With all of this talk about measuring low current though, it almost seems like some technicians have forgotten about using a current probe to measure higher current circuits. The fact is that high current measurement is a very useful tool. Let’s examine some of the possibilities that it offers.

Figure 1
A typical high current probe with large jaws for clamping around large conductors.

To measure high current you will need a high current probe, sometimes called a high amp probe. Figure 1 shows a typical high current probe. This type of probe is built to measure high current levels, usually in the hundreds of amps. Low current probes on the other hand, are designed to measure extremely low levels of current, down to only a few milli-amps. In fact, low current probes usually aren’t even physically large enough to clamp around a high current conductor such as a battery cable. 

The current probe you select should also be of the type that is intended for use with a multi-meter or labscope. Some probes have their own digital numeric display and can’t be hooked up to a meter. These stand-alone probes are great tools but they can’t provide the same amount of information as a probe used with a labscope or graphing multi-meter. Selecting the right probe for the job is essential.

Both high and low current probes output a voltage in response to current flow through a conductor. Therefore, it must be connected to a voltage input on your meter. Some meters may have an input labeled AMPS. This is for an intrusive low current test, DO NOT connect a current probe to this input. The voltage output by the probe has a specific calibration. Most high current probes (but not all) use a calibration of 1mV per amp. This means that every milli-volt displayed on the meter represents one amp. By the way, most probes use a battery to generate their output voltage. Always be sure that this battery is fresh and strong.

What is Current?

Another consideration when measuring current flow is to be fully certain that you understand what it represents. If it’s not clear to you, think of current as the "volume" of electricity flowing through the circuit. This is similar to the volume of water flowing through a pipe. You’re probably very familiar with measuring voltage. Voltage can be thought of as the "pressure" of the electricity in the circuit. Using the liquid analogy again, voltage is similar to the pressure of water in a pipe. 

Knowing the circuit voltage and current flow, you can determine the amount of power the circuit is consuming. This value is usually expressed in Watts and can be calculated by multiplying the voltage (in volts) by the current flow (in Amps). This power directly relates to the amount of work that the circuit is performing. Since the voltage in most automotive circuits is relatively constant we can normally just look at current flow to get a good idea of the amount of work being done. This has some very powerful implications for automotive diagnosis.

OK, now that I’ve touched on what current flow represents and the tools needed to measure it, let’s look at some of the applications where current measurement is useful. To determine the applications, we must remember that we are focusing on high current levels. There aren’t too many high current circuits in today’s vehicles. In fact there are really only two: the starter system and the charging system. I suspect that we will encounter more in the future as electric hybrid and pure electric vehicles become more common. For now though, we’ll have to focus just on the starting and charging systems.

Charged up

For the first example, let’s take a look at how we can use a low current probe to test the charging system. In the past you’ve probably wheeled some sort of battery and charging system tester over to the car to do this. That’s no longer necessary in many cases if you own a lab scope or graphing multi-meter and a high current probe. Take a look at figure 2.

Figure 2
Charging system voltage and current.

The signal is a bit noisy, making it difficult to determine the exact current value but pinpoint accuracy isn’t really necessary in this case. To perform this test, the voltage leads were simply hooked to the battery and the current probe was placed around the negative battery cable. During the test, all of the lights, the stereo, and the A/C were turned on to created a load on the charging system.

Figure 2 was captured using the Snap-on Vantage Power graphing meter. It was a heck of a lot easier to use than wheeling over that big charging system tester. If you want to look even deeper into the charging system, you can use your meter to look at the alternator diode pattern. After all, a bad diode can cause serious problems. 

The easiest way to look at a diode pattern is to set your meter to AC input coupling then connect the leads to the alternator output and ground. After adjusting the time base and voltage level you should get a nice display of the diode pattern. I used the Snap-on Vantage for this article though and it doesn’t offer AC input coupling. To overcome this I used an AC filter from AES. This filter removes the DC portion of the signal and allows you to focus on the always-changing portion (and you thought AC only stood for alternating current). In addition, I decided to stick with the current probe theme and use my low current probe to look at the diode pattern.

Figure 3
Alternator Ripple

Starter Circuits

So what about starter current? No problem, the high current probe has it covered. I’m sure that most everyone is familiar with using a starting/charging analyzer to look at starter draw. You’ve probably found many bad starter motors and other problems this way. Older analyzers had an analog display that used a needle to indicate voltage and current while newer ones have a digital numeric display. Today’s labscopes and graphing meters though offer a whole new level of detail compared to traditional equipment when used for this type of testing.

Figure 4
Starter Current

If the current had remained excessively high or low, we would know that there is a problem with the circuit such as a bad starter or resistance from corrosion or loose connections, etc. Don’t forget that most lab scopes and graphing meters have 2 and sometimes even 4 measurement channels. You can easily measure and display the system voltage and current simultaneously.

There is one more useful piece of information that starter current provides that you may have never even realized. That information is relative compression. Think about it, every time a piston is forced up against compression, it takes work. Current represents work doesn’t it? Well, it stands to reason that if you have a way to observe current on a small time scale then you should see the effect that each cylinder’s compression stroke has on starter current while the engine is cranking. The current probe and labscope or graphing multi-meter and a high current probe are the tools that can do this.

Performing a relative compression test is easy. You simply select a time and voltage range on your meter that will give you the desired display, disable the engine from starting then crank. Figure 5 shows an example of a relative compression test from a GM V-6.

Figure 5
Relative Compression

What would you see on the display if one cylinder had low compression? Lower compression requires less work to overcome so there will be a lower current requirement for that cylinder. This should result in one of the peaks being shorter on the display. Figure 6 shows what one cylinder with low compression looks like.

Figure 6
Relative Compression with one bad cylinder

Relative compression testing is very useful. I remember working in a shop where the manager insisted that we perform a compression test whenever we did a "tune-up" (I hate the term tune-up by the way. What is there to "tune" any more?). This was a good idea except for the fact that he expected you to have the tune-up done in half an hour. Sometimes it takes that long just to remove one spark plug. The relative compression test is the perfect time saving solution for this sort of thing. It’s also useful when you suspect a weak cylinder but you aren’t sure which one it is. You can perform a relative compression test to isolate the offending cylinder then do a traditional compression test on it. These are only a couple applications, I’m sure that you can think of many more.

Triggernometry

So you can spot a weak cylinder but how do you no which one it is? Once again, labscopes and graphing meters make this easy by using an external trigger (Figure 7).

Figure 7
Two examples of external triggers

In figure 6, the Vantage was set to trigger off of the RPM pickup that was placed on the number one plug wire. This means that every time the screen is updated, the #1 cylinder is first on the screen. All you have to do is follow the firing order to determine the particular cylinder represented by each peak. 

Since the external trigger is placed on a plug wire, the ignition system must be functional while performing this test. In order to keep the car from starting then, the fuel system must be disabled. Disabling the fuel system also minimizes any dieseling effects that can cause invalid results when doing a relative compression test on a hot motor. 

External triggers can be used with just about any meter that has more than one input channel. They are usually available form the company that manufactured the meter and from the aftermarket as well. It’s a very useful accessory; I urge you to buy one and learn how to use it.

Managing it all

There’s one other not so obvious benefit to using your labscope or graphing meter for this type of testing That benefit is information. You’ll notice that I captured and saved the images from every test for this article. You can do the same using a PC and a screen capture program (figure 8).

Figure 8
The AES Wave Information Management Software

There’s one last thing that I need to mention. All of the examples shown here were captured using the Snap-on Vantage Power Graphing Meter. I chose to use it simply because I wanted to learn a little more about it, not because it’s the only tool that can do this sort of thing. The fact is, just about any labscope or graphing multi-meter can be used for this kind of testing. There is absolutely no reason not to use yours.

What’s the bottom line? That’s pretty much up to you. There’s no excuse for buying the powerful diagnostic equipment available today and then letting it collect dust in your toolbox. Take advantage of what’s out there. High current measurement is a very useful and powerful tool. It’s only a small part of the enormous potential that today’s diagnostic tools offer though; don’t be afraid to explore it all.

Written by Chuck Walker

Chuck is an applications specialist here at AES. He is an ASE certified Master Automobile Technician and holds a California Smog Check Technician license. He has extensive field experience solving tough driveability problems and diagnosing & repairing automotive electrical, electronic and computer systems.

Copyright 2000 by AES (Automotive Electronics Services)