Foreign Service

There's more than one way to ensure that a voltmeter is adequately grounded. Sooner or later, a careless ground lead hookup will cause an inaccurate reading, which may lead to a costly misdiagnosis.

Back in my teens, one of my mentors joked that shortcuts weren't a problem unless they caused problems. This homespun advice has stuck with me and served me well since then. Trouble is, I sometimes learned the difference between risky shortcuts and proper procedure the hard way. I admit that I often copied things - good and bad - that older mechanics were doing.

Grounding a voltmeter incorrecdy is a common example of a potentially costly shortcut. This month, I'll explain why a good ground is vitally important and how easy it is to create an effective, reliable ground on any vehicle.

This topic may ring a bell with regular MOTOR readers; I cited it in my December 2010 column. There, I described how my buddy saw a grossly inaccurate reading on a Corolla vapor pressure sensor. He had grounded his voltmeter to the car body instead of to the battery or engine. Grounding it directly to the negative battery terminal solved the problem.

In truth, many technicians do the same thing: They connect the voltmeter's negative lead to the nearest metal part on the car. Sometimes they luck out because this point happens to be a suitable ground. But if it isn't, they may or may not think twice when they see strange voltage readings. They may relocate the meters ground lead and retest. However, some guys follow erroneous voltage measurements and run themselves in a circle - not to mention throw good parts after bad at the vehicle's symptoms. I learned a long time ago to plan my own luck by grounding the meter correctly the first time, every time.

Voltage is electrical potential or pressure. Many techs know that a voltmeter is an electrical pressure gauge. But they may not understand that it's also a subtraction calculator because it always shows the voltage difference between its two leads. For example, every tech has connected a voltmeter to a fully charged battery at one time or another. Suppose that battery's voltage is 12.60 volts at the positive terminal and 0 volts at the negative terminal. This means that a voltmeter connected to that battery will have 12.60 volts on one test lead, 0 volts on the other. The meter simply subtracts the voltage on one test lead from the other and displays the result. Therefore, the meter in this example subtracts 0 from 12.60 and displays 12.60 (12.60 volts less 0 volts equals 12.60).

Remember that ground, which measures 0 volts, should be the meter's reference point when you're performing basic voltage measurements. If it's not, then the meter can't measure voltage accurately.

Recently, some techs I met at a class I was teaching asked me about meter hookups. As I mentioned a moment ago, I routinely ground a voltmeter directly to the negative battery terminal. Mind you, this takes a wee bit more effort but it's proven to be well worth the trouble.

The challenge here is that the common voltmeter ground lead isn't nearly long enough to reach the battery. Solve this by fabricating or buying a lead that is long enough for the task. Whether you search on the internet or look locally, there seem to be oodles of sources for test leads. Here, I'll suggest two sources I'm familiar with. First, Waekon (wtvw.waekon.com) offers a kit containing coiled test leads that extend to 16 ft. or more. The coiled construction lets you stretch each test lead a little or a lot. Unlike a variety of leads that I've tried, these are built for the harsh shop environment. Look for kit number OTL01.

The second option is Automotive Electronics Services (www.aeswave. com). Not only does this company have a selection of extra-long test leads, but its technicians also make them to order. Once again, these test leads are made for the rigors of auto shop use.

Finally, you may be able to make your own grounding lead and do it very affordably. The Rube Goldberg setup shown in the photo on page 14 is just one example of the grounding leads I've made over the years. First, I recommend locating a large, strong alligator clamp. This enables you to quickly and securely grab a negative battery terminal, something you can't always do easily with puny alligator clips. Some electrical parts and equipment suppliers sell the same alligator clamps you find on battery chargers and jumper cables. But look around the shop or the junk bin; you may be able to pilfer usable clamps from discarded jumper cables or a broken battery charger.

Next, locate a suitable length (6 to 8 ft. or more) of wire and attach it securely to the big alligator clamp. Obviously, the longer the vehicles you service, the longer the grounding lead ought to be. I've used wire gauges in the range of 10 to 14 successfully on these grounding leads. Technically, these choices may be overkill but the cost of this scrap wire was always unbeatable - free! The 14-gauge stuff is more maneuverable in some test situations.

Then fabricate a reliable attaching point on the other end of your grounding lead. Some guys solder a standard male banana terminal onto the grounding lead and plug it directly into the voltmeter. Other techs solder a female banana terminal onto the long lead and then plug a common voltmeter lead into it. My preference has been to carefully loop the end of the grounding lead and solder it so it can't loosen up or unravel. Any of the common alligator clips you find in a digital voltmeter kit can securely grip this little soldered loop.

Last but not least, I've never attempted to bundle or wrap up these grounding leads. Instead, I put them on a pegboard hook and let them hang there. This simple step eliminates or minimizes the need to unroll, unwrap or straighten the grounding lead when you need it.

There's more than one way to ensure that a voltmeter is adequately grounded. Until now, you may have lucked out grounding the meter anywhere you want. But experience shows that sooner or later, a careless hookup will cause a costly misdiagnosis. A safe, secure ground is always less hassle in the long run.

[Sidebar]

This is the latest extra long grounding lead I made. I found the big alligator clamp and the length of white, 12-gauge wire in someone's electrical scrap bin. (Can't beat that price!) I looped and soldered the end of the grounding lead (north of the clamp) so It's easy to secure the common, smaller alligator clips to it. I also added a separate ground lead here with a banana terminal (lower left) in case I need to use it for a separate meter or scope hookup on the car.

[Sidebar]

I've always preferred a larger alligator clamp for the business end of my grounding lead. It's easier to connect quickly- and most of all, securely-to a negative battery terminal.

[Author Affiliation]

Dan Marinucci

dmarinucci@motor.com