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Discussion Starter #1 (Edited)
I’m starting this thread to collect short essays on electrical topics that commonly show up here in one guise or another. Quick snippets which describe concepts or situations that are common to a number of different potential mods or real problems. One concept per snippet! I’ll maintain a running index of them up here in this first post, so you can go straight to the ones that discuss the situation you’re interested in.

We're trying to keep this purely an information thread. If you have an electrical question, please ask it out in its own thread in the appropriate forum.And here are links to some other threads that can be counted on for good information of an electrical nature.
The FAQ-like thread Common problems, need your help! has an electrical section that points to answers to many of the common electrical problems encountered by Forester owners.

My tutorials on relays, diodes, multimeters, and LEDs get a good bit deeper into how to work with these components and tools.

My sources thread contains recommendations for a number of proven good sources of electrical components and tools.​


OK, chime in here, electrical guys! We need more snippets!
 

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Discussion Starter #2 (Edited)
A quick summary of electrical units and their relationship.

What’s a Watt, and who's up at Ohm?

Volt How hard the electricity is pushing. Similar to pounds per square inch in a pipe. You measure it by putting a meter across two points in a circuit. It’s the V in Ohm’s Law. The National Electric Code prescribes the wiring criteria for Voltages above 50 Volts. Below that there are no rules (other than the Laws of Physics). Even at that, automotive engineers can manage to break the rules.

Amp (Ampère) A measure of current, the flow rate of electricity. Similar to gallons per minute. You measure it by inserting a meter into a circuit so that the current flows through it. (That can be dangerous with high voltages or high currents. You can measure current much more safely with a clamp meter, though DC clamp meters are still kind of expensive. AC ones have gotten down into the affordable range.) It’s the I in Ohm’s Law.

You will see current ratings used in two different fashions. On a device that consumes power, its rating specifies how much current it will draw at its operating voltage. On a device that delivers power, it's a rating of how much it can provide when asked. It's not a specification of how much it will push through the circuit, unless the device is explicitly described as a current source. These are rare and specialized.

Watt The amount of power the electricity is delivering. It could also be described as the flow rate of energy. Watts equals Volts times Amps, so you can get the same amount of power from lots of Amps at low voltage (the starter motor) or from not many Amps at a higher voltage (a big AC motor that plugs into the wall). But due to the math of the situation, the power you lose in wiring and other incidental connections is proportional to the square of the current. That's why power transmission lines run at such high voltages, and consequently at an unbelievably low current.

Watt-Hour One of the various ways of measuring the amount of energy consumed by something over time (or available to be consumed). A Watt of power delivers a Watt-Hour of energy in one hour. The energy in batteries is usually expressed in Amp-Hours. At first glance it would appear that those units don’t work out right. A Watt is Volts times Amps, so a Watt-Hour would be Volts times Amps times Hours and not just Amps times Hours. But it’s OK, because the expressed voltage of the battery is an implicit factor in understanding this measure. But consider the Amp-Hour rating of a battery to be a general guideline. These things aren't exact.

Ohm Resistance. Similar to the cross-sectional area of a pipe, only backwards. At the same water pressure you don’t get as much water through a small pipe as through a larger one. It’s the R in Ohm’s Law.

Mho I threw this one in for the fun of it. It’s Ohm spelled backwards, and it’s a unit of conductance. People who study different types of conducting material wanted a unit that was more manageable than fractions of an Ohm, and which went up in value when things got better, so they invented their own. Who says engineers can't have fun! Mhos = 1/Ohms. Those without humor prefer to use the official SI unit of measure, the Siemens, which equals one Mho.

Ohm’s Law expresses the relationship among voltage, current, and resistance, in a circuit that contains only resistive components. No capacitors, no inductors, no non-linear elements like diodes. Just resistance. It’s simple: Voltage = Current times Resistance. V = I*R. Simple algebra rearranges this into I = V/R and R = V/I. The unknowing sometimes call these representations “Ohm’s three laws!” Any way you look at it, if you specify two values you have determined the third one. Everything is in nice linear proportions. Keep the resistor, double the voltage, and you’ve got double the current. And so on.
 

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Discussion Starter #3 (Edited)
Prefixes

Some electrical units are handy to use just the way they are, but others only fit practical situations in very large or very small quantities. So we use prefixes to designate multipliers or dividers, typically in powers of a thousand. It’s a lot easier and safer than counting the zeros to the left or right of the decimal point.

Here are the prefixes most commonly used in basic electronics and IT, along with some of the places where you might tend to encounter them.


Pico (p): Trillionth - Pretty darn small​
Nano ( n): Billionth - Pico, nano and micro are common multipliers for the Farad, a unit of capacitance so large as to hardly ever be seen. (I'm dating myself with this comment. It used to be said that you needed a pickup truck to deliver a Farad. Today's ultracapacitors have indeed reached the Farad mark. One common application is to help provide a reserve of power to tide a sound system over a brief peak in volume.)​
Micro (u, or µ if you have the font): Millionth - Ditto​
Milli (m): Thousandth - Commonly used with Amps in modest applications. Things that draw actual Amps with no prefix are things that mean business!​
Kilo (k): Thousand - Commonly used with Ohms, Hertz, Watts, and Bytes or Bits. Volts too, but not in cars.​
Mega (M): Million - Likewise.​
Giga (G): Billion - Commonly used with Hertz, Bits, or Bytes​
Tera (T): Trillion - Commonly used with Bits or Bytes​
Ziga (ZZ): Zillion - Used anywhere you want to use it.​


OK, I lied about Ziga. But it’s still one of my favorites when I feel the need to exaggerate. You can see the entire list of 20 official prefixes here. Zetta, zepto, Yotta, yocto ... too far out!

See the exchange starting here for a view of how prefixes can be used in misleading marketing.
 

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Discussion Starter #4 (Edited)
Roll your own resistor - Parallel and series resistors

Resistors only come in industry standard values of resistance and power handling ability (Wattage). And the more Watts you need to handle, the smaller the number of choices of resistance values there are (and the greater the cost and the scarcer the sources). So sometimes you have to manufacture your own special resistor by combining two or more in series or in parallel to get the resistance and/or Wattage that you require.

Two resistors in series is easy: their combined resistance is just the two of them added together. Two in parallel needs a formula: (R1 * R2) / (R1 + R2). Note that the result of parallel resistors will always be less than the resistance of either, but resistors in series will produce a higher value.

You can also use resistors in series or parallel to create greater power handling capability, AKA greater Wattage. Computations here get a little tricky. Let’s take it case by case.

Resistors in series all have the same current flowing through them. A shorthand computation to the Watts consumed by each resistor is I squared times its R.

(Derivation using Ohm’s Law: Power = V*I = (I*R)*I = I^2*R)​

A similar situation exists with parallel resistors, except that here it’s the voltage that’s the same across all of them. The shorthand formula for power consumption here is V squared divided by R.

(Derivation using Ohm’s Law: Power = V*I = V*(V/R) = V^2/R)​

Once you’ve figured out the power that each resistor can expect to consume (and turn into heat), double it and buy a resistor of no smaller than that Wattage.

A quick shortcut: You can double the Wattage by connecting two resistors of half the desired value in series, or two of twice the desired value in parallel.
 

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Discussion Starter #5 (Edited)
What’s all this low side switching stuff, anyhow?

There are two basic ways to control the electricity that flows through a circuit. ‘High side switching’ (sometimes called ‘positive switching’) puts a switch (or a relay or some other sort of controller) between the source of power (the battery in our cars) and the device that’s being controlled (‘the load,’ we call it). The circuit is completed by connecting the other side of the load to ground and thus back to the negative terminal of the battery. In ‘low side switching’ (‘negative switching’ or ‘ground switching’) the battery connects directly to the load (through a fuse of course, I hope), and the switch is located between the other side of the load and ground.

They both work fine, though low side switching is against the National Electrical Code in higher voltage situations such as home AC wiring, as it can put electricity in places where those unfamiliar with the wiring might not expect to find it.

Why then do we see it showing up so often in cars? The major reasons, quite intertwined, are complexity and economics. It starts with the nearly universal practice of connecting everything back to negative by way of the car’s body and frame—the ‘ground’ (‘earth’ in Brit-speaking countries) to borrow a term from the earliest days of electricity, when the real dirt of the earth was used as part of communications circuits. The wetter it was the better it was! This saved many miles of wire. Use the car’s own metal as ground (it conducts much better than dirt), and already you’ve eliminated half the wires that would otherwise be needed to wire up the car. But now this introduces some interesting situations, complete with additional impact in both complexity and economics, often leading to the choice between high or low side switching.

The classic illustration of low side switching is the switches that turn on the inside lights when the doors are open. Take one apart and you’ll often find just one wire connected to it. Before the days of dimmers and other fancy controllers, that wire would go straight up to the low side of the dome light bulb, as would a wire from each of the other door switches. Or perhaps some if not all of them would be strung out together, with one wire then going up to the light. The other side of each switch went to ground, usually directly through its mounting in the door frame without even using any wire. The high side of the dome light went straight to the battery. So when any one of the switches closed, the circuit to ground was completed and the light came on. We got the dome light to work at the cost of as many wires as there are doors, running between the light and the switches. If we chose to employ high side switching it would have taken twice as many—wires from the battery to the switches, and another set of wires up to the light. Pennies count in the design of mass produced items.

In the other direction, you would usually find high side switching employed in controlling loads that are located some distance from their switch, again for reasons of economics. Tail lights, backup lights, and directionals are common examples. One wire from the switch out to the load; return via ground.

Now for the big caution: aftermarket devices! The directions that come with these frequently make the implicit assumption that your car uses high side switching most everywhere. You will often see instructions such as “Tap into the high beam wire.” That won’t work for us, as our Foresters (except the 2009-2013 SH series) use low side switching on the headlights. The so called ‘high beam wire’ comes off of the (electrically) low side of the high beam filament. It’s switched to ground to turn on the high beams, and when they’re off it still connects back through the bulb to the battery, offering an intriguing ‘sneak path’ or ‘back feed’ that electrons have been known to take advantage of under certain conditions.

So, any time you see simplistic directions like “Tap into the x wire” on an aftermarket device, analyze the circuits first—the circuit you’re tapping into and the circuit that the aftermarket device appears to require. Check things out with a meter. Assure yourself that the ‘x wire’ behaves the way the aftermarket people figured it would (probably high side switched) before wiring things up that way. If you need to use a low side switched situation to control something else (such as driving lights), you can usually succeed by connecting a relay directly across the existing, switched device, so that the relay will close under whatever the conditions might be that turn on that device. No need to worry about which terminal is ground or where the source of power might be; if the existing wiring can turn on that device, it will also turn on your relay. We call this setup ‘floating’.

Don't succumb to the urge to double up on the use of whatever the low side switch might be, in order to control two separate things powered by two separate circuits. This can lead to all sorts of puzzling interactions. Invest a dollar in that relay or, under some low-current circumstances, use a couple of diodes to provide the required isolation.

Here are a few other threads that discuss specific aspects of this situation.


The thread Help! Driving Lights Won't Dip With High Beams walks you through an actual worked example of how low side switching confounded a member’s attempt to install aftermarket driving lights, and how we made it work. And here’s a similar one.

My thread What happens when a headlight fuse blows has a diagram that illustrates some of the anomalies that can be brought about by low side switching and mixed circuits. You can see why it’s outlawed by the National Electric Code.​


The “What's all this ... stuff, anyhow?” title of this post is used in homage to the late Bob Pease, MIT '61, a respected designer of analog integrated circuits and writer of many “What's all this ... stuff, anyhow?” articles for EDN Magazine. An engineer!
 

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Discussion Starter #6 (Edited)
Series and Parallel

I used these words loosely in previous posts, but they probably deserve a snippet of their very own, for those who are just getting into this.

Series
Two or more components are strung out like a daisy chain, the negative end of one connected to the positive end of the next in the chain. (Maybe elephants in the circus parade, connected trunk to tail, is an even better illustration.) Since current goes through a component, each one in the chain experiences the same current flow. Voltage, however, is shared across all the components in the chain, in proportion to the ratio of the component’s resistance to the sum of all the resistances in the chain. This circuit is sometimes called a Voltage Divider.​
Vc = V * Rc / (Sum of all R)​

Where Vc is the Voltage across a component C and Rc is its resistance, and V is the Voltage applied to the entire string.​

Series is something you use when you want to drive a number of lower voltage components from a higher voltage source.

Parallel
The positive ends of two or more components are all connected together, as are the negative ends. Since voltage goes across a component, all the parallel components see the same voltage. Current coming in one end splits, heading through each component in amounts inversely proportional to the individual resistances. The smaller the resistance, the greater the current.​
The current through each component is simply the common voltage divided by the component’s own resistance.​

Parallel is something you use when you want to drive a number of components of the same voltage rating from the same source.

Connecting things in parallel is easy, and something very common. Your car, your house, your neighborhood—they’re all just very large collections of things wired in parallel. All you have to do is be sure you have enough current available to drive all of them. A series connection requires careful analysis—lots of Ohm’s Law and the like—to address the current and voltage requirements of everything in the string. It gets especially complicated if something branches off part way through. Other than manufacturing some sort of custom assembly of LEDs, it’s probably not something you’d tend to do in a car application. You can read more about working with LEDs at this thread.

Here’s an example of both. I was stationed in Germany in the Army. Electricity there is 240 Volts. It was Christmas time and I wanted to put up a tree. But I already had American Christmas tree bulbs (the larger 120 volt type), and I didn’t want to invest in a 240 - 120 Volt transformer just for that purpose. So I bought a German plug and two American sockets, wired the two sockets in series, and connected the resulting series chain to the German plug. Then I plugged an identical string of American lights into each of the sockets. Since the resistance of each string was equal, they split the 240 Volts evenly between them, and both were happy.

However, I did have to be careful to be on the lookout for burned out bulbs. Each string was a set of parallel 120 Volt bulbs. So if a bulb burned out it would raise the effective resistance of its string, causing it to hog more of the applied 240 Volts. This would encourage other bulbs in that string to blow out sooner than the ones in the other string, further increasing the first string’s resistance and hence its voltage. The phenomenon is known as runaway!
 

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The Modfather
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Something to keep in mind when looking at the Service Manual wiring diagrams. Wire colour will not always be consistent with the manual, however pin position on the plug usually is. See attached.

Additional input from bbottomley: Good info, Peaty. I like to keep reference info tight, so with Peaty's implied permission I'll add this data about color codes right here in the same post.

You will see letter codes on every connection point in a Subaru wiring diagram, which indicate the (expected) color of the wire at that point. One code means a solid color; two mean a background color (first code) with a stripe on it (second code). Here are the codes:
L Blue
B Black (Means ground, most of the time anyway)
Y Yellow
G Green
R Red
W White
Br Brown
Lg Light green
Gr Gray
P Pink
Or Orange
Lb Light Blue
Sb Sky Blue (Appears to be same as Lb in SH models. Thanks to member Rustoz for the update.)
V Violet

SA Single interior conductor in a shielded cable
SB Outer shield in a shielded cable, usually grounded​

Note that these are Subaru-unique codes, not to be confused with colors in the National Electric Code, where black means HOT.
 

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Underhood Electrical and Water

Everything under the hood needs to be waterproof, watertight, or submersible.

Plastic electrical tape is unlikely to give you any water sealing. It might only be water-resistant.
Apply a sealing type tape or compound first, then wrap with tape.
Black rubber will fuse to everything and be difficult to remove if needed.
Gray silicone tape (3M #30?) will only fuse to itself and be easy to remove. Both will perform well.
Heavywall shrink tubing with sealant is best.

Waterproof means unaffected by the entry of water, - some fans, and some connectors.
Watertight means it keeps water out, - coil, and some connectors.
Submersible means it can be underwater/liquid, - fuel pump, and some connectors.
Some components could qualify for all three, or only one.

Water-resistant does not do much for you under the hood. It is only limits the entry of water. It is OK for things that have occasional exposure to water, and then get a chance to dry out, like tents.
Leather might be water-resistant, or even waterproof.
 

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Finding Shorts

Finding Shorts

Instead of blowing lots of fuses until a fire starts, use pins to make the fuseholder terminals accessible.
Do not let the pins contact each other. I made an extender from a blown fuse, solder, and wire.

Connect a 12V test light to the pins. If the light is full brightness, you still have a short.
This puts a lamp with plenty of resistance in series with the short circuit.
The lamp is an indicator and current limiter, and is replacing the fuse.

You can then disconnect stuff and observe the light.
 
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