Distributor Less Ignition Systems 2o364g

A Brief Evolution of Ignition Systems Early automotive ignition systems relied entirely on mechanical components to properly supply an ignition voltage to a cylinder of multi cylinder engines. These were composed of a single coil and a mechanical power distribution device called a distributor. The distributor was made up of components which switched low and high voltages. These items are called the cap, rotor and points. The points were operated by a cam mounted on the side of the shaft of the distributor. The opening and closing of the points would charge and discharge the coil primary winding. The rotor, mounted on the end of the distributor shaft, and cap would conduct the high voltage output of the coil secondary to the appropriate spark plug. In essence it distributes the voltage to the cylinders. This system served its purpose well. However due to the mechanical nature of it, the parts were prone to wear and early failure. The points were subject to high flyback voltage from the primary when they opened and the magnetic field collapsed. The rotor and cap were subject to the high voltage produce on the secondary side of the coil. Since all of the parts operated with an air gap, arcing would occur and their conducting surfaces would burn and pit. This would degrade their performance and required regular maintenance. Additionally the mechanical timing advance parts in the distributor were not very accurate. A Step In The Right Direction

Transistorized ignition systems were first introduced in the mid to late 70's. These essentially replaced the breaker points in the distributor with a transistor for a coil driver and a sensor in the distributor for timing. This eliminated much of the headache of setting point gap and dwell angle. Although this was a significant improvement in reliability, the mechanical timing advance mechanism was still in place and the cap and rotor still required regular replacement for optimum performance. Distributorless Ignition Systems (DIS) were introduced in the early 90's. In a nutshell a DIS is a transistorized ignition system that uses a small computer to determine which plug to fire based on input it receives from various sensors mounted on the engine. These fully electronic systems utilize multiple ignition coils. At the correct moment of crank rotation the computer will switch a driver transistor which will in turn cause the correct coil to fire. Many DIS systems use what is called wasted spark, where one coil fires a pair of spark plugs that are opposite one another in the firing order. Many newer systems utilize a coil over plug strategy and have a separate coil for each spark plug. Systems utilizing a coil per plug strategy often mount the coil directly on top of the spark plug. In order to determine proper spark advance, sensors such as coolant temperature, air intake (MAP or MAF) and throttle position as well as crank speed are used. Most modern systems will also utilize a knock sensor. This is a piezo crystal device that can detect ignition knock much better that a human ear can. Engines equipped with knock sensors

can run at the very edge of knock safely. They can also make minor changes for fuel quality. Last but not least, fully electronic DIS also have special timing for when the engine is being cranked. DIS have eliminated almost all of the maintenance that was associated with an ignition system by doing away with the mechanical distributor. This eliminated the cap and rotor and the mechanical advance mechanism inside the distributor. (There were actually some transistorized, mechanical distributor ignition systems that did not have a mechanical advance mechanism. They relied on a computer to fire the coil. The distributor had a hall sensor that would signal the computer.) In many cases, there aren't any spark plug leads either. As a result, distributorless ignition systems are pretty reliable. With that said, it does not mean they are trouble-free. Failures can and do occur for various reasons. Knowing how to diagnose common DIS problems can save you a lot of time, money and aggravation.

Troubleshooting Ignition Problems Depending on the coil configuration, if the system uses plug leads simply disconnect a spark plug wire from a spark plug, crank the engine and check for spark. On coil over plug (direct fire) systems, there are no plug wires so you have to remove a coil and use a plug wire or an adapter tool to check for a spark. Alternatively, remove the spark plug or use an extra spark plug and insert it into the boot on the end of the coil. Then ground the plug to the engine. A good order to procede is: • • • • • • •

Check the fuse(s) Check power Check for spark Check plug leads Check coils Check ignition module coil outputs Check input sensors

To perform these tests a Volt Ohm meter is needed. These are also refered to as DVM (Digital Volt Meter) DMM (Digital Multi Meter) or VOM etc. Digital meters are prefered since they are generaly more accurate and have a high input impedance as compared to an analog meter. Analog meters do have their place so keep one on hand in the tool chest. If all the fuses and power have checked to be good, check for spark on one cylinder. If no spark is found, check another cylinder. No spark in all cylinders could indicate a failed

DIS control module or crankshaft position sensor. A car equipped with electronic fuel injection may use the same crankshaft position sensor signal to trigger the fuel injectors. So, if there’s no spark and no injector events, the problem is likely in the crank position sensor. If the engine uses a camshaft position sensor to trigger fuel injection events, this will not be the case. No spark in only one cylinder or in two cylinders that share a coil could indicate a coil has probably failed. However it is still possible that the coil driver in the DIS module went bad. Plug Lead Check

If the system uses leads to carry voltage to the spark plugs check their condition by removing the wires from the spark plugs and measuring resistance through the plug wires. Depending on the brand of wires these could range from a few hundred ohms a maximum of 8,000 ohms of resistance per foot for the plug wires. On a coil over plug setup, there will often be a rubber boot that may have a resistor built in. This boot is connected to the insulator cone of the secondary side of the coil. Remove the boot and the resistance and compare it to the appropriate specification for that make and model of car. Don’t forget that a worn or fouled spark plug will act just like a weak or bad coil. Anytime you find an ignition problem that is isolated to a single cylinder, be sure to remove and inspect the spark plug to rule out those possibilities. Coil Checks

DIS coils are tested in the same fashion as any other ignition coils. First, isolate the coil pack by disconnecting all the wires. Use an ohmmeter set to the low range. Connect the ohmmeter leads across the coil’s primary terminals, and compare the primary resistance reading to specifications (typically less than 2 ohms). Then connect the ohmmeter leads across the coils’ secondary terminals and compare the secondary resistance reading to specifications (typically 6,000-30,000 ohms). The difference in the readings of the primary and secondary are due to the number of windings each has. If readings are outside the specified range, the coil is defective and needs to be replaced. Ignition Module & Sensor Checks

A simple and inexpensive test that will show if a DIS module is receiving a crank position signal is to connect a halogen headlamp to the terminals that mate the DIS module to the coils. A headlamp is used since it draws more of a load than a simple test lamp. If the headlamp flashes when the engine is cranked, the DIS module and crankshaft position sensor circuit are functioning. Therefore, the problem is in the coils. If the headlamp doesn’t flash, or there’s no voltage to the module or coil pack when the engine is cranked, the problem is most likely in the crankshaft sensor circuit. On newer vehicles, a bad crank position sensor will usually set a fault code, so use a scan tool to check for a code. Or, check the crank sensor itself. Magnetic crank sensors can be tested

by unplugging the electrical connector and checking resistance between the appropriate terminals. If resistance is not within spec, the sensor is bad and will need to be replaced. Magnetic crank position sensors produce an alternating current (AC) when the engine is cranked so a voltage output check is another test that can be performed. These sensors are typicly a coil of wire around an iron core. With the sensor connected, read the output voltage across the appropriate module terminals while cranking the engine. If you see at least 20 mV on the AC scale, the sensor is good, meaning the fault is probably in the module. If the output voltage is low, remove the sensor and inspect the end of it for rust or debris (magnetic sensors will attract iron and steel particles). Clean the sensor, reinstall it and test again. Make sure it has the proper air gap (if adjustable) because the spacing between the end of the sensor and the reluctor wheel or notches in the crankshaft will affect the sensor’s output voltage. If the air gap is correct and output is still low, replace the sensor. Hall effect sensors typically have three terminals; one is for power, one for ground and one for the output signal. These are usually a semiconductor device that detects disturbances in a magnetic field. The sensor must have voltage supply and ground to produce a signal, so check these terminals first with a voltmeter. Sensor output can be checked by unplugging the DIS module and cranking the engine to see if the sensor produces a voltage signal. The voltmeter needle should jump each time a shutter blade es through the Hall effect switch. If observed on an oscilloscope, you should see a square waveform. The absence of a signal would tell you the sensor has failed. If the engine starts and runs but does not run well, the control module may be r eceiving bad information from one or more of the engine’s sensors. A MAP/MAF sensor with a low output voltage or a coolant sensor that reads cold all the time will allow more spark advance than normal. This, in turn, may cause knock problems when the engine is under load. A faulty knock sensor, if the system incorporates one, can also cause this problem. A faulty coolant sensor can also cause increased emissions and poor fuel economy by causing the engine to run too rich. High MAP/MAF output voltage can have the opposite effect and cause the spark control system to retard timing more than normal. Retarded timing will reduce performance and fuel economy.