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BASIC TRAINING
In recent issues, we have covered the K-Jetronic or
CIS fuel injection system in great detail. From the basic
version we've already discussed, K-Jetronic evolved
into a feedback system called K-Lambda. This article
discusses the basic operation of the Lambda version
of K-Jetronic.
Remember that everything you learned about basic
K still applies to the Lambda-equipped systems. You
may even want to review the K-Jetronic material in our
April and May issues before you dive into the Lambda
system.
Lambda Sensor
The Lambda story begins in the exhaust manifold
where the exhaust gas watchdog lives. The watchdog
is an oxygen sensor that sniffs the oxygen content of
the exhaust gases. This watchdog barks voltage signals
to a system control computer (the ECU) according to
the amount of oxygen he sniffs. Here are the ways the
watchdog reacts:
• RICHER MIXTURE -> LOWER OXYGEN - HIGHER
VOLTAGE SIGNAL TO THE ECU
• LEANER MIXTURE - HIGHER OXYGEN
LOWER VOLTAGE SIGNAL TO THE ECU.
-
Tricking The Dog
Any exhaust leak ahead of the oxygen sensor can
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drive the system RICH. A cracked EGR pipe, cracked
air injection manifold, or a leaking exhaust manifold
gasket not only lets exhaust gas leak out, but these
leaks, in turn, allow negative exhaust backpressure
pulses to draw in additional oxygen. Our watchdog
will assume that this intruding oxygen is a LEAN con
dition and he'll bark LEAN to the ECU. Then the ECU
will incorrectly RICHEN the mixture.
A defective spark plug wire or fouled spark plug
will fail to fire the air/fuel charge in a given cylinder.
This incomplete firing means that raw fuel (hydrocar
bons) and oxygen have not combined in combustion.
This means we have a lot of leftover oxygen and left
over hydrocarbons. Our watchdog will ignore the
hydrocarbons, but the extra oxygen will make the dog
howl that the mixture is LEAN! So the ECU will send
a RICH command to correct what it thinks is a LEAN
condition. Richening the mixture on an engine that's
already dumping unburned fuel could cause a
Chernobyl-style meltdown of the catalytic converter.
The presence of leaded fuel will put a muzzle on
our exhaust puppy. The oxygen sensor's voltage signal
will be much lower-than-normal or nonexistent,de
pending upon the amount of lead present. This causes
the ECU to issue a RICH command.
The exact opposite happens when silicone vapors
hit the sensor. The vapors turn our puppy into a mad
dog and the mad dog really howls. It sends a higher
voltage (RICH) signal that causes the ECU to LEAN the
mixture out too much.
A normal oxygen sensor also responds quickly to
changes in exhaust mixture. Just as a dog gets old and
may take longer to wake up to an intruder, the tailpipe
puppy may get lazy too. He barks loudly enough for
the ECU to hear him, he just doesn't bark often enough
to keep the mixture properly adjusted. On some
domestic feedback systems, the sensor's responsive
ness (how often the dog barks) is called cross-counts.
The Lambda system can't detect when the watch
dog gets lazy. You have to test the oxygen sensor with
your volt-ohmmeter (VOM) to be sure that the puppy
barks instantly when the need arises and keeps bark
ing often enough to alert the ECU.
Lambda Control Unit (ECU)
Measuring at the ECU lets you check both the sensor
and its wiring up to the ECU. Checking voltage down
near the sensor pigtail won't identify any unwanted
resistance (a broken wire or a dirty connection) be
tween the sensor and the ECU.
The oxygen sensor wire's connection at the ECU
can become a problem once corrosion sets in. The con
nections at the ECU must be clean, dry, and tight.
Many a computer has been unplugged and another
one plugged in to correct a problem. The R and R
process itself—not the new computer—cured the car.
That is, unplugging and plugging the connector wiped
the connections clean.
The ECU listens to the voltage message from the
tailpipe puppy and then makes a command to correct
the condition. Remember: command corrects condi-
I
Distributor (O
Fuel Inlet
Main Pressure Regulator
Fuel Return Line
This schematic shows how the frequency valve is tied into
the normal K-Jetronic fuel distributor plumbing. When the
ECU grounds one terminal of the frequency valve, the valve
opens. With the valve open, some of the lower chamber fuel
pressure vents back to the fuel tank.
tion. The command the ECU makes is always opposite
the signal sent by the oxygen sensor. If the exhaust is
LEAN, the ECU makes a RICH command. Therefore:
• LEAN EXHAUST causes a RICH COMMAND;
Every time the exhaust gas becomes RICH, the oxy
gen sensor sends a voltage signal greater than about
.5 volt to its master, the ECU. When the voltage signal
is less than about .5 volt, the signal means LEAN. Just
remember:
• LEAN = LOW VOLTAGE SIGNAL (less than .5 volt);
• RICH = HIGH VOLTAGE SIGNAL (as high as 1 volt).
The oxygen sensor signal usually enters the ECU at
pin connector number 2. This is where you want to
take your voltage measurements. Use a high-impe
dance voltmeter or you'll get inaccurate readings.
• RICH EXHAUST causes a LEAN COMMAND.
A part called the frequency valve carries out the
ECU's orders to richen or lean out the mixture. We*U
explain the frequency valve in a moment.
Open Loop/Closed Loop
When the engine is first started, the ECU allows
the fuel system to act like a non-Lambda system. This
is called open loop. In open loop, the ECU won't listen
to the oxygen sensor's signals. In fact, the ECU won't
listen to the exhaust watchdog until three things
happen:
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Here's a closer, more simplified look at the frequency
valve's effect on the fuel distributor. When the frequency
valve is closed, the lower chamber pressure is greater than
the upper chamber pressure. This greater pressure pushes
the diaphragm upward, restricting the fuel outlet port to
the injector.
1) A thermal switch in the coolant passage opens its
connection to ground. Breaking this connection to
ground tells the ECU that the engine's warm enough
to go into closed loop.
2) The oxygen sensor gets hot enough to begin
generating voltage signals. The watchdog has to wake
up before he can begin barking!
3) A certain amount of time passes by. Once the puppy
wakes up, the thermal switch opens, and a few minutes
go by, the Lambda system goes into the closed loop
phase of mixture control. Then the ECU takes the con
trols and steers the air/fuel mixture toward
stoichiometry.
The lambda system uses a feedback loop to closely
control the mixture during closed loop operation.
Feedback loops have been used for years in home
heating systems. The room thermostat senses the tem
perature of the room and sends a message to the fur
nace. Too cold? The thermostat calls for more warm
air. Too warm? The thermostat turns the furnace off
for a while. The process is repeated as necessary to
maintain the desired temperature.
The Lambda feedback loop, or closed loop, is
essentially the same. The oxygen sensor is the ther
mostat; the furnace's controller is the ECU. Instead of
controlling the amount of warm air produced, we're
controlling the amount of fuel that's injected.
Then what would typify an open loop system?
Think of the air conditioner from some 1960s'-vintage
cars. There was an ON button and an OFF button. The
air conditioner did the best it could do with only two
settings, but sometimes it got too cold. Other times,
it didn't cool well enough. This is exactly the way a
fuel system works in open loop. Sometimes it gives
too much fuel, other times it doesn't give enough.
Think about the home heating example again. Now
imagine that the house is old, and that all of the win
dow and door weatherstrips are shrunken, cracked,
and leaking. Ix>ts of cold air is leaking in. Even though
the furnace is running full blast on HOT, the room still
doesn't warm up. The heating system just can't pro
vide enough heat to overcome the cold drafts.
This is exactly like an engine with leaking injector
seals, leaking manifold gaskets, etc. The oxygen sen
sor is telling the ECU to richen the mixture. The ECU
is keeping the frequency valve turned on as much as
it can, but it still can't overcome all the air leaks. The
Lambda system is buried at its full-rich position (90
percent duty cycle), but the engine is still too lean.
The opposite happens when the leaks are fuel in
stead of air. Leaking injectors can bury the system at
its lean limit (15 percent duty cycle) and the engine
will still be too rich. These air/fuel problems are out
side the window or range of the ECU's control!
Frequency Valve
The frequency valve is nothing more than an elec
trically controlled injector writh a hose connected to
each end. One hose connects the frequency valve to
the- lower chamber of the fuel distributor. The other
hose connects it to the fuel tank return line.
Think about that K-Jetronic fuel distributor again.
The amount of fuel delivered to the injectors depends
upon the pressure imbalance between the upper and
lower fuel distributor chambers. Anything that
changes this pressure imbalance changes the amount
of fuel flowing to the injectors and out of the injectors.
If we use the frequency valve to bleed off some of
the pressure in the lower fuel distributor chamber, a
spring in the upper chamber pushes a diaphragm
downward. As the diaphragm flexes downward, it ex
poses more of the fuel distributor outlet ports and more
fuel flows to the injectors.
Unlike some other feedback fuel systems you may
have seen, the longer the frequency valve remains
open, the richer the Bosch Lambda system runs. The
greater the duty cycle (dwell) of the frequency valve,
the longer the frequency valve is open. The longer the
valve is open, the more lower-chamber fuel pressure
it vents off to the tank. The lower the lower chamber
pressure becomes, the more fuel sprays from the in
jectors. To sum up the results:
• HIGHER DUTY CYCLE results in a RICHER
MIXTURE;
• LOWER
MIXTURE.
DUTY
CYCLE
results
in
a
LEANER
Note that this is opposite the reaction on domestic
feedback fuel systems.
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Remember that the duty cycle is the on/off ratio of
the frequency valve. Battery voltage constantly feeds
one frequency valve terminal. The ECU pulses the
other terminal—the ground terminal. If you compare
this to a set of conventional ignition points, the ECU
acts like the points. That is, it operates a coil by mak
counterclockwise to lean the mixture. Remember to:
• turn the screw CLOCKWISE TO GO RICHER;
• turn the screw COUNTERCLOCKWISE TO GO
LEANER.
ing and breaking the coil's ground circuit.
In closed loop phase, the ECU controls fuel mix
ture by changing the dwell of the frequency valve.
Duty Cycle and Dwell
Think of conventional ignition theory for a mo
ment. Dwell is the number of degrees that the points
stay closed. The longer the points stay closed com
pared to total distributor rotation, the greater the dwell.
The points still open the same number of times per
engine revolution. But points-closed duration (also
called primary ignition "on" time) is what we measure
when we measure dwell.
The same holds true for the frequency valve. It
opens 10 times per second, but its duration—how long
it's open—is what changes. Frequency valve duration
is similar to primary ignition "on" time or points-
closed time.
A dwell meter is really a duration meter. It does
the same thing a duty-cycle meter does. As long as you
remember to put the dwrell meter on the 90-degree/
four-cylinder scale, it is interchangeable with the duty-
cycle meter. A 50/50 duty cycle reading on the 100 per
cent duty cycle meter equals a 45-degree reading on
the dwrell meter. This midrange setting is critical. This
is where we set the Lambda mixture adjustment when
we turn the mixture-adjusting screw.
To use a dwell meter on a Lambda system, just
remember that:
DUTY CYCLE x 90 percent = DWELL.
When the system is in open loop, the duty cycle
or the on time to off time of the frequency valve is
50/50. Once the system goes into closed loop, we want
the mixture to remain in that window or range of
50/50. That's why we adjust the mixture screw to keep
the on/off ratio at the 50/50 mark. By doing this, we
let the ECU have enough space in its mixture adjusting
range to properly control minor imperfections that oc
cur in the air/fuel ratio. If we connect the duty meter
to the system and read 65 percent duty, we know that
the ECU is holding the frequency valve open (ground
ing it) 65 percent of the time. The ECU is closing the
valve 35 percent of the time. Look back over what
you've already read so far. Can you see that the mix
ture is lean? Can you understand that the Lambda's
ECU is driving the system rich to compensate?
Let's assume that you don't find any mechanical
problems that are causing a lean condition. You can
remove the adjustment plug and turn the CO adjusting
screw clockwise to richen the mixture back to the
50-percent duty mark. If the meter was reading 40 per
cent and you didn't find any problems, turn the screw
When the frequency valve opens, it reduces pressure in the
lower chamber. Then the spring in the upper chamber
pushes the diaphragm downward and increases the open
ing at the fuel outlet port. This increases fuel flow to the
injector. The more fuel the injector receives, the richer the
mixture becomes.
CO Adjustments
To check the basic CO setting, we need to eliminate
the oxygen sensor and put the system into open loop.
Connect an infrared exhaust analyzer to the exhaust
test port. The engine oil must be warm, the idle speed
must be correct, the air injection must be disabled,
and all electrical accessories must be off. Don't forget
to disconnect and plug the charcoal canister purge
hose.
After correcting the idle speed, make your final ad
justment as you would on a non-Lambda K-Jetronic car.
Reconnect the oxygen sensor when you're done and
check for proper oxygen sensor response. If the specs
call for setting the duty cycle, connect a duty cycle
meter to the car's test connector. Or you can use
something such as Thexton's test harness P/N 391 to
tap right into the system at the frequency valve con
nector. Leave the oxygen sensor hooked up. If you're
using a duty cycle meter, look for a reading of 50 per
cent. With a dwell meter set to its 90 degree/fourcylinder scale, you'll want a 45 degree reading.
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Remember that once the ECU gets a signal from the oxygen sensor, the ECU makes a command to correct the mixture con
dition. If you follow the actions and reactions around this chart, you'll get a feeling for how a closed loop feedback fuel
system works.
If the meter reads greater than 60 percent, the mix
ture is set too lean. Turn the adjustment screw clock
wise to richen it. If the meter reads less than 40 per
cent, turn the screw counterclockwise. Remove the
3 mm adjusting tool, cover the access hole, and rev
the engine slightly between adjustments. The idle
speed will have to be right on the money for your ad
justments to work. These adjustments bring the mix
ture within the range that the ECU's capable of
controlling.
If the duty cycle or dwell reading is fixed and the
reading isn't oscillating around the 50/50 mark, the
system is stuck in open loop. The reading should fluc
tuate about 15-20 percent above and below the 50/50
mark. If it doesn't fluctuate—or if you can't adjust the
mixture—check the oxygen sensor and the thermal
switch. One or both of them is keeping the system in
open loop. If they both test okay, then there's a prob
lem in the wiring or in the ECU.
That covers the basic operation of the Lambda ver
sion of K-Jetronic fuel injection. For an in-depth look
at troubleshooting this system, tune in to next month's
Basic Training!
—By Dre Brungardt