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Automotive Electrical Basics

After reading many of the excellent “Basic of Basics” topics authored and posted by “MartinSR” over in the body forum, and reading and replying to numerous posts in the electrical forum, I thought it was about time to write a “Basics” of my own.

I make no claims to being an “expert”, nor am I a professional automotive electrical specialist. To be frank, the newer cars that you have to “plug in” to diagnose are beyond what my old head can handle, but I feel I have a good knowledge of DC circuitry, and troubleshooting skills. So….

A lot of us without hesitation will completely disassemble an engine, transmission, or other intricate and precise mechanical device, repair it, and put it back together. But when it comes to why a lamp won’t light, or [multiple quote] “weird turn-signal, horn, name something” problems occur, a lot of us throw up hands in frustration. I hope this will help you to get an understanding of the elusive “Eddie Electron”, and his peculiar behavior. I’ll apologize in advance for being “too simplistic” but I write how I teach/explain. My descriptions will deal with DC circuitry, in negative ground situations.

In dealing with automotive electricals, the thing to remember is the word “circuit”. Note it’s similarity to the word “circle”! A complete DC electrical circuit MUST have the following components (at a minimum) to operate:
1) Source of Power (Battery)
2) A “To” Path (a way to get the electricity from the Source to the Load - wires.)
3) Load (a device that uses/consumes electricity – bulb, motor)
4) A “From” Path (a way to get spent electricity from the Load, back to the Source – Ground).

(It’s the same basics as internal combustion – Suck, Squeeze, Bang, Blow – if you have all 4, at the right time, it WILL run!)

Circuit 101:
In easy terms the positive/full “juice” leaves the battery (source), travels down the wire (“To” path), into the light bulb (load). The lamp uses the “juice” (load) and the negative/empty “juice” travels back (“From” path) to the battery (source) to be replenished. This makes a complete circle or circuit.
Any break or marginal connection in this circuit will result in something wrong or not working properly. It doesn’t matter what type of electrical circuit you are dealing with (lights, horn, stereo), they all have these 4 basic components.
A basic thing to remember is that BOTH paths must be of EQUAL capacity for the circuit to work properly! Example: The load asks for 150 boxes of “juice” to do its work, but has only 75 boxes worth of road (path) to send it back – the light won’t light – or it will find another road to send the boxes back (screwing up other things along the way). This brings us to the next topic, Grounding.

Ground, Grounds, or Grounding.
The cars we love/hate use a negative ground system. The negative side of the battery is attached by a cable to the body/frame of the car, and uses the frame, or steel/metal components of the car as a path back to the battery (source). The entire car becomes our “From” path back to the battery. This is why our wiring harnesses typically contain 1 wire for a given circuit.

Example: Positive juice leaves the battery or fuse block via a wire, it might go through a switch, or relay down another wire to a bulb, then….??? The base/case of the bulb is screwed into a socket, which is attached via metal contact, by bolts or friction to the metal of the car, which is attached to the negative side of the battery. A complete circuit! (An exception might be a lamp socket attached to plastic or other poor conductor – then a negative or “from” wire will be run to the nearest convenient metal object to complete the circuit.)

The problematic part of automotive electrics is usually the negative or Ground side of the circuit. It relies upon the mechanical contact between electrically conductive parts to do its job. Remember the “equal” example? If you don’t have good connections on the “from” side, the “to” side won’t work.

A good ground consists of clean metal to metal contact between parts all the way back to the negative post of the battery! This would include scraping off all paint or corrosion, putting a “star” washer (it bites into the metal) under the connection, and tightening.

Paint does not conduct electricity! Powder Coat does not conduct! Grease or Oil does not conduct! Rust/Corrosion does not conduct electricity! Twisting a bare wire around the nearest convenient screw and tightening it down does not make a good ground!

You may have spend zillions restoring your engine bay and don’t want to scratch the perfect satin black GM-correct paint, but if the ground wire/star washer from the headlight harness doesn’t bite hard into steel, expect problems!
The guys on the assembly line (and engineers) didn’t expect our objects of desire to last 30+ years. They designed and built the circuits for (guessing) a 10 year service life. “Joe 6-Pack” rams his 20,437th self-tapping screw of the week into the radiator support of the 489,783rd Chevelle that week. Back then the star washer did its job, bit into the steel and provided the conductivity needed for the circuit to work. After 30 years, billions of vibrations and years of corrosion, the connection may need attention. Remember – metal to metal contact!

On the “Positive” side
With a negative ground system, we rely upon wires to carry the positive ½ of a circuit to the “load”. The same connection practices apply here as well, but with a few precautions.

All of the “metal to metal” contact rules still apply! You must have good clean connections! The caveat is that the connection must be protected from accidental “grounding” or contact to the negative side by insulation – plastic, tape, etc. This is why the wiring or connectors are encased in rubber, plastic, or similarly protected from the ground or negative side (entire car) of the circuit.

A Break in the Action – Switches
Like the title says, that is all a switch is – a break in the circuit! A switch (in general terms) has two states – open or closed. A switch is inserted in a circuit to either enable or prevent a “load” from working. Switches can be inserted in either the positive or negative “path” of a circuit – it doesn’t matter – it still breaks the circuit! When closed current flows, when open the circuit is broken (remember the rules of a circuit). **Important** The mechanical contacts of a switch must able to equal or exceed the load of the circuit/conductors that it is inserted into!

Relays:
A relay is nothing more than a heavy duty “remote control” toggle switch. By activating a relay, you energize an electromagnetic coil that either engages or releases a set of heavy-duty contacts. You use a small device to turn a HUGE device on or off.
Example: The starting circuit. A starter solenoid is just a relay on steroids. The key switch in the dashboard or column couldn’t possibly handle the current needed to crank over your 11.5:1 BB! The switch in the column merely makes/breaks the circuit to a BIGGER heavy-duty switch that energizes the starter motor.

Fuses – Fusible links:
A fuse is a nothing more than a sacrificial element in an electrical circuit. It is a device that lives in line (like a switch, but without a choice) within a circuit, and when something goes wrong will self-destruct and save the other components. It is sized/rated for the maximum current a circuit is designed to handle (plus a bit of a cushion). If a fuse repeatedly blows, something is WRONG.


Troubleshooting:
This can get ugly. The first thing I can recommend is to take your “tried and true” probe/test light, find the biggest BFH in your toolbox, and release some anger with the hammer on the test light! Beat the darn thing into atoms with the BFH.
Now that your mind is clear, invest some bucks into a decent Digital Volt/Ohm meter. Find a unit that has a “beeper” for continuity, and a plus would be alligator clip adapters for the probes. Gift Ideas – ask for one.
Learn to use this thing! It’s a wonderful device, and it won’t blow up or smoke the circuits you’re testing the way a test-light will!

Attach the Blk/neg lead of your new Digital VOM to a good ground (re-read “good ground”) dial it to the DC Volts / 20 scale and start checking your circuit with the positive/Red probe! Nearly every circuit in our cars starts from “hot”, goes through switches or relays, to a “load”, then back to source via “ground”. Probe the +,Red, Pos. lead to each side of every device in the “chain” or circuit. It should read out between 11 and 14.00 Volts (depending on the car’s battery condition). When the meter goes OPEN, 0.0 or OL (it depends on the meter) you’ve found the problem – a break in the circuit – a defective or open part of the circuit.

The continuity “beeper” is a great tool as well. You can check fuses (that visually look good), or verifying the circuit path is intact. Example: De-energize the circuit, and set the meter to continuity/beep and touch the probes to each side of a device – if good it will beep. Follow the circuit path beeping your way along.

I invite criticisms or comments to this. This dialog is in response to the many pleas for help concerning electrical “gremlins” that have been posted here.
 

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Dude! You're not supposed to tell everyone! Just let 'em keep thinking it's all some sort of voodoo! ;)

Glad you took the time for that. That's a long one. Good stuff.
 

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Thanks alot! That was well worth the time it took you to write it! Even an electrically-challenged individual like myself can follow that explanation. Good work!
 

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I've been telling people for years...GET RID OF THAT TEST LIGHT AND BUY A DECENT VOM OR DIGITAL METER!! I bought my first VTVM in 1960 at the age of 13 and have never been without a meter....not in MY field!
 

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Excellent write up.

One issue that has drove me nuts in the past is the "good" connection problem. I was troubleshooting a problem and was in the process of tracing the 12 volts thru various connections and swithes.

Boy, I had 12 volts everywhere but the relay would'nt close. Thought I had a bad relay.

What I was not considering was that I had 12 volts but practically no amps. A fuseable link had burned almost all the way thru but retain one or two very small wires. SOOOOO, I could get 12 volts but the circuit could not carry enough amps to get the job done.

Maybe you could expand your discussion to include how to tell if your circuit is adequate, if you have a good ground etc (in laymen terms, you know, the terms I can understand)

Thanks again for a good write up.
 

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Nice write up! You explain things really well. You might want to add that when checking continuity, you should have the power source removed so you don't blow the internal fuse to the VOM. Also, when checking voltage, you usually see 0.0 volts or a very low mV reading and not "OPEN" or "OL". My meter shows OL only when checking continuity with an open circuit. I am just trying to help your already excellent write up. Please ignore or correct me if I'm wrong.
 

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as beancounter stated he had 12 volts everywhere but it wouldnt work..I think everyone should learn to use thier test light along with thier dvom...a voltmeter may show 12 volts on the circuit but it doesnt give any indication of the amps that the curcuit can carry. If you have 12 volts on your dvom and you touch your test light to it and it wont light and your volts drop to 0 that shows that the curcuit cant carry the load,due to high resistance.. I think everyone needs to keep this in mind as they are checking circuits...also learn to do a voltage drop test with your dvom to identify bad circuits....Steve
 

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Discussion Starter #9
The reasoning behind ditching the testlight is that it WILL blow up sensitive circuits - it's just too easy to grab it and start probing around on a newer vehicle and POOF! (cost me a lot of $$ for a Porsche F/I computer!) Just break the habit and learn how to use all the features of a VOM.
A VOM will show a problem with the circuit under load - you'll read a lower voltage after or at the output of the suspect device.
 

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Originally posted by beancounter68:
What I was not considering was that I had 12 volts but practically no amps. A fuseable link had burned almost all the way thru but retain one or two very small wires. SOOOOO, I could get 12 volts but the circuit could not carry enough amps to get the job done.
If you were measuring the voltage while trying to energize the relay then you should have seen the problem. This problem only occurs when you measure the wiring without the load connected.

You CAN NOT have a circuit that has the correct voltage but isn't producing the required current. If the supply wire is not capable of supplying the required current the voltage will drop when the load is applied.

Peter
 

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Actually a test light is a very handy tool to have. If you have a short to ground somewhere all you do is pop the fuse out and put the test light in its place then start pulling on connectors. Thats the only thing that I can think of right now where I would use it. I know that Ford says to use test light for this reason.
 

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Originally posted by LYTEMUP:
Actually a test light is a very handy tool to have. If you have a short to ground somewhere all you do is pop the fuse out and put the test light in its place then start pulling on connectors. Thats the only thing that I can think of right now where I would use it. I know that Ford says to use test light for this reason.
I carry my trusty DVM in my glove compartment. I also keep a cheap VOM in my small tool box in the trunk. Never without them...ever.
 

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Originally posted by LYTEMUP:
Actually a test light is a very handy tool to have. If you have a short to ground somewhere all you do is pop the fuse out and put the test light in its place then start pulling on connectors. Thats the only thing that I can think of right now where I would use it. I know that Ford says to use test light for this reason.
Part of the problem with the Ford idea is that it may not show something like a pinched wire. There may not be enough current to operate the light bulb or the light bulb may be so dim that you can't see it coming on.
The idea will work for a "dead short" or a "hard short" to ground. These are very low resistance, near 0 ohms, shorts.
If you have a something like a pinched wire or a relay that's partially shorted inside you have a higher resistance to ground. It's not 0 ohms but something higher.
The light won't come on 'cause it needs some current and a high enough voltage to work. The meter won't care.
 

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Discussion Starter #14
Thank You all for your comments!

I have made some edits to my original text, incorporating some of your suggestions.

Believe it or not electrical troubleshooting is something I enjoy! Electricity is a very black/white thing. The pioneers in the field (Ohm, Volta, Ampere, Watt, Farad) realized that this force is "controllable", and came up with the mathematical formulae that we use today.

When I get the "jones" for another "Basics", I'll put fingers to keyboard and do another.
 

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Originally posted by John D:
Believe it or not electrical troubleshooting is something I enjoy!
It's what I do for a living, and I love it. :D

I only wish I could teach it as well as others do (I've had to be a "teacher's aid" recently, and probably will have to teach it in the future).

You did a great job explaining auto electrics online. I look forward to your "next installment".
 

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I disagree with those who say not to use a test light. I have many test lights and I have wired Vat 40's to have a center tap test light on the A/B full field switch. There are test lights that have 2 wires positive and negative. The bulb is connected to ground on one side and through a resister to positive on the other side. This way the bulb is half bright when not connected or connected to an open circuit. When connected to 12v the light goes bright and when connected to 0v/gnd the light goes out. I have other lights that also use two wires but turn red when grounded and green when connected to juice. You can then choose to either juice or ground the circuit and there is also a seperate ground wire provided so you can both provide juice and ground. Combine all that with long 25 foot wires and you just can't beat that for basic troubleshooting. No it is not "computer save" but they make test lights that are. In all the years I have used my test lights I have never had ANY problems.

If you are doing a "wiggle test" to look for a short to ground I suggest either a headlight as a load across the fuse or a door buzzer. That way you can either see or hear when the short to ground goes away.

When tracing "short to grounds" I personally use my MAC 60A gauge in series with my remote starter switch to juice the circuit via a seperate dedicated juice wire with no fuse. Then I pulse the circuit for just a second and trace the current with an inductive amp meter or "short tracer" I have not found one of these circuit breakers that work satisfactorily for me. USE CAUTION.

Don't misunderstand this to say that a voltmeter is bad, they are not. All I am saying is that each has it's place. I had a diesel motor once that would not shut off when you turn the key off. The test light turned off the the motor kept running. Well there were some radios on the rail truck that were cross feeding and there was 0.1v on the fuel cut solenoid. That was enough to keep it activated and the test light did not show it.

Furthermore most of this talk is about 64-73 chevelles and you would be hard pressed to show an example where a test light would be bad here. I think everyone should have at least a basic test light in the glove box at all times.

Just my .02

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http://www.thexton.com/catalog/electric/elec_1.htm#No.%20121
http://www.thexton.com/catalog/diagnose/diag_2.htm#137
Full field adapter
http://www.thexton.com/catalog/charging/char_1.htm#No.%20304
 

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Discussion Starter #19
Chevellenut:
Thanks for the comments/suggestions! Yes, it's true that none of the circuits on the year of cars we deal with ('64 - '75ish) should be harmed with a common testlight. But, why not break a bad habit and use a better tool for the job? It's just too easy to forget and use a light on your daily and toast an expensive item. This is the point I was trying to get across - learning new procedures ;) .
 
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