Volkswagen New Beetle. Manual - part 164

 

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Volkswagen New Beetle. Manual - part 164

 

 

fully operate the noid light BUT NOT ENOUGH TO OPERATE THE INJECTOR.  

If this is not clear, picture a battery with a lot of corrosion on the terminals. Say there is enough corrosion that 
the starter motor will not operate; it only clicks. Now imagine turning on the headlights (with the ignition in the 
RUN position). You find they light normally and are fully bright. This is the same idea as noid light: There is a 
problem, but enough amp flow exists to operate the headlights ("noid light"), but not the starter motor 
("injector").  

How do you identify and avoid all these situations? By using the correct type of noid light. This requires that 
you understanding the types of injector circuits that your noid lights are designed for. There are three. They are: 

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Systems with a voltage controlled injector driver. Another way to say it: The noid light is designed for a 
circuit with a "high" resistance injector (generally 12 ohms or above).  

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Systems with a current controlled injector driver. Another way to say it: The noid light is designed for a 
circuit with a low resistance injector (generally less than 12 ohms) without an external injector resistor.  

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Systems with a voltage controlled injector driver and an external injector resistor. Another way of saying 
it: The noid light is designed for a circuit with a low resistance injector (generally less than 12 ohms) and 
an external injector resistor.  

If you are not sure which type of circuit your noid light is designed for, plug it into a known good car and check 
out the results. If it flashes normally during cranking, determine the circuit type by finding out injector 
resistance and if an external injector resistor is used. You now know enough to identify the type of injector 
circuit. Label the noid light appropriately.  

Next time you need to use a noid light for diagnosis, determine what type of injector circuit you are dealing 
with and select the appropriate noid light.  

Of course, if you suspect a no-pulse condition you could plug in any one whose connector fit without fear of 
misdiagnosis. This is because it is unimportant if the flashing light is dim or bright. It is only important that it 
flashes.  

In any cases of doubt regarding the use of a noid light, a lab scope will overcome all inherent weaknesses.  

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 

NOTE:

Some noid lights can meet both the second and third categories 
simultaneously. 

 

1998 Chevrolet Pickup C1500 

GENERAL INFORMATION Waveforms - Injector Pattern Tutorial

  

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 second, all measurements in between are 
averaged. Because a potential voltage drop is visible for such a small amount of time, it gets "averaged out", 
causing you to miss it.  

Only a DVOM that has a "min-max" function that checks EVERY MILLISECOND will catch this fault 
consistently (if used in that mode). The Fluke 87 among others has this capability.  

A "min-max" DVOM with a lower frequency of checking (100 millisecond) can miss the fault because it will 
probably check when the injector is not on. This is especially true with current controlled driver circuits. The 
Fluke 88, among others fall into this category.  

Outside of using a Fluke 87 (or equivalent) in the 1 mS "min-max" mode, the only way to catch a voltage drop 
fault is with a lab scope. You will be able to see a voltage drop as it happens.  

One final note. It is important to be aware that an injector circuit with a solenoid resistor will always show a 
voltage drop when the circuit is energized. This is somewhat obvious and normal; it is a designed-in voltage 
drop. What can be unexpected is what we already covered--a voltage drop disappears when the circuit is 

 

1998 Chevrolet Pickup C1500 

GENERAL INFORMATION Waveforms - Injector Pattern Tutorial

  

unloaded. The unloaded injector circuit will show normal battery voltage at the injector. Remember this and do 
not get confused.  

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:  

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They only work on voltage controlled injector drivers (e.g "Saturated Switch"), NOT on current 
controlled injector drivers (e.g. "Peak & Hold").  

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A few unusual conditions can cause inaccurate readings.  

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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 cause inaccurate readings. This is because of a 
DVOM's slow display rate; roughly two to five times a second. As we covered earlier, measurements in 
between display updates get averaged. So conditions like skipped injector pulses or intermittent long/short 
injector pulses tend to get "averaged out", which will cause you to miss important details.  

The last limitation is that varying engine speeds can result in inaccurate readings. This is caused by the quickly 
shifting injector on-time as the engine load varies, or the RPM moves from a state of acceleration to 
stabilization, or similar situations. It too is caused by the averaging of all measurements in between DVOM 
display periods. You can avoid this by checking on-time when there are no RPM or load changes.  

A lab scope allows you to overcome each one of these limitations.  

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.  

 

1998 Chevrolet Pickup C1500 

GENERAL INFORMATION Waveforms - Injector Pattern Tutorial

  

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° 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 everything together it does not miss anything (though this is 
also a severe weakness that we will look at later). If an injector has a fault where it occasionally skips a pulse, 
the meter registers it and the reading changes accordingly.  

Let's go back to figuring out dwell/duty readings by using injector on-time specification. This is not generally 
practical, but we will cover it for completeness. You NEED to know three things:  

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Injector mS on-time specification.  

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Engine RPM when specification is valid.  

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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° of crankshaft rotation, a bank-fire that injects every 720°, or an SFI that injects every 
720°. 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. 

 

1998 Chevrolet Pickup C1500 

GENERAL INFORMATION Waveforms - Injector Pattern Tutorial

  

 

 

 

 

 

 

 

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