Current Ramping... What is that? Is it like off road biking?

Current ramping technology is by far the most powerful diagnostic strategy the a modern technician can employ. It is a strategy, but also involves the right equipment and not necessarily a big expense on the technician's side. See what it's all about...

 

THEORY OF OPERATION

Current ramping is one of the most powerful diagnostic techniques available to the modern automotive technician. It is by far the fastest and least intrusive way of assessing the working condition of an electronic circuit. It is also very unlikely, given enough knowledge and experience, to be confused or misguided by the results of a current ramping waveform analysis. Of course, as in any technical procedure, it also has certain drawbacks, which we will get into later.

Current is the amount of electrons flowing through a conductor (wire). An analogy can be drawn from a water pipe. The bigger the diameter of the pipe the more water that can flow through it. By the same token, the thicker (lower wire gauge number) the electrical wire the more electrons that can flow though it. It is virtually impossible to have a circuit or component with the correct current waveform and be shorted or open. Hence the inherent power of this technique. A shorted or open circuit (and anything in between) will draw more or less current than that needed. Such an excess or lack of current will show as a specific waveform trace on the oscilloscope. The ability to read these scope waveforms will speed up the diagnostic process and provide you with higher returns, due to the time saved.

In order to take advantage of current ramping, two pieces of equipment are needed. An oscilloscope (DSO) and a low/ high amperage clamp-on amp probe are a definite must. Current ramping takes advantage of the latest advances in electronic equipment technology. Only a few years ago, it was impossible to adapt these procedures for automotive use because of the lack of available and affordable equipment. The clamp-on amp probe is a device that converts an electromagnetic signal into a voltage signal that the scope can plot on the screen. It is important to know that all electrical/ electronic wiring have a magnetic field around it, whenever it is in operation. For example, a cranking starter has current flowing to it, so does a cooling fan, an ignition coil, a solenoid, etc. In all these cases, the magnetic field around the wires that go to such components is directly proportional to the amount of current flowing though them. In other words, a starter draws more current than a cooling fan. Therefore, the magnetic field picked up by the amp probe is also bigger and so is the voltage amplitude (height of the waveform). The amp probe converts the wireís magnetic field into a voltage output for the scope. By analyzing this waveforms, current ramping techniques can be applied to almost any electrical/ electronic device. The main concept to remember in current ramping is that the oscilloscope, through the current probe, is actually measuring the magnetic field around the wiring of the particular circuit you want to analyze. Current ramping will not pick-up voltage related issues with a particular circuit. This technique does exactly what it is called. It is a current measuring procedure which works by picking up the magnetic field around an electrical wire.

CURRENT RAMPING A FUEL PUMP

In essence, fuel pumps are electric motors. Electric motors work by flowing an electric current to the coil windings through a set of carbon contacts. These coil windings have a set of contact point called commutators. Every time the rotating coil windings rotate, the carbon contacts make a different connection, which actually shows up on the current waveform.

By analyzing this waveform, we can deduce a couple of details about a fuel pump motor. First, a determination has to be made as to the amount of commutators on an electric motor. It is virtually impossible to know such a detail on all the possible fuel pumps out on the market today. The technique to find the amount of commutators in a motor will be explained later on, but for now it is important to know that most fuel pumps have 8 commutators.

With such information, it is possible to determine the speed of the motor and by doing so, the condition of the fuel pump. By simply freezing the waveform and measuring the time it takes to make 8 current humps (8 commutators), all we have to do is divide 60, 000 by such figure. It takes 60 seconds to a minute and 1, 000 mS to a second. 60 sec * 1000 mS = 60,000 mS. There are 60, 000 mS in one minute, which is why we always divide by 60, 000. This technique can actually be applied to any electric motor. By knowing the rotational speed and current draw of a fuel pump motor, we can determine its condition. A faster than normal fuel pump, with low current draw, points to a lack of resistance in the fuel flow. A defective fuel pressure regulator letting too much fuel return back to the tank, a worn out pump impeller itself, a clogged suction filter sock, etc, can all lead to a fast spinning fuel pump. On the other hand a slow fuel pump with high current draw points to a restriction in the fuel lines. A clogged fuel filter, restricted fuel pressure regulator, etc, will slow down the fuel pump, since it has to push the fuel a lot harder.

In cases where the specific amount of commutators is not known, the use of fairly high screen definition scope is needed. By actually looking for a repeating pattern in the humping fuel pump waveform, the exact amount of commutators can be arrived at. Not all oscilloscopes have the high screen resolution needed for this technique and no matter which scope is used, it should always have the cursor measuring feature so as to measure time between the two cursor lines. Power graphing multi-meters can also be used so long as the specific amount of commutators is known, since they lack the screen definition to detect a waveform repeating pattern.

Whenever a current waveform is needed, the best and fastest place to get it is usually by jumping the fuel pump fuse with a wire and clamping on with the clamp-on amp probe, right at the jumper wire. Be aware that this fuse should only be feeding the fuel pump. If any other component is tied to this circuit, you will also be reading its current draw and the reading would be useless. The fuel pump relay is also a good place so long as it is readily accessible.

 

In the previous illustration, the repeating nature of a fuel pump waveform was shown. Once a repeating pattern is found, the time is measured between one set of repeating humps and the fuel pumps speed can be calculated. In the case above, the pump has 4 commutators. It takes 5.3mS for one complete revolution. By dividing 60, 000/5.3 mS a figure of 10714 RPM is arrived at. This is normal for a carbureted engine. It is important to note that speed specifications are virtually impossible to find for this test. Time and experience will dictate the success that you will enjoy with this technique.

NOTE: Always measure current to the fuel pump with the engine running. An engine not running has by average 12.6 volts while a running engine has around 14.3 volts. By taking a current reading with the engine off the reading will indicate a bad fuel pump when in fact itís not. This is because the lower voltage also means lower amperage hence weaker magnetic field around the fuel pump electrical feed wire.

IGNITION COIL CURRENT RAMPING WAVEFORM ANALYSIS

 

Current ramping techniques can be used to virtually analyze any electrical device. This diagnostic technique becomes even more powerful when checking ignition coils in order to find shorted coil elements, which are causing specific misfire codes. Given todayís coil-on-plug (COP) ignition systems, where it is virtually impossible to have access to the coilís primary or secondary, a current ramping analysis of the ignition coil is both fast and conclusive. By just jumping the particular fuse that feeds the ignition coils with a fused straight wire and using the clamp-on amp probe, a quick determination can be made as to the general health of the coil and spark output. A shorted ignition coil will show up as a fast vertical line on the coilís current waveform. Ignition coils need to reach saturation in a slow timely manner. A sudden vertical line in the waveform will surely point to a shorted or semi-shorted ignition coil.

CURRENT RAMPING AN INJECTOR

In the latest engine designs, it seems that tighter and tighter electronic component arrangement is the rule. This is another area in which current ramping can really shine. If access to the injector wiring is not possible, by simply jumping the main injector power feed fuse an injector current ramping waveform can be obtained. MPFI injectors operate at just under 1 amp of current. For this reason, a fairly good quality low-amp probe is needed if you are to look deep into the injector waveform. Such analysis, as when the injector pinttle is opening (to detect clogged injector) or an ECM injector driver failure are only achievable with a high quality DSO and low-amp probe. Clogged or stuck shut injectors are sometimes fairly hard to detect using a current waveform. And will usually only affect the mechanical part of the injector and not necessarily the interior electrical coil windings.

CURRENT RAMPING COMPRESSION TEST

Yet another useful current ramping technique is the current compression test. By using a high amperage probe and clamping around either battery cable it is possible to measure the starterís cranking amperage. The starter will draw less current if a particular cylinder lacks compression at cranking time. These starter cranking current variations are picked up and measured by the amp probe and are plotted on the scopeís screen. A quick current compression test will quickly (within a minute) identify an offending cylinder without the need of more time consuming compression gauge tests. This test will not do away with the compression gauge but will point directly to the bad cylinder. Afterwards all further test can be directed to that cylinder if necessary.

As you have seen in this article, current ramping techniques can be applied to fuel pumps, coils and starters with a high degree of success. This technique however could potentially be used with any electrical circuit like cooling fan motors, window motors, actuators and solenoids, etc. Use of your imagination and experience will lead you to a better and faster diagnosis.

copyright © by Mandy Concepcion, Automotive Diagnostics and Publishing

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copyright 2011 Mandy Concepcion, Automotive Diagnostics and Publishing