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
wire’s 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 it’s 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 today’s coil-on-plug (COP)
ignition systems, where it is virtually impossible to have access to the
coil’s 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 coil’s 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 starter’s 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 scope’s 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
