Going With the Flow

Using a low amp clamp can add bullets to your diagnostic gun belt.

Today's cars are getting more and more electronic, with computer modules controlling engine management systems to automatically turning on the wipers at the first hint of rain. Diagnosing problems in these circuits can be involved, and the more testing techniques you can add to your diagnostic "gun belt," the better.

Basic electrical theory tells us that current flow is impacted by voltage and circuit resistance. If only there was a way to monitor and measure that current flow. Because current flow in a circuit is the same on both sides of the load, measuring current through the wires sure would be easier than trying to reach the individual components. I bet there would be a number of diagnostic uses for a tool that could do that. Wait a minute! There is such a tool. It's called a low amp current clamp, and its use can help isolate and diagnose a number of electrical circuit problems.

How It Works and Tips for Use

Current flow through a conductor creates a magnetic field around that conductor. A low amp clamp is designed to react to that magnetic field and produce a voltage signal that is either displayed directly on the tool or used as a source of signal for a conventional Digital Volt-Ohm Meter (DVOM) or Digital Storage Oscilloscope (DSO). Some current flows we want to monitor are very fast in their operation; ignition coils and injectors come immediately to mind. When the low amp clamp is paired with the speed of a modern scope, even these events can be tracked and examined with ease.

Before we dive into specific testing methods, let's take a look at some tips and techniques in the general use of this powerful tool, using a common component for low amp inspection: the fuel pump.

The low amp clamp is powered by a battery, typically 9 volts, and most have some type of indicator telling you the battery condition. Make sure the battery is healthy to ensure accurate results.
Make sure the jaw clamps fit completely around the wire to be tested. An open jaw will provide an inaccurate reading, if any.
If your test point is at the fuse, use a fused jumper when crossing the fuse terminals to protect the circuit being tested.
Most tools have a "zeroing" function to match the tool to the DVOM or DSO. Zero the reading before you activate the circuit being tested.
Your reading may be negative or positive, depending on the orientation of the clamp to the circuit flow. That's OK; if you want to change it, just turn the clamp around.
Each tool has a range of measuring capability and a maximum input rating. Do not exceed the rating or the clamp will be damaged. If trying out a new circuit measurement, start with the highest range first, then work your way down.
The clamp typically lists a conversion factor for each range. For example, the one used in this story has a conversion factor of 0.001v (1 millivolt) equals 0.010 amp (10 milliamps) on its low scale, and 0.001v equals 0.1 amp on its high scale. When taking your measurement, multiply the reading times the appropriate factor to read actual current flow in amps.

For better results with a DVOM, make sure the DVOM battery is in good condition and use the manual range function to select a three decimal place reading. Your zero reading should look like this: 0.000v.
The engine compartment, with a running engine, is a very noisy place electrically. Keep the clamp and its leads as far away from electro-magnetic field (EMF) sources as you can, especially when attached to a scope. (Note: Even the fluorescent lighting in your shop can bleed through a scope pattern. Know how your equipment responds to these EMF sources before condemning the pattern.)
When used with a scope that has a "glitch" feature, turning this feature off can clarify the pattern.
Want to "zoom in" on a pattern, but don't have a sensitive enough scope? No problem — use a wire jumper looped on itself and clamp your tool around it. Just remember that the readings you get will be increased by a factor corresponding to the number of turns in your homemade amplifier. For example, if your coil has five windings, the amperage reading on your tool will be five times more than actual.

Sometimes a scope pattern will move off the screen as you try to decrease your scope's voltage readings. That's due to the DC component of the signal. Use your scope's AC coupling feature to eliminate the DC and focus on the pattern.
Use Ohm's Law (Voltage = Amperage x Resistance) to check your current flow reading against the resistance specifications for the component you are testing. This conversion won't be exact, as there are other factors in play. However, a significantly lower reading than normal can indicate a voltage drop issue in the circuit, and point you to the need for more pinpoint testing. Current flow also indicates the circuit is complete — even if the component isn't working — and can help isolate a failed component.
Some circuits share power at a single fuse, making them a parallel circuit. Be aware that this type of circuit may impact your readings or scope patterns. Sometimes, like in an injector circuit, this can be a plus, allowing you to see all the injectors at once, and spotting weaknesses with a single test.

Like any other diagnostic tool, the more you use it, the more comfortable you'll become with it. Measure known good circuits and components for the vehicles you commonly service to build your own "reference library" and when that bad one comes in, you'll know it right away. So go grab that low amp clamp gathering dust in your toolbox and add a few "bullets" to your "gun belt."

Pete Meier is an ASE CMAT, member of iATN, and full-time tech in Tampa, Fla. His experience reaches back more than 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model. You can contact Meier directly at www.autoservicetech.com .