It helps to understand a little about electronics to work on and diagnose electrical problems on cars. It is much more important today than it was when I was a line mechanic. In every dealership and shop I worked in, it was evident that most mechanics and even more body men didn't even know the basics. The factory training classes we attended every so often covered a little but not enough to help. Every mechanic hated electrical problems for two reasons.
One was the way we were paid. We received 50% of the labor rate. Which was less on warranty work and you were paid on how long it took to fix the problem. Unless it took you an unusual long time to fix something. In that case the service manager usually had to get out his hopped up pen and do some fancy paper work to get the mechanic some pay. A line mechanic had to be able to beat the flat rate (the time the factory said it took to do a job) by about 50% to make up for rusted bolts and jobs that no one could even match the factory flat rate. So electrical work was dreaded.
Like most mechanics in the late 50s and 60s I also struggled with my share of these problems. Until one day I was working on a customer's car and talking to him. (Back in those days a customer could go back and talk to the mechanic) He owned one of the largest radio and TV repair shops in Jax and I was getting interested in electronics. I had been taking a home course from RCA on radio and TV repair and was at about lesson 11 of 44. (basic electronics) He invited me to stop by his shop so I took him up on that and I was impressed. It was a well lit shop with nice work benches around all of the walls and padded bar stools and lots of test equipment over each bench. And air conditioned. I was tired of getting covered in grease and skinned knuckles and hot all summer. I asked how much a radio and TV repairman could make and he said a man with two or three years experience could make about half of what I was making. Needless to say I stopped taking any more lessons. But what I did learn enabled me to breeze through most of the electrical problems on the cars I was working on. The trouble with that, was that I was finishing up my electrical problem cars quickly but still only got paid for the straight time. This did enable me to more quickly go back to get another car which may have a better flat rate that I could beat. This however got cut off because the service manager noted how quick I was getting the electrical problems fixed and he started loading all the electrical problems on me. Across from me was a new mechanic working on good jobs and making twice what I was making. I told the service manager that I didn't think I should be punished because I spent time at night studying electronics and all the good paying jobs were going to mechanics who didn't study and learn how to fix the electric problem cars. He agreed and started dividing up the electrical work again. I offered them my work books from RCA but none wanted to study.
I still can't fix a radio nor a TV but it did help me with my car electronics. So much that I was later able to design and build a test box to spot intermittent faults in a electronic injection ECM /ECU or any electrical component on a car. An intermittent fault is the hardest electrical problem any mechanic can get. It didn't take long for the car manufactures to start installing fault codes in their ECMs and with the government mandated code readers, My box stopped selling.
It doesn't matter which you believe, as to which way current moves in a conductor. The loose electrons move from one atom to the next or weather it is the hole that moves the other way. As long as you know that current always moves in a circle. All you need to do is to remember Ohms Law (E over I R see below) and half the work is done. Voltage is the force or push and current (amps) is the quantity that makes the trip. Resistors choke the flow. Diodes allow flow in one direction but not back. Transistors are like switches that allow or disallow current to flow by a trigger which can be voltage or a ground depending on what kind of transistor. A "load" is what you are trying to operate.
All generators and alternators only produce AC current (Alternating current). I see some eyebrows go up on that statement. A generator produces alternating current which is turned into pulsating DC current at the commutator and brushes. An alternator produces AC current and is changed to DC by a diode pack inside the alt. or a rectifier outside of the alt.
Amperage is 100% dependent on load. Voltage is dependent on the source or pusher. Everything is a conductor of electricity even nothing is a conductor of electricity. Metal is a good conductor so it does not take a high voltage to force current to pass through it. Rare metals conduct current better than most metals and aluminum will conduct current but is a relative poor conductor. Copper is mostly used for wire and other conductors.
Even though copper is a good conductor you can measure a resistance in a copper wire with a voltmeter. Apply a load to a long copper wire and a voltage power source, then use a volt meter to test a voltage drop by connecting the volt meter to each end of that wire when a load is applied. You will show a low voltage. The thinner the wire used the higher voltage you will note. This is because even copper wire has resistance.
Amps times Volts equals Watts. Watts is a measurement of power. A transformer transforms electric from high voltage and low amperage to high amperage and low voltage. Or a transformer can do the opposite. Like the transformer on the light pole near your house. Another transformer turns high amperage and low voltage to very high voltage and very low amperage like the ignition coil on your car.
Your house electric is AC (alternating current) It changes polarity (positive and negative) about 60 times a second. The electric in your car coil is DC (direct current) It always goes in the same direction. Your alternator produces AC current but you can't charge your battery with AC current so it is necessary to change the AC to DC.
When you make a conductor (wire) wrap in circles and apply power to it, an electromagnetic field is built up in the loops or coils. When you disconnect the power supply, the electromagnetic field is still there and looks for a place to go. If you have another coil of wire close but not in contact the field will transfer to the other coil of wire and produce a current for a moment in the other wire. These two coils of wires are called a "Primary" and a "Secondary". There can be more than one "Secondary" so a field can transfer to both "Secondary" coils.
Automotive Ignition coils can be powered by 12 volts or any voltage down to 6 volts and produce voltages in the secondary from 25,000 volts on up to 60,000 volts and higher. In an old pointed ignition (called Kettering Ignition) it was common to see a 12 volt coil produce about 35,000 volts in the secondary. As the voltage would build in the secondary the spark plug gap was in the circle of the secondary so the only way current could travel in that circle was to jump that gap in the plug. If the gap were close like .025 in, the secondary may jump that gap when it built to 25,000 volts. But if you change the gap to .040 in, then the field would have to build to maybe 35,000 volts before it could jump that wider gap. When you increase the pressure it becomes more difficult for the spark to jump the gap, so a higher compression engine requires a higher voltage in the secondary than a low compression engine. A rich fuel mixture makes it easier for the spark to jump the gap and a lean mixture makes it difficult to jump the gap. When the field in the coil transfers to the secondary windings, all of the field does not go. This leaves a charge in the primary and it too looks for a place to go. This charge can be as high as 300 volts and in a pointed ignition system that 300 volts will jump back across the points. To cut down on this current jumping back across the contact points a condenser (capacitor) is installed to bleed off much of this 300 volts. Electronic ignition systems have this built in.
When cars were forced to clean up their exhaust emissions in 1958, they leaned out the mixture which then forced them to use high energy coils to jump the plug gap. To add to this when they leaned out the fuel mixture there were not enough fuel molecules between the gap to keep a fire lit. So they were forced to widen the plug gap even further to light enough fuel to start a burn. With higher energy ignition they had to use better insulation on the plug wires and even better material in the distributor cap, rotor and wires. The long plug wires were a problem and a better system had to be devised so they came up with "Direct Ignition" where they eliminated the distributor and mounted coils directly over the plugs. They could put a coil over each plug and use a trigger from a pickup on the flywheel or use a trigger wheel on the front pulley. Some used a dual secondary and fired two plugs at a time. Even though no two cylinders were on TDC of the compression stroke at one time it didn't matter because they would fire one cylinder on the compression stroke and the other on the exhaust stroke and the next time they were reversed. This way they could use 4 coils on a V-8 engine.
Direct Ignition has several advantages over a distributor based ignition system. When a distributor is driven off of the crankshaft, there is a small amount of free play between the gears that drive that distributor. This free play means the actual firing of the spark can not be held steady at a specific time. Since max horse power is achieved at just before detonation, the set timing must be retarded slightly so as not to get too close to that detonation point. Detonation can quickly destroy a piston, especially at high RPM where the detonation is difficult to hear. A distributor that is driven off of a camshaft is even worse off because not only is there free play in it's drive gears but now you have the added free play in the cam drive. It is even worse when the ignition is pointed because of the free play in the distributor shaft which causes the points to open at a different time as the shaft moves around however slight. Also, on a distributor ignition the coil fires and that current must travel down a coil wire to a distributor cap, then transfer through a carbon or copper brush to a rotor and then jump a gap back to a post in the cap again and transfer to a plug wire and up the plug wire to a spark plug to jump another gap. Add to this the plug wires need to be resistance wire to help cut down the electronic pulse sent out to things like radios and computers in the car. Along with all the wearable parts a distributor system has, it does not make cense to have a distributor when direct ignition is available.
Most electric motors on cars are the type that use a permanent magnet as the field. Most of these types of motors will run in the opposite direction when the power and ground leads are reversed. This is especially useful to owners of the older British cars because they were originally positive ground. In the late 50s and 60s most of the MGs made were shipped to the US and since the US had everything negative ground like radios, many MG owners wanted to use alternate radios in their MG. In the dealership we were continuously asked to change the polarity of their MG so they could use negative ground equipment like radios, tape players and other accessories.
This was easy to do on the MGs before the mid 60s because all we had to do was polarize the generator to operate as a negative ground and reverse the coil leads. Later when the MG started installing electric tachs, we had to open up the tach and reverse a red and black lead inside the tach and we discovered that even though the heater fan seemed to work in either direction, the blades in the cage of the fan worked better in the correct direction so we started reversing those leads too.
As I stated before, Wattage is a measurement of power and most light bulbs are rated by wattage. When you install a bulb of different wattage, you change the load. If you go to a higher wattage bulb, you get a brighter light but it uses a higher amperage. Remember, amperage is 100% dependent on load. That wire that handled the lower amperage was ok, but now the higher Wattage bulb requires more amperage so now that wire is pushed to it's limit to handle the higher amperage. The switches and the fuse is also stressed. It was not noted as much of a problem on most of the MGs except where a high wattage amplifier for stereo radios or some two way radios were installed. Higher wattage headlight bulbs were being installed without much trouble. But the problems were noted with fuses and generator output at low RPM on cars that used the high wattage amplifiers etc. Another problem noted was the turn signals. The flasher unit that makes the lights flash, worked on a resistance of the bulbs and when the bulb wattage was changed it made the flasher change speed. A customer would complain that something was wrong with the car and didn't relate it to him putting in a higher wattage bulb in one side. Now his flasher would flash at a different speed on one side more than the other side and he didn't relate it to him installing a new bulb in one side.
I quit working on cars in the late 70s but did stay in touch with many mechanics while I worked as a tech line answer-man for a import car parts importer and distributor (WorldParts Corp.) I had complaints about the last MGBs from BLM that had decreased the size of the wires from 14 ga. This made it a problem when high wattage bulbs were used and in some cases even with the recommended bulbs. The resistance of the wire was so much as to make the bulbs burn dimly. Relays are used to correct problems like this. You can trigger a relay with a very low amp current and the contacts in the relay then make one or more connections. The connections can be high amp circuits. A starter relay is an example of this. A relay can be used to disconnect a circuit also. They also may have both and even several connections. A simple relay may just have 4 pins. A power for the small electro magnet, a ground for that magnet and two contacts. Most relay connections today are labeled 85 & 86 for the two magnet connections and 30 & 87 for the contacts. 30 is normally the power for the contact and 87 is the load (whatever you are operating). If you see a 5 pin relay and the numbers are 85, 86, 30, 87 & 87a, it is usually a relay that operates two items or two loads. An older British 4 pin relay may have different pin IDs. W1 and W2 will be the magnet and C1 and C2 will be the contacts. C1 the load and C2 the power. Relays can be used to take the high amp load off of a switch or even a wire. So a light duty switch can then operate a load that uses a very high amp load.
I used the version "E over I R" "E" being Electromotive force or "Voltage" and "I" being Amperage and "R" being "Resistance"
(R x I = E) (E / R = I) (E / I = R) [Resistance times Amperage = Voltage] [Voltage divided by Resistance = Amperage] and [Voltage divided by Amperage = Resistance]
With this, if you know two of them, you can find the third with this formula.
Keep in mind that resistance changes in a conductor with heat. This is used today in many electronic fuel injection systems to control the amount of fuel injected.
There is a lot more electronics in cars today than just a few years ago so the mechanic needs to be more electronics educated. Some cars today have computers that send signals to other computers to do a job and thus become more difficult to test. The car manufactures were racing down different roads with their on board computers and at first only the dealerships had the test equipment to diagnose problems. The government mandated that they will have a more uniform test connection. This enabled independent companies to manufacture scanners that could retrieve codes from the computer. With electric cars on the horizon there will be even more high tech electronic components in cars and the auto mechanic will need to be skilled even more in electronics.
Over the years working in dealerships I have worked out a procedure that worded for me. I had to follow rules that I had laid out for myself.
1. Never work on the electrical system of any car without a wiring diagram.
2. Never try to diagnose an electrical problem by symptoms.
3. Listen to a customer's complaint but delete everything except what item doesn't operate.
4. Try to never fall into the "Trap" (Oh! yea, I've had this happen before so I know what it is).
5. Never replace a part until you know what is wrong for sure.
6. Always TEST to come to a conclusion.
7. Always start testing at the "Load" (some mechanics start at the power source)
8. When several electrical problems are present, pick ONLY ONE circuit to test at a time.
9. Any time you have problems with heavy amp circuits (starter / Alternator) use "voltage drop tests on all cables (+) & (-).
10. When all else fails, call that lady in Haiti that knows Voodoo.