Testing for Available Voltage

This lesson is about the easiest and most important electrical diagnostic test you can perform.  A voltage available test allows you to discover where voltage is present, how much voltage is present, and if your circuit is complete or not.

The first thing you will need is the proper tools for the job.  There are several options out there for you to pick from.  Each brand offers distinct advantages, and each tool type has it perks. 

If you are doing this for a career, I would highly recommend looking at the Recommended Tool List to make sure you have what you need.  For this test, I have provided a compiled list of tools that you could will need, a short description of each one, and a link to buying one on Amazon.  I have found that Amazon offers the best prices on about anything you may need for any project. They are listed from most useful in this situation to least useful.

The Power Probe 3 - This is basically a logic probe, a voltmeter, and a jumper built into one. It will allow you to read voltage, find grounds, find power sources, and even apply power/ground to the circuit with a switch.  Click the name to go to the article I have written about the Power Probe 3.  It is easily the best test probe available.

Voltmeter - A meter that allows you to read voltage. They come in different designs such as a analog voltmeter, digital voltmeter, and DVOM (digital volt and ohm meter).  This test requires only the voltmeter function, but a DVOM is the best. Click the link above for a cheaper model, but if you like the better brand check out the Fluke 115.

Test Light - Unfortunately, a test light can only tell you if you have a power source or a common ground. I do not recommend using a test light at all because it does not tell you an actual voltage measurement, but it is the cheapest testing method available.

Once you have picked up all of the necessary tools, you will need to understand a bit of basic knowledge about electricity. All terms can be defined in the Electrical Dictionary i have written.  Automobiles make use of DC voltage and mostly use a 12V system with the exception being hybrids.  All references in this guide are for the 12V system. There are several rules electricity must follow.  Electrons and protons are not like people - they can not break or bend rules.

Electricity always takes the easiest path to ground. If it gets the opportunity to go straight to its destination (ground is ALWAYS its destination) it will in a flash - literally.  If it goes straight to ground without first passing through a load, then it has directly shorted out.  Direct shorts are basically like putting a wrench across the battery terminals. If the circuit is fused, it will certainly blow the fuse.  If it has no fuse (like a wrench), it will melt the wire, cause a fire, or potentially blow up the battery.

A rule you should be familiar with.. DC voltage will always be the same throughout the whole connected part of a open circuit. Assuming the break in the open circuit is after the load, voltage measured after a load in a open circuit will still read source voltage.

Voltage read after a load in a closed circuit will always read less than original voltage.  How much less is determined by how many loads, type of circuit configuration, and how much resistance each load/ or wire has. Ohms Law will explain the rest of that to you.