Test Equipment and Meters
A word about the meter you use....
Be careful how you use meters and test equipment. What you measure is only as good as how you use your meter. Learning how to measure is important.
The two most common errors are:
1.) using the wrong connection points. Think about what you really want to know, and always read directly across what you want to know! For example, if you want to know battery voltage, read across or between the posts coming out of the battery. If you want to read voltage to the vehicle, read between the vehicle chassis and the positive wire you are concerned with. Don't read the battery posts and assume it is the same elsewhere, and don't read the vehicle wiring, or even between the battery post connectors that attach to the posts, and assume that is the battery voltage
2.) using the wrong meter setting or range. Use the closest range or scale to what you want to read. If your meter has 2, 5, 20, 50, and 200 volt scales, don't use the 50 volt scale to read 12 volts. Use the 20 volt scale. If you are reading the TPS, you should try to use the 5 volt scale.
One thing to avoid is neglecting to include important connections or leads where there might be voltage drops. The best place to read voltages is always directly across the terminals of the power source if you are evaluating something supplying power (like the condition of a battery or alternator), or directly across the load if you are measuring something that consumes power or uses power from somewhere else in the system. A headlight or starter uses power, they are loads on the electrical system.
Designers who put cars and electrical systems together usually understand voltage drops and how sensitive a system is to connection problems. They rarely (although they sometimes do) make wiring mistakes, and many subtle things they do are for a good reason. For example, the computer in a car has to accurately read some pretty low sensor voltages on some computer inputs. When a computer has to read a low voltage accurately, the sensor ground wire is only solidly grounded at one point! Sometimes there is a floating ground lead from the computer ground reference to the sensor, and the only ground is at the computer. At other times, the sensor is grounded and the computer port for that sensor floats from ground, and is grounded only at the sensor.
This is similar to what high-end low-level audio systems do. To avoid picking up hum and noise, low level audio lines in well-designed audio systems are grounded only at one point. If they are grounded at multiple points, alternator whine and other noises can creep in.
Buying a Test Meter
The best places to buy meters are large electronic wholesalers. You will almost always get a much better meter for less money than buying a meter at an auto parts supplier. A "professional" meter might cost over $150 from an electronics supplier like MCM Electronics. A very good mechanic's or hobbyist's meter generally is in the $40 to $100 range. MCM's Tenma brand meters are a very good value. Whatever you purchase, be sure to read the tolerance specifications. For $100 you can purchase a "clamp-on meter". A "clamp on meter" is not only suitable for measuring voltages, it will also measure starter and alternator currents up to 1000 amperes DC! You can test almost anything in the electrical system with a meter like that!
Keep in mind what you read is only within about 1% of what the meter says, unless you buy an exceptionally good (more expensive) meter. If you follow the guidelines below, and use the CORRECT voltages and measurement connection methods, a 1% meter is great. You don't care if a voltage is 1.01 volts or 0.99 volts. A 1-2% meter is generally far better than required for automotive use.