Volkswagen Corrado (1993 year). Manual - part 216

 

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Volkswagen Corrado (1993 year). Manual - part 216

 

 

WAVEFORMS - INJECTOR PATTERN TUTORIAL 

         OVERVIEW OF DVOM

         A DVOM is typically used to check injector resistance and
available voltage at the injector. Some techs also use it check
injector on-time either with a built-in feature or by using the
dwell/duty function.
         There are situations where the DVOM performs these checks
dependably, and other situations where it can deceive you. It is
important to be aware of these strengths and weaknesses. We will cover
the topics above in the following text.

         Checking Injector Resistance
         If a short in an injector coil winding is constant, an
ohmmeter will accurately identify the lower resistance. The same is
true with an open winding. Unfortunately, an intermittent short is an
exception. A faulty injector with an intermittent short will show
"good" if the ohmmeter cannot force the short to occur during testing.
         Alcohol in fuel typically causes an intermittent short,
happening only when the injector coil is hot and loaded by a current
high enough to jump the air gap between two bare windings or to break
down any oxides that may have formed between them.
         When you measure resistance with an ohmmeter, you are only
applying a small current of a few milliamps. This is nowhere near
enough to load the coil sufficiently to detect most problems. As a
result, most resistance checks identify intermittently shorted
injectors as being normal.
         There are two methods to get around this limitation. The
first is to purchase an tool that checks injector coil windings under
full load. The Kent-Moore J-39021 is such a tool, though there are
others. The Kent-Moore costs around $240 at the time of this writing
and works on many different manufacturer's systems.
         The second method is to use a lab scope. Remember, a lab
scope allows you to see the regular operation of a circuit in real
time. If an injector is having an short or intermittent short, the lab
scope will show it.

         Checking Available Voltage At the Injector
         Verifying a fuel injector has the proper voltage to operate
correctly is good diagnostic technique. Finding an open circuit on the
feed circuit like a broken wire or connector is an accurate check with
a DVOM. Unfortunately, finding an intermittent or excessive resistance
problem with a DVOM is unreliable.
         Let's explore this drawback. Remember that a voltage drop due
to excessive resistance will only occur when a circuit is operating?
Since the injector circuit is only operating for a few milliseconds at
a time, a DVOM will only see a potential fault for a few milliseconds.
The remaining 90+% of the time the unloaded injector circuit will show
normal battery voltage.
         Since DVOMs update their display roughly two to five times a

WAVEFORMS - INJECTOR PATTERN TUTORIAL 

         Checking Injector On-Time With Built-In Function
         Several DVOMs have a feature that allows them to measure
injector on-time (mS pulse width). While they are accurate and fast to
hookup, they have three limitations you should be aware of:

      *  They only work on voltage controlled injector drivers (e.g
         "Saturated Switch"), NOT on current controlled injector
          drivers (e.g. "Peak & Hold").
      *  A few unusual conditions can cause inaccurate readings.
      *  Varying engine speeds can result in inaccurate readings.

         Regarding the first limitation, DVOMs need a well-defined
injector pulse in order to determine when the injector turns ON and
OFF. Voltage controlled drivers provide this because of their simple
switch-like operation. They completely close the circuit for the
entire duration of the pulse. This is easy for the DVOM to interpret.
         The other type of driver, the current controlled type, start
off well by completely closing the circuit (until the injector pintle
opens), but then they throttle back the voltage/current for the
duration of the pulse. The DVOM understands the beginning of the pulse
but it cannot figure out the throttling action. In other words, it
cannot distinguish the throttling from an open circuit (de-energized)
condition.
         Yet current controlled injectors will still yield a
millisecond on-time reading on these DVOMs. You will find it is also
always the same, regardless of the operating conditions. This is
because it is only measuring the initial completely-closed circuit on-
time, which always takes the same amount of time (to lift the injector
pintle off its seat). So even though you get a reading, it is useless.
         The second limitation is that a few erratic conditions can

WAVEFORMS - INJECTOR PATTERN TUTORIAL 

         Checking Injector On-Time With Dwell Or Duty
         If no tool is available to directly measure injector
millisecond on-time measurement, some techs use a simple DVOM dwell or
duty cycle functions as a replacement.
         While this is an approach of last resort, it does provide
benefits. We will discuss the strengths and weaknesses in a moment,
but first we will look at how a duty cycle meter and dwell meter work.

         How A Duty Cycle Meter and Dwell Meter Work
         All readings are obtained by comparing how long something has
been OFF to how long it has been ON in a fixed time period. A dwell
meter and duty cycle meter actually come up with the same answers
using different scales. You can convert freely between them. See
RELATIONSHIP BETWEEN DWELL & DUTY CYCLE READINGS TABLE.
         The DVOM display updates roughly one time a second, although
some DVOMs can be a little faster or slower. All measurements during
this update period are tallied inside the DVOM as ON time or OFF time,
and then the total ratio is displayed as either a percentage (duty
cycle) or degrees (dwell meter).
         For example, let's say a DVOM had an update rate of exactly 1
second (1000 milliseconds). Let's also say that it has been
measuring/tallying an injector circuit that had been ON a total of 250
mS out of the 1000 mS. That is a ratio of one-quarter, which would be
displayed as 25% duty cycle or 15

ø

 dwell (six-cylinder scale). Note

that most duty cycle meters can reverse the readings by selecting the
positive or negative slope to trigger on. If this reading were
reversed, a duty cycle meter would display 75%.

         Strengths of Dwell/Duty Meter
         The obvious strength of a dwell/duty meter is that you can
compare injector on-time against a known-good reading. This is the
only practical way to use a dwell/duty meter, but requires you to have
known-good values to compare against.
         Another strength is that you can roughly convert injector mS
on-time into dwell reading with some computations.
         A final strength is that because the meter averages

WAVEFORMS - INJECTOR PATTERN TUTORIAL 

      *  Injector mS on-time specification.
      *  Engine RPM when specification is valid.
      *  How many times the injectors fire per crankshaft revolution.

         The first two are self-explanatory. The last one may require
some research into whether it is a bank-fire type that injects every
360

ø

 of crankshaft rotation, a bank-fire that injects every 720

ø

, or

an SFI that injects every 720

ø

. Many manufacturers do not release this

data so you may have to figure it out yourself with a frequency meter.
         Here are the four complete steps to convert millisecond on-
time:
         1) Determine the injector pulse width and RPM it was obtained
at. Let's say the specification is for one millisecond of on-time at a
hot idle of 600 RPM.
         2) Determine injector firing method for the complete 4 stroke
cycle. Let's say this is a 360

ø

 bank-fired, meaning an injector fires

each and every crankshaft revolution.
         3) Determine how many times the injector will fire at the
specified engine speed (600 RPM) in a fixed time period. We will use
100 milliseconds because it is easy to use.
         Six hundred crankshaft Revolutions Per Minute (RPM) divided
by 60 seconds equals 10 revolutions per second.
         Multiplying 10 times .100 yields one; the crankshaft turns
one time in 100 milliseconds. With exactly one crankshaft rotation in
100 milliseconds, we know that the injector fires exactly one time.
         4) Determine the ratio of injector on-time vs. off-time in
the fixed time period, then figure duty cycle and/or dwell. The
injector fires one time for a total of one millisecond in any given
100 millisecond period.
         One hundred minus one equals 99. We have a 99% duty cycle. If
we wanted to know the dwell (on 6 cylinder scale), multiple 99% times
.6; this equals 59.4

ø

 dwell.

         Weaknesses of Dwell/Duty Meter
         The weaknesses are significant. First, there is no one-to-one
correspondence to actual mS on-time. No manufacturer releases
dwell/duty data, and it is time-consuming to convert the mS on-time
readings. Besides, there can be a large degree of error because the
conversion forces you to assume that the injector(s) are always firing
at the same rate for the same period of time. This can be a dangerous
assumption.
         Second, all level of detail is lost in the averaging process.
This is the primary weakness. You cannot see the details you need to
make a confident diagnosis.

 

 

 

 

 

 

 

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