Electro-fairy-wizardry question 

"AestheticsInMotion" (aestheticsinmotion) 02/24/2018 at 23:18 • Filed to: None

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I’m going to hook up a voltage gauge in my car tomorrow if the weather doesn’t suck. Negative wire needs to be grounded, and positive wire... Spliced into a constant 12v source? That sound right?

The internets told me to avoid an ignition wire because that would lead to a bad reading, and the internets also told me to pick a wire that will allow me to get a read while I’m turning the key to see the battery/alternator/fairy-prison health while cranking.

That’s all well and good... But uh. I don’t know how to do that. Just find any 12v constant and I’m good..? I have wire taps and I imagine they’re easy enough to use.

The internets also mentioned using this handy dandy tool below to check which wires have power when the engine is off (and when the key is turned to accessories, I imagine..?). But uh... Despite keeping it in my tool box I don’t have the slightest clue how to use this. Anyone want to give a quick “digital multimeter for dummies” lesson?

Tl:Dr - how da fuq do I wire a voltage gauge.

Bonus round! My belt squeel is getting rediculous. It’s only on startup, for anywhere from 20 seconds to two minutes. If I go for a drive, shut my car off and turn it on again within like five minutes there’s no startup squeel. Cold seems to make it a bit worse, maybe. The number of accessories running definitely seems to make it worse. Throttle position and/or rpm greatly increase the sound. Whether or not the clutch is in seems to effect things as well.

What’s your diagnose? I’m thinking alternator, timing belt or accessory belt pulley...

StudyStudyStudy > AestheticsInMotion 02/24/2018 at 23:39

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If the squeal isn’t from a pulley, you can check via the old extension against the pulley accessory trick (careful not to actually put it on something moving), it could be a glazed belt. Once a belt starts slipping the surface is not as grippy on first start, so it will slip for a bit before it catches up to temp which will melt the belt a bit from the slip friction, which will compound the problem as time goes on.

You can pull a common wire that gets power after ignition. The gauge ones are handy because they they are off when the ignition is off so they won’t cause a drain, although that is pretty minimal, and they are also inside the cabin which makes it fairly easy.

If you wanted the most direct source you would get a wire from the battery straight, but you have to be careful since that is a very very live wire and if you don’t sheath it well and it shorts it will send all the amps through whatever it is shorting to, if you are lucky the wire will melt and break. If you are unlucky it will get red hot and slice into other wires and kill the car (just putting a little bit of the fear of god into you :D).

For the multimeter you will be using DC20 setting to detect voltage below 20v. Put the black end which should be on the ground port against something metal that is bolted to the car, the red probe you will use to find an ignition active wire. You can do this by jamming it into a plug or using the tip to bore a little bit into the wiring if you can’t find a good angle. Make sure to tape afterwards. Don’t worry you are only detecting voltage so you won’t be able to fry anything unless you bridge contacts.

Let me know if you need a hand, I’ve got quite a decent wiring kit.

LOREM IPSUM > AestheticsInMotion 02/24/2018 at 23:41

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If you wire the positive to constant voltage, it will stay on at all times, even with the key off, and will be a drain on your battery. How much of a drain, I don’t know, but probably more than you want to have going at all times.

If you want to be able to use it with the ignition off, wire it to constant power but put a switch on the positive lead so it can be turned on and off manually regardless of ignition position.

Easier though would be to find a fuse in the fuse box that only comes on when the ignition is turned on and wire it to that location. Your car may even have an open fuse location for accessories, but if not you can get adapters that will plug into an existing fuse location and it gives you a tab to connect a wire connector to while still retaining the fuse and existing circuit.

I wouldn’t worry about voltage drop too much under the circumstances. So long as you tap a good 12v lead the amount of displayed voltage drop is going to be negligible for your purposes imo.

As for the squeal, I’d replace the belt first. If no go, probably do the tensioner as a formality. If still no good, diagnosis with a stethoscope or just replace everything that spins on the front of the engine.

Full of the sound of the Gran Fury, signifying nothing. > AestheticsInMotion 02/24/2018 at 23:44

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Well, you might want to put a switch in-line with the positive connection otherwise it will be a slow drain on the battery.

I’ve got the same display around here that I meant to install a few years ago, but somehow I misplaced the shifter bezel for my car in which I was going to place the voltmeter. I’ve looked for it for years and still haven’t found it. I’m not going to pay retail for a replacement assembly, and finding this part for a Mazda5 with a manual transmission in the US is, well, impossible. I’ve got one from an automatic, and I’ve got the boot from an E34 5-series, and I just need to get busy and create one of these myself.

i bought one of these and love it. It even shows the current draw on the USB ports when something’s plugged in. Well worth the $10.

AestheticsInMotion > StudyStudyStudy 02/24/2018 at 23:56

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Thank you! Yeah I’d prefer to avoid going to the battery and potentially burning my car down... Don’t worry, I already have the fear in me! I’ve got the space, and at some point I really want to fabricate some kind of battery cage and relocate it to the engine bay. It’s such a pain in the trunk...

I would love to get some help with some projects at some point. Maybe do a tech day kind of thing? I’ve got gauges to wire up, a non-functioning speedometer to look into, plenty of other issues, along with a few more parts to throw in

AestheticsInMotion > LOREM IPSUM 02/24/2018 at 23:57

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I’ll check the fuse box. But I might just end up using a switch because I have a few laying around and have wanted to use them for awhile...

AestheticsInMotion > Full of the sound of the Gran Fury, signifying nothing. 02/24/2018 at 23:59

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Interesting. My only issue with these things is in how quickly portable electronics evolve, and soon enough the charging capabilities can’t even come close to keeping your gadgets going. I realize these are cheap.... But I want something with some degree of future proofing...

traitor joe > AestheticsInMotion 02/25/2018 at 00:01

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Easiest wire to splice into is often the one going to the radio. Radios have connections to constant 12v and running 12v and many will have wiring diagrams printed on the back (or available on the internet).

Full of the sound of the Gran Fury, signifying nothing. > AestheticsInMotion 02/25/2018 at 00:04

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Well, I can’t see any USB devices taking more than 2.1A in the near future (my big-ass phone only takes 1.2A, and I haven’t tried an iPad in the car yet), so I think you’d be good for years. Think of it as a voltmeter that just happens to have a few sockets built in. Plus, if it does get outdated, you don’t have a hole in your dash to cover up somehow.

StudyStudyStudy > AestheticsInMotion 02/25/2018 at 00:33

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Moving the battery back into the bay seems a bit backwards especially if you are thinking turbo in the future, but a battery box is pretty easy to make, buy, or modify. If it is inconvenient just for jumping I know they make external jump port kits, but with the offloading you do maybe not the best idea.

Whenever you want, I’m usually free on weekends, my next planned event is an autocross two weeks from now. Speedometer might be tricky if it is cable driven, there may not be much we can do. Gauges and such are pretty simple if we have the internet.

Dogsatemypants > AestheticsInMotion 02/25/2018 at 03:29

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Good rule of thumb. If you have an accessory/lighter socket you have found your wire. For the multimeter you would set the dial to dcv the closest voltage setting over 12. Black probe to ground point, red probe to test point.

If you can remove the socket, it will be a million times easier to wire it inline. The other thing you can do is jump a wire in on the fuse block or relay block. There should be a fuse labeled acc or something like that. The hvac control relay would only draw power with ignition power on.

Dogsatemypants > AestheticsInMotion 02/25/2018 at 03:37

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Jalopnick hackerspace?

Take some time to read a few basic rules about low voltage dc wiring. Resistance and impedence are good to know about. You dont have to bust out the calculator for every project, but it will help you in figuring out what kind of wire to use. Don’t be afraid to make mistakes. I fried a couple of things on my first car figuring stuff out.