relays. I can't tell you how many times just scraping the corrosion off a ground terminal brought everything back to life - corrosion is a major problem in snowy areas, as they salt the sh!t out of the roads, at least around here they do (which, alas, was the demise of my beloved Toyota...).nopArigato, Aiji-san! Arigato.

author: SeanB
author_rank: Super Contributor
timestamp: October 17, 2013, 06:17:40 pm
content: It is a Corsa, so the location of the fuse boxes are in the owners manual, which should still be in the glove compartment still shrink wrapped. Typically in the footwells and under the dashboard, and another inside the engine compartment.i should do the same with the work vehicle, but there it is the wiring loom which is rather hard to get to, seeing as it is behind a steel dashboard.

author: geraldjhg
author_rank: Regular Contributor
timestamp: October 18, 2013, 01:23:11 am
content: hiuse an analog meter or a scopelong cables on 1 meg input DMM are bad newsput 10k res in parallel with meter and re measureor look with a battery operated scope andat least youl see what the meter interprets in acid use a 20k ohm per volt analog meter and keep calm

author: Monkeh
author_rank: Super Contributor
timestamp: October 18, 2013, 01:28:48 am
content: Quote from: geraldjhg on October 18, 2013, 01:23:11 am

author: c4757p
author_rank: Super Contributor
timestamp: October 18, 2013, 01:31:12 am
content: Why would you need to get a whole new meter when you can just apply a bit of thought? If you see this happening, then you know the line is floating. Problem solved.And don't go sticking a scope on it like a twit. The meter's quite enough.