Forget all those name brand high-cost meters. I've been in tv / vcr repair for over 40 years, and have acquired over a dozen digital voltmeters, mostly because I misplace them and I'm always looking for one that doesn't have dead batteries. My first meter was a Simpson, the size of a shoe box, that cost $400.00. Next came three or four from Radio Shack for around 40 bucks each. I'm still using those after 25 years. They're about twice the size of a smart cell phone and do more than the Simpson. Next came similar meters from a mail order tv parts store for 20 bucks each. They still work fine but I can't find 'em right now. The latest ones I've gotten are the $5.00 specials from Harbor Freight Tools. One developed a problem where it reads the wrong voltage, but the other three or four work fine. They even have sockets for testing transistors, but I've never used them.
That brings up the next point. Don't waste your money on features you'll never use. The worst one by far is "auto-ranging". I've never owned one and never will. The first problem I found when using a friend's was it takes way too long for it to select the correct range. I like to check resistors, sometimes a few hundred at a time, before I sort them into various drawers. With each one you have to wait for the meter to try one scale, then another, and another, until it finds the one with the highest accuracy. That can take one or two seconds. That doesn't sound like a lot, but that is pretty painful when you're doing a lot of repetitive testing. It's much faster to sort the resistors into groups that all call for the same range on the meter, then test all of those at once.
The other problem is you can overlook the range the meter selected. There have been times when using another friend's high-class meter that I thought I found a wire to have 2 ohms when double-checking later it turned out to be 2 megohms, or in effect, an open circuit due to a corroded connector terminal. Sure, you can say it was my fault for not noticing the range the meter was on, but the bottom line is that mistake cost us a lot of valuable time, and it wouldn't have happened with a cheaper meter.
Digital voltmeters, (all voltmeters, actually), also have a rating called "ohms per volt". When you connect it to a circuit, it appears like a resistor to that circuit. That can affect how the circuit works in a lot of electronic and computer circuits, but for most automotive circuits you'll never notice that. You pay a lot extra for a meter that has a higher ohms-per-volt rating. That's something you don't need. Even the cheapy Harbor Freight meters will have a rating of at least 100,000 ohms per volt, and 1 megohm per volt is more typical. To measure 12 volts, you have to put the meter on the 20 volt scale. That means the meter will be the equivalent of inserting a 20 meg resistor, and for most automotive circuits that is in effect an open circuit. It's hard to find resistors much higher than 20 megohms.
The more expensive digital meters made specifically for mechanics will have a Hertz scale. There are some practical applications for that, but after specializing in automotive electrical for 16 years, and teaching it for nine years, I never needed or used that function. All digital meters will read AC voltage, but they're meant to measure a 60 hertz sine wave. Most computer signals on cars are square waves, and they're much higher in frequency. In fact, a meter made to respond to 60 Hz sine waves won't even respond to signals over 1000 Hz, and certainly not a car's computer signals that are in the megahertz range.
The types of places you'd measure frequency, (hertz), is for an anti-lock brake wheel speed sensor, or, ... As Ford is always different than everyone else, their mass air flow sensors. Both of those readings are irrelevant. If you're testing a wheel speed sensor, you're interested in whether you have something or nothing. No one is concerned with the exact frequency because you'd have to spin the other wheels by hand at exactly the same speed at the same time to compare them. Instead, we connect a scanner and view all four wheel speeds while driving the car. On Ford's mass air flow sensors, you'll figure out from the scanner data and diagnostic fault codes if there's a problem. If you read the frequency with a meter and you get 152 hertz, is that good or bad? Typically, if that value is even listed in the service manual, it will say it's supposed to be between 130 and 160 hertz. In other words, do you have something or nothing? No two sensors will read exactly the same, and it's up to the Engine Computer to learn the characteristics of every sensor. The bottom line is you're likely to never use the hertz scale, and you shouldn't waste your money on those kinds of features you won't use.
GM is famous for producing diagnostic equipment for their dealerships or for requiring their mechanics to buy specialized equipment, that helps them diagnose new circuits or systems that haven't been seen before. With just a little experience, the mechanics throw that stuff away or never use it again. You'll find tons of that equipment at auctions when dealerships go out of business, but no one buys it because it's out-dated so soon. Digital meters with a hertz scale was one of those requirements, but in response to that perceived demand, some meter manufacturers still put it on their products. All other manufacturers provide a few hours of training on how those new circuits work and how to diagnose them rather than saddle their dealers and mechanics with the burden of buying stuff that will be obsolete in less than a year.
Lately I bought a pile of digital voltmeters on eBay for less than three dollars each. Obviously this isn't what you're looking for as they're just little gauges you can plant in your dash, but it shows how horribly inexpensive it is to build them. If you buy a Klein or Snapon meter, you can be sure 95 percent of what you paid is profit. You may get added features like a rubber shock-absorbing case or a carrying handle, but those don't help you get the job done or add to your productivity.
The things I've come to value are how the probes plug in and the power switch. On some meters you have to rotate the range selector knob from the "off" position to the range you want. I prefer an on / off switch so I can leave the meter set to the range I normally use. Most meters use probes with banana jacks for the plugs. Replacement probes are easy to find and are relatively inexpensive. I have had a little trouble with those that have a plastic tube molded around the plug. Not sure what the purpose of those tubes are or who came up with that idea, but you just have to be careful that plastic doesn't get folded over and push the plug out while you think you're taking measurements.
Also look at what the meter uses for an over-range indication. Most display a single "1" on the left, "OL", or the lower half of the digit "1". I've seen some where the display goes totally blank. To me, when both of my arms are up inside a dash board and I'm twisting to read the meter, that means the batteries are dead or I forgot to turn the meter on. A blank display for over-range is confusing.
Another thing to consider is most digital meters have scales in ranges of "2", meaning you select between the "2 volt" scale, the "20 volt" scale, or the "200 volt" scale. You'd use the 20 volt scale to read any voltage between 0 and 20 volts, obviously including 12 volts on cars, but what if you're working on a truck with a 24 volt electrical system? You'd have to switch to the 200 volt scale, and that means losing one decimal place of accuracy. All of my older Radio Shack meters use scales of "3". For 24 volt circuits, which are commonly found in tvs, I can use the 30 volt scale and retain that degree of accuracy after the decimal point.
Most meters are going to have an internal fuse for the lower current ranges. You want to be able to change that fuse by sliding off the battery cover. When you need to take a measurement right now, you don't want to have to fiddle around removing screws to take the back cover off to replace a fuse.
If possible, look at the display before you buy a meter to see if it's intuitive or if you have to struggle or squint to interpret it. My friend has a Snapon high-class meter with huge digits, but the "V", "AC", and ohm symbol are so tiny to need a magnifying glass to see them. By yourself that might not be a big deal since you know where you placed the range switch, but when you're working with another person, figuring out what the meter is telling you becomes a slow and tedious extra step. It's another one of many distractions that can make you lose your train of thought.
I like looking at digital meters to see what's new on the market, and for the most part, I've never been tempted to buy one from an auto parts store, hardware store, Sears, or tool truck because they don't represent a good value. They're way too over-priced. For the best features for the money, I would send a friend to Walmart, Radio Shack, or I'd look online on eBay or one of the electrical parts stores. I haven't bought tv parts in a number of years, but when I did, some of my favorite suppliers were Suburban Electronics, Dalbani, Parts Express, MCM, and All Electronics.
My 20-dollar meters came from Dalbani in the early '90s. I found one in my tool box that I had forgotten to turn off, and it sat there for at least two weeks without killing the batteries. That brings me to my last point. Most of what I have use four "AA" batteries which are very inexpensive. Try to avoid meters that use 9-volt transistor batteries unless you don't care about the cost of replacing the battery.
Thursday, October 9th, 2014 AT 4:30 PM