Diagnostic Tips

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 during cranking.
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 4-6"Hg 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 ECU's

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 off.
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.

FUEL INJECTION - 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 emission laws.

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 problem.
NO SPARK HOT - ignition module failure or trigger failure.
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:

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.