DIAGNOSTICS - helpful hints:
The most important aspect of diagnosing what is wrong with a modern vehicle
is to accurately determine WHAT IS ACTUALLY WRONG. You must induce the
fault so that you can determine what is wrong. If you cannot do that
then you are JUST GUESSING. The chances are that you will be wrong and
a large bill will ensue and an unhappy owner. To determine what is actually
wrong you need develop some simple but accurate test procedures - too
many repeat tests because they don't trust the test. A basic understanding
of the principals of how an engine works is important. Appreciate that
it does not matter the brand, or model or engine size, they all work
on the same principal - suck, puff, bang, blow. They all need spark in
sufficient quantity and at the right time. They all need fuel in the
combustion chamber in the correct amount. They all need compression,
an inlet manifold that is not falling off and a camshaft that is turning
at the right time. The following checklist may help:
The majority of NO GO problems will have nothing to do with the computer,
but more to do with the condition of the engine. Don't overlook things
like spark plugs, leads, cap & rotor button, battery earth straps, inlet manifolds loose,
EGR valves jammed open, lack of oil, lack of service, etc.
Keep it simple - if no spark, always check that the distributor shaft is
turning - the cam belt hasn't broken.
To check spark take a spark plug lead off any spark plug and devise a way
(be aware spark generated in the modern system can hurt and even kill in
certain circumstances) where the metal end of the lead is approx. 10mm from
an engine earth (metal manifold or block). Every engine should develop at
least 20Kv (10mm in atmosphere) at the plug lead or at least 30Kv (22mm in
atmosphere) from the coil lead. A normal spark plug uses 4Kv in atmosphere
- not a good test.
If you have spark then have you sufficient fuel? After a few moments cranking,
remove the spark plug and it should be damp. Saturated indicates it is flooding.
If dry, then check to see if any petrol comes out the return line during
cranking - you should have approx. 1 litre per minute normally, or half that
If you've got spark and fuel and it won't go, then it is mechanical. Don't
forget the exhaust - a blocked exhaust or warn valve guides (lifters pump-up)
will stop an engine. During cranking, expect 2-3"Hg of steady vacuum and
steady vacuum for a 4 cylinder. An initial peak of vacuum then a drop back
to atmosphere during cranking is indicative of a blocked exhaust.
Inadequate mixture or a faulty idle speed device can cause a surging idle.
Appreciate the oxygen sensor may be confusing the ECU because it is reacting
too slowly - disconnecting can prove otherwise. Similarly, disconnecting
the idle speed device will prove - don't reconnect idle device with ignition
on - danger of spikes.
For intermittent problems, only test when the fault condition is happening
or induced. Otherwise you will be wasting your time.
Don't assume it is the computer causing the problem - 70% of problems are
mechanical, nothing to do with the computer.
SOME DO'S & DON'TS WHEN WORKING WITH
Don't start or run the engine with the battery disconnected - spike the
ECU. When jump-starting another engine don't hook the battery cables up
back to front - it will certainly destroy the ECU.
When jump-starting avoid overcharging damage when removing the battery
cables too quickly after start-up.
When doing compression test, disable the fuel pump or injectors to avoid
flooding. Only use a digital multimeter with high impedance - you get what
you pay for. Don't remove harness plug from ECU without first turning ignition
Don't remove sensor input harness plugs (except injectors) with engine
running unless it is a protected circuit.
Avoid high electromagnetic fields & RF signals anywhere near ECU - from
electric welders particularly - remove ECU.
Don't substitute voltage to a sensor without first knowing what the correct
voltage is - some operate on 2v, 5v, 8v, 10v and battery.
- the basics.
The purpose of electronic fuel injection (EFI) is to meter the correct
amount of atomised fuel at the back of the inlet valve. The computer
(ECU) that controls the operation is programmed to assume that the
engine is in NORMAL CONDITION, that there is SUFFICIENT FUEL at the
CORRECT PRESSURE & VOLUME and there is
SUFFICIENT SPARK at the CORRECT TIME. If the engine is badly worn
then any load signal to the ECU will reflect this lack of suck and will correspondingly
incorrectly calculate the amount of fuel required and the engine
will run poorly.
The injectors are controlled by the ECU in much the same way as a
coil. In other words, power from the ignition powers-up the injector,
which is energised to open when the ECU switches the -ve side of
the injector to ground. This happens within the ECU. You can expect
this to be around 2msec at idle. You will need an LED test light,
or multimeter reading duty cycle to measure whether or not the injector
is switching. The amount of fuel delivered to the engine is determined
by how long the ECU holds the injector open - referred to as "injector
open-time". That "open-time" is
calculated by the ECU from several inputs:
The TRIGGER signal - usually from the ignition source
will tell the ECU when to turn the injector on.
The LOAD signal from the air flow sensor or MAP
sensor. The ECU needs to know how much air is drawn in by the engine.
The incorrect signal will mean an incorrect injector open-time.
A jammed air flow meter flap, or backfire damaged air mass meter,
or a KV air flow sensor where the intake ducting has come loose,
or the vacuum line fallen off a MAP sensor will all have significant
affect on the ECU calculating the open-time. You should learn to
test each of the 4 different load sensors so that you can learn
what sort of output voltage or frequency to expect.
The COOLANT TEMPERATURE sensor. More fuel is required
for a cold engine (choke) than for hot engine. This sensor has
a significant affect on how the engine runs. Most common problem
here is when the thermostat fails and the engine does not run
at operating temperature and the ECU increases the open-time for
the colder temperature which is too rich for the conditions. Corrosion
of the harness plug will increase resistance and make the ECU
put more fuel in. The AIR TEMPERATURE sensor
has a similar role as the CTS, but to a lesser affect. The denser
the air the more fuel required and vice versa.
The THROTTLE POSITION SWITCH for acceleration
enrichment - when you put your foot down the ECU will need to
increase momentarily the injector open-time. Otherwise a lean
mixture with the rapid increase in air will lead to a flat spot
or hesitation. The ECU can also use a signal for wide-open throttle
during cranking to indicate the engine may be flooded and to
reduce injector open-time during cranking by say 70%.
The THROTTLE POSITION SWITCH for idle enrichment
- we need a slightly richer mixture at idle. The ECU can also
use this to shut down the injectors when it sees idle at say
over 2000rpm - called deceleration injector shut-off. The ECU
may also have an idle speed motor it needs to control when the
engine is at idle.
The THROTTLE POSITION SWITCH for cruise. The
ECU will need to know when cruise is happening so that it can
optimise the air fuel ratio.
The SYSTEM VOLTAGE. All
systems employ a compensation adjustment for the increase in
time it takes to fully open an injector when system voltage reduces.
Some manufacturers use system voltage compensation for cranking
enrichment - the bigger the fall in voltage during a cold cranking,
then the longer the injector open-time, compared to little or
no voltage drop when the engine is hot and spins over rapidly,
requiring very little fuel to start. A poor battery, corroded
earths, high current draw starter motor, faulty alternator, even
a slipping fan belt can all have significant affect on fuel consumption.
The OXYGEN SENSOR in the exhaust tells the
ECU what the mixture is. The ECU uses this during cruise, at
operating temperature and usually once the vehicle is over
65Kph to finely adjust the mixture. In more recent times the
ECU uses the information stored in its memory to bias injector
open time during idle, cold warm-up and in some cases for acceleration
enrichment. The oxygen sensor is playing an increasingly important
role in monitoring mixture under the increasingly stringent
The SPEED SENSOR is used by the ECU to control
transmission change points, for use with the oxygen sensor to
ensure emission standards are maintained, for use particularly
with front wheel drive vehicles to ensure smooth engine breaking
(read injector shut-off) in stop start city driving, etc. Most
vehicles now employ some engine shut down limit for high-speed
protection primarily of the vehicle components. Be aware some
manufacturer models (eg. Nissan Pintara TRX 2.4) will not allow
the engine to revved above 3000rpm after 30 seconds of not seeing
a speed signal - quite a problem when the speed fails.
ELECTRONIC IGNITION - the basics.
Appreciate that nothing has really changed since points ignition.
To generate a big spark to burn the fuel/air mixture in the
combustion chamber we switch the primary windings of the coil
to ground by an electronic switch (transistor) rather than
a mechanical switch (points). We can generate a bigger spark
today than before because we have variable dwell - the period
the points are switched to ground to saturate the coil primary
windings. We do that by programming a microprocessor (chip)
inside an ignition module, for example. You will note the modern
coil is smaller than the old oil filled type and has a very
low primary winding resistance usually 0.5 to 1.0 ohms. This
promotes faster saturation. Generally expect 10 to 20 degrees
of dwell at idle and approx. 40 degrees at 2000 rpm. We still
use roughly the same amount of timing as we used to - but we
can vary it far more accurately than before in reply to various
inputs (engine rpm, temperature, load, acceleration, etc.).
Whereas before we used bumps on the distributor shaft to move
the points, today we still need something to tell the transistor
when to switch the coil. That something will be a trigger device
connected to the engine crankshaft or camshaft. For distributed
systems (see below for distributorless ignition systems) it
will more than likely be inside the distributor just like before.
The difference is the trigger will be either a magnetic or
inductive pick-up, an LED (light emitting diode) or Hall effect
triggers (forgetting about the Lucas Opus system in V12 Jaguars).
WHAT GOES WRONG WITH ELECTRONIC IGNITION
NO TRIGGER - usually crank angle sensor.
COIL BEING SWITCHED BUT NO SPARK - coil failure.
INTERMITTENT MISFIRE - usually plug leads
from lean mixtures.
INTERMITTENT MISFIRE - ignition module not
switching cleanly - check earth.
INTERMITTENT MISFIRE - ignition module switch
NO SPARK HOT - ignition module failure or
SPARK ONLY DURING CRANK - module failure.
TESTING A MAGNETIC or INDUCTIVE TRIGGER TRIGGER
- identified by 2 wires and no power KOEO. Usually have resistance
between these 2 wires of between 200-300 ohms and up to 1000
for Bosch style systems. Sometimes a 3rd wire is used as an
earthing shield only. They generate an alternating current
(AC) by inducing a voltage when an iron stator (attached to
the distributor shaft) passes the magnetic pick-up (usually
fixed to the distributor base plate). Remove and hold in hand,
spin the shaft and measure the induced voltage between the
2 signal wires. Generally expect greater than 0.4v ac (for
most Toyota/Mazda systems you will see well in excess of 1.0vac).
If using a lab-scope, expect a peak of plus 1.5v, and minus
1v. The clearance between the stators & pick-up (usually factory
of 10-15 thou) limits the induced voltage - if you close down
the gap to within 2thou (sufficient to just see light between
stator & pick-up) the voltage will increase and may be sufficient
to solve your problem. These triggers are susceptible to heat
& vibration problems after 150Km.
WHAT GOES WRONG WITH INDUCTIVE TRIGGERS:
NO GO insufficient induced voltage - replace.
FAIL HOT - common for them to fail hot, especially
10 mins after shut down. Cooling will recover.
POLE OUT- intermittent failure/engine miss
as engine rpm rises because of worn bushes in distributor.
TESTING AN L.E.D. TRIGGER - identified by
having battery power & ground to drive the trigger, and usually
a 5v output from the computer is switched off & on (which the
computer will read as a rising or falling voltage to initiate
the trigger). Generally LED systems are used in a crank angle
sensor (CAS) which is usually a black plastic device inserted
into a distributor and containing 2 signal outputs resulting
in a total of 4 wires. Nissan & Isuzu are the main users of the
LED system. Basically a thin metal plate is attached to the distributor
shaft. This plate has slots cut in the surface to allow the LED
to shine through to a receiver which indicates top dead centre,
the number of cylinders or the position of the crank shaft. With
the female 4-pin harness plug removed from the sensor, test each
of the 4 wires of the female plug with KOEO. You should have
12v power (usually from an efi relay), 0v ground (from main earth),
and two 5v outputs from the ECU - if not then go and find out
why. Note; when the plug is attached to the CAS, back probing
these 2 outputs can give incorrect readings depending on the
location of the slots. Remove the distributor from the engine,
earth out the distributor body against the engine, and with the
harness plug reattached, back-probe the 2 signal voltages (the
5v signals). For each rotation of the distributor shaft, expect
one signal to switch off & on
equal to the number of cylinders and the other to either
switch once or 360 times. The switching should be a clean
5v on then 0.0v off. A lab-scope will confirm the quality
and shape of the signal outputs.
WHAT GOES WRONG WITH LED TRIGGERS:
NO GO will not trigger at least one LED
- fails old age - replace.
FAIL HOT - old age - replace
INTERMITTENT FAIL - from collapsed top bush
spraying metal particles (rust) and shorting LED out.
INTERMITTENT FAIL - water damage or corrosion
of wiring connections.
INTERMITTENT SWITCHING KOEO - LED has failed
- (coil fire/pump start) - replace.
INTERMITTENT FAIL AS RPM RISES - suspect
cheap CAS failing.
TESTING HALL EFFECT TRIGGER - for testing
purposes, treat this exactly the same as the LED system.
Commonly used by Mitsubishi, Mazda, Honda, Ford, Holden, etc.
they are identified by having battery power & ground to
drive the trigger, and usually a 5v output from the computer
or ignition module. Hall systems are used in a CAS but can
be identified by the "chopper" plate attached to the distributor
shaft. They usually contain 2 signal outputs resulting in a
total of 4 wires. Basically a thin metal "chopper" plate
is attached to the distributor shaft. As this plate passes
through the Hall cell a magnetic circuit is interrupted which
switches the 5v output off & on.
With the female 4-pin harness plug removed from the sensor, test
each of the 4 wires of the female plug with KOEO. You should
have 12v power (usually from an efi relay), 0v ground (from main
earth), and two 5v outputs from the ECU - if not then go and
find out why. Note; when the plug is attached to the CAS, back
probing these 2 outputs can give incorrect readings depending
on the location of the slots. Remove the distributor from the
engine, earth out the distributor body against the engine, and
back-probe the 2 signal voltages (the 5v signals). For each rotation
of the distributor shaft, expect one signal to switch off & on
equal to the number of cylinders and the other to switch once.
The switching should be a clean 5v on then 0.0v off. A lab-scope
will confirm the quality and shape of the signal outputs. Generally
Hall effect crank sensors have developed a reputation for reliability.
WHAT GOES WRONG WITH HALL SENSORS:
- WILL NOT TRIGGER - old age - replace.
- OLD AGE - replace.
- WORN SHAFT BUSH allows chopper plate to
short out Hall cell - can damage computer.
- HALL CELL collapses during "on" period
resulting in double triggering spark/injection.
DISTRIBUTORLESS IGNITION - the basics.
DIS is a recent development of "normal" distributor electronic
ignition in a full engine management computerised system. Rather than distribute
spark to individual cylinders from one or two coils, here we fire each spark
plug direct by its own individual coil (as in Saab 9000/Nissan N13 EXA) often
described as a "coil-over" system. Alternatively we may use one coil
to fire 2 companion spark plugs - referred to as Direct Fire Ignition (DFI)
as in the V6 Holden Commodore/AU Falcon. In both cases we don't have to fire
each coil more than once per engine revolution. That leaves an enormous amount
of time to saturate the coil, especially at higher engine rpm. That means
bigger dwell numbers, higher coil output to burn leaner mixtures - better
fuel efficiency & power. Coil output is typically doubled to 60,000v
to 100,000v - spark can now jump 10cm in atmosphere! Like conventional
system, we need a trigger (inductive, LED or Hall) usually
mounted on crank or camshaft. The trigger will signal either
an ignition module (as in V6 Commodore) or direct to the
ECU (as in Falcon EF/AU/BA). The module (or compute)r will
calculate when to fire (timing) with what dwell (depending
on the system) and switch each coil in the correct sequence
by switching the -ve side of each coil primary winding
to ground. Testing can therefore be treated in much the same
way as for a conventional ignition system.
WHAT GOES WRONG WITH DIS:
NO TRIGGER - usually crank angle sensor.
COIL BEING SWITCHED BUT NO SPARK - coil
INTERMITTENT MISFIRE - usually plug
leads from lean mixtures.
INTERMITTENT MISFIRE - ignition module
not switching cleanly - check earth.
INTERMITTENT MISFIRE - ignition module
NO SPARK - ignition module failure.
ENGINE MANAGEMENT SYSTEMS
The EMS is the combination of computer controlled ignition
timing and fuel injection in the one computer. EMS has been
around for many years - probably the first advanced EMS system
that Australia saw was in the 1986 Holden VL Commodore 6 cylinder.
Working on EMS is easier than fuel only or ignition only systems
because an input sensor fault will show up on both fuel delivery
and ignition timing.