When troubleshooting and repairing appliances you will always be measuring voltage, current, resistance, and temperature. The test instruments that are absolutely necessary for you to have from the very beginning are a Digital Multimeter (DMM), a clamp-on meter, and a remote reading thermocouple meter. Before I get into talking about each of those instruments, I want to advise you to but the best, the highest quality meters that your budget will permit. As an example, take DMMs, you can get one today for less than $10, or you can easily spend $300 or $400 for one. I do not recommend either extreme. A $10 meter may do the job at first, but it will fail you when you need it most. Those $300 to $400 meters will have bells and whistles that you will never use unless you get into prototype circuit engineering and designing.
A Digital Multimeter (DMM) versus an Analog Multimeter (VOM).
If your budget only permits purchasing one or the other, buy a quality DMM. I still have my old Simpson 260 Volt-Ohm-Miliammeter (VOM) that I bought back in the late 1960s in my shop but seldom use it except for testing motor starting and motor running capacitors. An analog meter does work better for testing these capacitors than any state of the art DMM. For anything else—measuring AC/DC voltages, resistance, and very small AC/DC current, nothing beats a good DMM.
Like most professionals, I have my favorites when it comes to tools and test equipment. One of my favorites and that of most professional electrician's is Klein Tools. The Klein Tools MM200 Auto Ranging Multimeter has a street value of around $60 but with a little shopping around they can be had for about $40.
This is an auto-ranging meter so you never have to worry about selecting the best range for measuring voltage, current or resistance. With the proper accessory probes you can even use this relatively inexpensive meter to accurately measure frequency, temperature and capacitance. Equip it with the right probe and this meter doubles as a remote reading thermocouple meter, although I prefer a dedicated thermocouple meter because I do not like tying my DMM up for extended temperature checking.
Digital Clamp-On Meter.
Modern clamp-on meters have many of the same features as a DMM, but where they become a necessity is in measuring the current being drawn by an appliance under test or the current being drawn by any electrical load. A DMM is limited to measuring currents of 10 Amperes or less whereas even a relatively inexpensive clamp-on can measure currents up to several hundred Amperes. The Klein Tools 600 Amp AC Digital Clamp Meter can measure current up to 600 Amperes and it has a street price of around $60.
The digital clamp-on meter excels in another way too when it comes to taking Amperage reading. With conventional meters, you have to cut into the circuit and connect the meter in series with one side of the line. With a clamp-on meter you do not have to connect physically in series with one side of the line, you simply clamp the jaws of the clamp-on meter around the unbroken conductor. The jaws of the conductor act as the core of a transformer and the magnetic field around the current carry conductor induces a voltage of the clamp-on pick-up coil. The current carrying conductor acts as the transformers primary winding and the clamp-on pick-up coil acts as the transformers secondary.
Remote Monitoring Thermocouple
An accurate, remote monitoring thermocouple thermometer becomes indispensable when servicing clothes dryer, electric ranges and built-in ovens. The problem was, until recently, those meter were inexpensive and inaccurate, or accurate and very expensive. The good meters were priced beyond what the average DIY electrician could afford. Then Cole-Palmer introduced their Remote Monitoring Thermocouple Thermometer with a street price of under $30. with most other thermocouple meters you could expect to pay that much for the thermocouple pick-up alone.
Now, for less than $30 you get everything that you need to start servicing conventional ovens and clothes like a pro. The Cole-Palmer comes with everything included—meter, thermocouple probe, and even the 9-Volt battery. The truth is that it does everything that my $450 thermocouple meter does and I do not have worry about it being damaged on the job.
Those are the three basic,must have test instruments. If you are serious about doing your own appliance repair and other electrical repairs around your home, you need to invest in these three test as soon as possible.