Hummer H2. Manual - part 159

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

  

feed circuit if you see big shifts and repair as necessary.  

Note that circuits with external injector resistors will not be any different because the resistor does not affect 
open circuit voltage.  

Point "B" is where the driver completes the circuit to ground. This point of the waveform should be a clean 
square point straight down with no rounded edges. It is during this period that current saturation of the injector 
windings is taking place and the driver is heavily stressed. Weak drivers will distort this vertical line.  

Point "C" represents the voltage drop across the injector windings. Point "C" should come very close to the 
ground reference point, but not quite touch. This is because the driver has a small amount of inherent resistance. 
Any significant offset from ground is an indication of a resistance problem on the ground circuit that needs 
repaired. You might miss this fault if you do not use the negative battery post for your Lab Scope hook-up, so it 
is HIGHLY recommended that you use the battery as your hook-up.  

The points between "B" and "D" represent the time in milliseconds that the injector is being energized or held 
open. This line at Point "C" should remain flat. Any distortion or upward bend indicates a ground problem, 
short problem, or a weak driver. Alert readers will catch that this is exactly opposite of the current controlled 
type drivers (explained in the next section), because they bend upwards at this point.  

How come the difference? Because of the total circuit resistance. Voltage controlled driver circuits have a high 
resistance of 12+ ohms that slows the building of the magnetic field in the injector. Hence, no counter voltage is 
built up and the line remains flat.  

On the other hand, the current controlled driver circuit has low resistance which allows for a rapid magnetic 
field build-up. This causes a slight inductive rise (created by the effects of counter voltage) and hence, the 
upward bend. You should not see that here with voltage controlled circuits.  

Point "D" represents the electrical condition of the injector windings. The height of this voltage spike (inductive 
kick) is proportional to the number of windings and the current flow through them. The more current flow and 
greater number of windings, the more potential for a greater inductive kick. The opposite is also true. The less 
current flow or fewer windings means less inductive kick. Typically you should see a minimum 35 volts at the 
top of Point "D".  

If you do see approximately 35 volts, it is because a zener diode is used with the driver to clamp the voltage. 
Make sure the beginning top of the spike is squared off, indicating the zener dumped the remainder of the spike. 
If it is not squared, that indicates the spike is not strong enough to make the zener fully dump, meaning the 
injector has a weak winding.  

If a zener diode is not used in the computer, the spike from a good injector will be 60 or more volts.  

Point "E" brings us to a very interesting section. As you can see, the voltage dissipates back to supply value 
after the peak of the inductive kick. Notice the slight hump? This is actually the mechanical injector pintle 
closing. Recall that moving an iron core through a magnetic field will create a voltage surge. The pintle is the 
iron core here.  

This pintle hump at Point "E" should occur near the end of the downward slope, and not afterwards. If it does 

 

1998 Chevrolet Pickup C1500 

GENERAL INFORMATION Waveforms - Injector Pattern Tutorial