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Discussion Starter #1 (Edited)
I'll start off with some very basic things here to hopefully take some of the mystery out of your cars electrical system. How to measure voltage and current in a car/truck using a multimeter. It doesn't matter if you are using a digital meter or an old style analog meter with a pointer the principles are the same. If you are using a cheap meter from some discount tools store the accuracy will not be as good, but for most of these general measurements even an ultra low cost meter will work.

Always, I repeat Always set your meter to the proper range and hook the leads to the correct plugs/jacks before taking any measurements.

When taking measurements you should have everything turned off (no light's, no ignition, no radio/stereo, no heater or A/C).

When you measure voltage or current in a vehicle it's not an exact science. Your battery voltage should be 12.6V. If it's 12.4, 12.5 or 12.7 volts that's perfectly acceptable. When you measure current the same hold true, a small variance is acceptable


Voltage
When sitting with everything turned off you battery voltage should be 12.6V. A voltage of under 12V indicates a bad cell on the battery.

When cranking the starter your battery voltage should be approximately 10V. A lower voltage indicated a discharged battery, charge the battery and retry. If after charging it still is below 10V, you probably have a bad battery. Option #2 take the battery/car and have a load test done, this is the best way to test a battery. Some discount auto parts stores will do this for free.

When your engine is running your battery voltage should be approximately 14V. Anything lower then 13.5V would indicate a problem with your charging system (generator, alternator or voltage regulator). For people with a one wire alternator, you will need to rev your engine for a second or two to over 2K RPM to excite the field on the alternator and start it generating voltage before taking this measurement.


Current
Better meters will be able to measure both AC and DC current, you always need to be sure and select ADC or Amps DC. Low end meters will only be able to measure DC (Direct Current) and that is what we have in an automobile. The average multimeter will only be able to measure a small amount of current, usually 10 Amps or less. An electrical short or large load could easily far surpass this and blow the fuse in the meter or even burn up the meter if it is not protected by a fuse.

When you measure current it is with everything turned off. You should see no current being drawn. However you may see 10 milliamps (0.01 amps) is 1/100 of an amp to 50 milliamps (0.05 amps) is 5/100's of an amp. These are very small amounts of current and are normal and if you have a aftermarket stereo/radio, MP3 player, clock, GPS or EFI connected you might see a little more. You could have a load up to 100 milliamps (0.10 amps) and this may be OK. If you have a load that draws more than 100 milliamps, start by pulling all the fuses from the fuse box and remeasure. If you still have a load drawing current, unhook the alternator and starter and remeasure. Same for aftermarket ignition systems, electricfans, HID headlights and relays to them, unhook and remeasure. Do this until you find what is causing your current draw/load.
Do not try and measure current this way with the engine running!


A few general words of caution:
Always unhook the negative side of a battery first. The reason for this is your car is a negative ground, so if your wrench slips and it touches a bracket or the body it will not spark and cause a fire or explosion. If you try and take the positive lead off first and short your wrench between the post and the body or engine bracket you will draw a nice big spark. Lead acid batteries when charging give off a small amount of hydrogen gas which is explosive.

In a pinch you can generally use a smaller value fuse or fuse from another location. Examples: You can probably use a 20A fuse in a 30A space for a short amount of time to get by. You can steal the radio fuse to get the wipers working. Never use a fuse with a high rating than what is called for by the circuit. And never, ever use aluminum foil or screw to replace a blown fuse.

I made a couple of diagrams to hopefully help you understand these few basic things.



 

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Good article, however you may want to post a caveat on Note#1. My friend who is not tech savvy, tried this but there was some missing ground(s) and the metal parts on his vehicle weren't all at true battery ground.....
 

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Food for thought -

Voltage is PRESSURE

Amps is FLOW, CURRENT, DUTY

Ohms is RESISTANCE TO FLOW, CURRENT, DUTY

Watts is volts multiplied by amps.
 
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