Volkswagen New Beetle. Manual - part 165

 

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Volkswagen New Beetle. Manual - part 165

 

 

2. Determine injector firing method for the complete 4 stroke cycle. Let's say this is a 360° 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° 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.  

Here is one example. Imagine a vehicle that has a faulty injector driver that occasionally skips an injector pulse. 
Every skipped pulse means that that cylinder does not fire, thus unburned O2 gets pushed into the exhaust and 
passes the O2 sensor. The O2 sensor indicates lean, so the computer fattens up the mixture to compensate for 
the supposed "lean" condition.  

A connected dwell/duty meter would see the fattened pulse width but would also see the skipped pulses. It 
would tally both and likely come back with a reading that indicated the "pulse width" was within specification 
because the rich mixture and missing pulses offset each other.  

This situation is not a far-fetched scenario. Some early GM 3800 engines were suffering from exactly this. The 
point is that a lack of detail could cause misdiagnosis.  

As you might have guessed, a lab scope would not miss this.  

RELATIONSHIP BETWEEN DWELL & DUTY CYCLE READINGS 

Dwell Meter 

(2)

Duty Cycle Meter 

1° 

1% 

15° 

25% 

30° 

50% 

45° 

75% 

60° 

100% 

(1)

These are just some examples for your understanding. It is okay to fill in the gaps.

 

1998 Chevrolet Pickup C1500 

GENERAL INFORMATION Waveforms - Injector Pattern Tutorial

  

THE TWO TYPES OF INJECTOR DRIVERS 

OVERVIEW 

There are two types of transistor driver circuits used to operate electric fuel injectors: voltage controlled and 
current controlled. The voltage controlled type is sometimes called a "saturated switch" driver, while the current 
controlled type is sometimes known as a "peak and hold" driver.  

The basic difference between the two is the total resistance of the injector circuit. Roughly speaking, if a 
particular leg in an injector circuit has total resistance of 12 or more ohms, a voltage control driver is used. If 
less than 12 ohms, a current control driver is used.  

It is a question of what is going to do the job of limiting the current flow in the injector circuit; the inherent 
"high" resistance in the injector circuit, or the transistor driver. Without some form of control, the current flow 
through the injector would cause the solenoid coil to overheat and result in a damaged injector.  

VOLTAGE CONTROLLED CIRCUIT ("SATURATED SWITCH") 

The voltage controlled driver inside the computer operates much like a simple switch because it does not need 
to worry about limiting current flow. Recall, this driver typically requires injector circuits with a total leg 
resistance of 12 or more ohms.  

The driver is either ON, closing/completing the circuit (eliminating the voltage-drop), or OFF, opening the 
circuit (causing a total voltage drop).  

Some manufacturers call it a "saturated switch" driver. This is because when switched ON, the driver allows the 
magnetic field in the injector to build to saturation. This is the same "saturation" property that you are familiar 
with for an ignition coil.  

There are two ways "high" resistance can be built into an injector circuit to limit current flow. One method uses 
an external solenoid resistor and a low resistance injector, while the other uses a high resistance injector without 
the solenoid resistor. See the left side of Fig. Fig. 1 .  

In terms of injection opening time, the external resistor voltage controlled circuit is somewhat faster than the 
voltage controlled high resistance injector circuit. The trend, however, seems to be moving toward use of this 
latter type of circuit due to its lower cost and reliability. The ECU can compensate for slower opening times by 
increasing injector pulse width accordingly.  

(2)

Dwell meter on the six-cylinder scale.

NOTE:

Never apply battery voltage directly across a low resistance injector. This will 
cause injector damage from solenoid coil overheating. 

 

1998 Chevrolet Pickup C1500 

GENERAL INFORMATION Waveforms - Injector Pattern Tutorial

  

Fig. 1: Injector Driver Types - Current and Voltage 

CURRENT CONTROLLED CIRCUIT ("PEAK & HOLD") 

The current controlled driver inside the computer is more complex than a voltage controlled driver because as 
the name implies, it has to limit current flow in addition to its ON-OFF switching function. Recall, this driver 
typically requires injector circuits with a total leg resistance of less than 12 ohms.  

Once the driver is turned ON, it will not limit current flow until enough time has passed for the injector pintle to 
open. This period is preset by the particular manufacturer/system based on the amount of current flow needed to 
open their injector. This is typically between two and six amps. Some manufacturers refer to this as the "peak" 
time, referring to the fact that current flow is allowed to "peak" (to open the injector).  

Once the injector pintle is open, the amp flow is considerably reduced for the rest of the pulse duration to 
protect the injector from overheating. This is okay because very little amperage is needed to hold the injector 
open, typically in the area of one amp or less. Some manufacturers refer to this as the "hold" time, meaning that 
just enough current is allowed through the circuit to "hold" the already-open injector open.  

There are a couple methods of reducing the current. The most common trims back the available voltage for the 
circuit, similar to turning down a light at home with a dimmer. 

 

1998 Chevrolet Pickup C1500 

GENERAL INFORMATION Waveforms - Injector Pattern Tutorial

  

The other method involves repeatedly cycling the circuit ON-OFF. It does this so fast that the magnetic field 
never collapses and the pintle stays open, but the current is still significantly reduced. See the right side of Fig. 
Fig. 1 for an illustration.  

The advantage to the current controlled driver circuit is the short time period from when the driver transistor 
goes ON to when the injector actually opens. This is a function of the speed with which current flow reaches its 
peak due to the low circuit resistance. Also, the injector closes faster when the driver turns OFF because of the 
lower holding current.  

THE TWO WAYS INJECTOR CIRCUITS ARE WIRED 

Like other circuits, injector circuits can be wired in one of two fundamental directions. The first method is to 
steadily power the injectors and have the computer driver switch the ground side of the circuit. Conversely, the 
injectors can be steadily grounded while the driver switches the power side of the circuit.  

There is no performance benefit to either method. Voltage controlled and current controlled drivers have been 
successfully implemented both ways.  

However, 95% percent of the systems are wired so the driver controls the ground side of the circuit. Only a 
handful of systems use the drivers on the power side of the circuit. Some examples of the latter are the 1970's 
Cadillac EFI system, early Jeep 4.0 EFI (Renix system), and Chrysler 1984-87 TBI.  

INTERPRETING INJECTOR WAVEFORMS 

INTERPRETING A VOLTAGE CONTROLLED PATTERN 

z

See Fig. 2 for pattern that the following text describes.  

Point "A" is where system voltage is supplied to the injector. A good hot run voltage is usually 13.5 or more 
volts. This point, commonly known as open circuit voltage, is critical because the injector will not get sufficient 
current saturation if there is a voltage shortfall. To obtain a good look at this precise point, you will need to shift 
your Lab Scope to five volts per division.  

You will find that some systems have slight voltage fluctuations here. This can occur if the injector feed wire is 
also used to power up other cycling components, like the ignition coil(s). Slight voltage fluctuations are normal 
and are no reason for concern. Major voltage fluctuations are a different story, however. Major voltage shifts on 
the injector feed line will create injector performance problems. Look for excessive resistance problems in the 

NOTE:

Never apply battery voltage directly across a low resistance injector. This will 
cause injector damage from solenoid coil overheating. 

NOTE:

Voltage controlled drivers are also known as "Saturated Switch" drivers. They 
typically require injector circuits with a total leg resistance of 12 ohms or more. 

NOTE:

This example is based on a constant power/switched ground circuit. 

 

1998 Chevrolet Pickup C1500 

GENERAL INFORMATION Waveforms - Injector Pattern Tutorial

  

 

 

 

 

 

 

 

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