The ignition in most E30 engines (M10, M20 and M40) is a standard coil-based system whereby a large electric voltage is generated and sent to each cylinder in turn by a mechanical distributor. In the more modern M42 engine, this system is replaced by coil packs. Inside each cylinder is a spark plug which, when fired, creates a powerful spark to ignite the fuel, and therefore generate power.
The timing of the ignition system is governed either by the rotation of a mechanical distributor, or by a central ECU. Timing governs when the spark fires for each cylinder, relative to the position of the piston in the cylinder and the cam shaft above it. Because the idea of combustion is to push a piston down, the spark needs to ignite the fuel before the piston gets to the top of its cylinder (BTDC), so that there is enough time to ignite all the fuel efficiently. As engine speed increases, the spark needs to fire earlier, known as advancing the spark, while slow-running or idling engines need the spark later, which is called retarding.
Engines with Motronic management use the ECU to control the timing of the spark to achieve the best performance, based on the parameters inside the ECU map.
On the inside wing of the engine bay sits a big silver or black cylinder known as the Coil. This enormous transformer receives electricity directly from the battery/alternator via an unfused link, and is capable of generating an enormous voltage and should therefore be treated with a certain degree of respect.
Underneath its protective rubber cap, the Coil has three connections; two small Primary terminals, and a large Secondary terminal known as the King Post.
The first Primary terminal, +, is numbered 15 and gets its power directly from the ignition switch. This green wire goes from the switch, through a plug and socket clipped half way down the steering column, across behind the dash to a two pin socket above the glovebox, fitted with a plug with a loop, (where the immobiliser should have been connected), up through the duct behind the glovebox into the fusebox. It passes straight through the fusebox with no connections, out to the 20 pin C101 engine loom plug beside the fusebox, then into the engine loom and to a coil. Inside the loom there is a single branch to the ECU.
The second Primary terminal, - , is numbered 1 and goes to the main ECU inside the engine loom. There is a single branch to the diagnostic socket.
If you want to test your coil, follow these steps:
- Disconnect all cables from the Coil
- Using a multimeter, measure the resistance between the two Primary terminals. You should get a reading of 0.5 Ohms.
- Now measure the resistance between terminal 15 and the King Post. You should get a reading of 5000 Ohms.
- To test it is receiving power, ensure all cables are correctly attached, and then use a bulb to ensure you are getting 12V between earth and terminal 15.
The King Post connects to the distributor via one large HT lead. To test the condition of this lead, measure its resistance. It should read 2000 Ohms.
The distributor is a rotating mechanism that receives incoming voltage from the coil and sends it to each spark plug in turn. The distributor turned a Rotor Arm like the hand of a clock, which in turn touched the terminals spread around the edge of the Distributor cap. As the Rotor made contact with the terminal, voltage was sent from the Coil to the Spark Plug.
On earlier Jetronic and M10 engines, the distributor was a mechanised gear that rotates in turn with the engine, mounted to the engine block itself. It was crucial for this mechanism to be lined up correctly with the rest of the engine, and the alignment of this system is known as the Timing.
With Motronic, the mechanism was removed and the Rotor was attached directly to the end of the cam shaft, reducing the amount of components used and the need to adjust the timing. Timing in Motronic engines is controlled completely by the ECU.
The Distributor Cap is a plastic form, usually coloured orange, with a number of thick cables (HT leads) protruding from it. The number of HT leads is equal to the number of cylinders in your engine plus one; the incoming lead from the coil.
The distributor cap collects these leads and arranges them in a circle to match the firing order of the engine. On the inside face of the distributor cap are solid metal terminals; these are designed to be stroked by the Rotor Arm as it makes its turn inside the distributor cap. In the centre a carbon contact delivers power through the distributor cap to the Rotor Arm.
Over time, this contact and the terminals wear down, reducing the voltage that passes through and weakening the spark to the edge. Eventually these terminals will fail, causing misfires and eventually stopping the engine from running.
A cracked distributor cap will also allow moisture into the system. As you should know by now, water and electricity shouldn't mix, and a cracked distributor cap must be replaced.
The Rotor Arm is the heart of the distributor. Just like the hand of a clock, it rotates around its central point, bridging the terminals as it moves. Its tip is almost an inch long, so the amount of time the rotor is in contact with any given terminal is relatively long; this allows the ECU to deliver either an advanced or retarded spark as it tracks the engine speed.
Over time, this tip will wear, to the point where it will fail to contact the terminals.
On early (pre-87) vehicles there is an impulse generator in the distributor, which is often the cause of ignition fault.
To test this unit, you need to measure its resistance. Remove the small 2 pin plug from the control unit on the firewall (Siemens unit) and check that the resistance between the 2 pins is around 1100 ohms. If it is not, check the wire between the control unit and the distributor is intact; this is most easily done by removing the distributor. Remove the retainer for the lead and remove the lead and check for continuity in both conductors. Then check the resistance across the two pins directly in the distributor. If this check at the distributor indicates a reading outside of +/- 1100 ohms, you will need to replace the impulse generator unit. If you have a bosch control unit with a single flat plug connecting under it, you check this resistance between pins 5 and 6.
If this checks ok, measure the air gap between the trigger wheel tabs (star-shaped wheel under the rotor arm) and the impulse generator tabs. The gaps should measure between 12 and 28 thou.
Ignition Control Module
On early (pre-87) models equipped with Jetronic engine management, an Ignition Control Module was fitted to control the ignition. Since this unit is an integrated unit, it requires dedicated testing equipment to check; however, they very rarely fail and therefore should not be considered suspect in the event of ignition failure.
The only checks you can do to the control unit are to ensure that it is getting a supply. On the Siemens unit, remove the large round 8 pin plug and check for voltage (ign on) between pins 3(+) and 6(-). Pins should be numbered inside the plug. On the Bosch unit, check between pins 4(+) and 2(-).
If you dont get voltage, you need to check your wiring. The negative pin ( 6/Siemens or 2/Bosch) should show continuity to ground and the positive pin (3/Siemens or 4/Bosch) should show continuity to the 15 (green wire) on the coil.
The High Tension leads deliver the voltage from the distributor to each spark plug. Because the distributor is placed at the front of the engine, the length of each HT lead is specific to the cylinder it feeds. Each HT lead has a specific plug on the distributor cap to control the firing order of the engine; mix up this order and the engine will either run badly or not at all.
While HT leads do wear out over time, after 20 years most E30 HT leads are still in perfectly usable condition. There is no advantage in replacing them with aftermarket components.
To test the condition of your HT leads, you can use a multimeter to measure their resistance. You should also physically inspect them for cracks and splits. To perform a sight test, place your car in a dark environment (a garage, or outside at night) with the bonnet open. Have an assistant crank the engine for you, and watch for small blue sparks.
If your HT leads are broken or cracked, replace them only with BMW originals.
The number of spark plugs directly corresponds to the number of cylinders. They are considered a standard service component, and should be replaced regularly.
Although Bosch plugs were fitted to the E30 when new, the recommended replacements are NGK BPR6ES
In their simplest form, coil packs are groups of individual coils, with one coil per cylinder. They replace the need for the Coil and Distributor in more modern injection engines. This combined pack allowed the ECU to directly control the timing of each spark per cylinder. The only E30 engine fitted with this arrangement was the M42. Later BMW engines such as the M50 and M52 used a coil on plug arrangement.
A Coil Pack consists of two coils of wire, the primary (or Low voltage coil) and the secondary coil (or High voltage HV coil). The primary coil is energised through the 12V input voltage from the battery/alternator, which causes a controlled current to flow in the primary coil. Once full current is reached the 12V charging circuit is opened which allows the coil to discharge into a capacitor, basically forming an LCR tuned circuit. This discharge creates an oscillating magnetic field, which then induces a voltage into the secondary coil of up to 20kV (20,000V). It is this secondary voltage that is then sent to the spark plug.
To check if your coil pack is in proper working order, measure the resistance across the two Primary terminals, just as you would on a normal coil. Compare this reading on all of your coil packs, and if any one pack shows an abnormal reading, it is probably faulty.
On the earliest systems such as the M10, ignition was primarily mechanically governed. This meant that when all components were installed they were designed to line up specifically, so that they would always run sychronised. A very primitive ECU governed fuelling with regard to cold starts, but very little was managed electronically regarding the ignition.
The first fuel injection system on the E30 was L-Jetronic, which used a proper ECU. However, this system still managed the fuelling only, and it wasn't until the Motronic upgrade of 1987 that fuelling and ignition was incorporated into one ECU. The early Motrnic 1.1 used two sensors mounted in the gearbox, to establish the position and speed of the engine and adjust the firing of the sparks accordingly. This solution was rather inelegant, and a year later Motronic 1.3 replaced these two sensors with one Crank Position Sensor (CPS), located on the crank pulley at the front of the engine.
By reading pulses from the CPS, Motronic activates ignition firing via a connection from the ECU to the coil, advancing or retarding the spark as needed. By reading a sensor on the last HT Lead, known as a CID, it is also able to fire the appropriate injector to improve fuel economy.
The easiest way to test if spark is present is to remove one of the spark plugs and rest it on the rocker cover of the engine. Make sure it is still connected to its HT lead. Then crank the engine and watch for a spark. You can also do this with a spare spark plug connected to an existing HT lead.
If no spark is present:
- First confirm the presence of Fuel
- If no fuel is present either, the immediate culprit is the Crank Position Sensor
If fuel is present and the CPS measures the appropriate resistance, follow these steps:
- Test the coil and replace if necessary
- Check your HT leads are firmly attached, especially the King Post lead to the distributor
- Connect a spark plug to the King Post HT Lead from the coil, and rest it on the rocker cover. Crank the engine and look for spark
- If spark is present, your distributor is most likely at fault
Cross-threaded Spark Plug
Because spark plugs are replaced regularly, it only takes one ham-fisted spanner monkey to force a spark plug in the wrong way, damaging the thread in the head. With a stripped or cross-threaded head, the seal between the plug and the head is not perfect, allowing intake and exhaust gas to blow out. Aside from the loss of compression, and therefore power, this can cause even worse damage to the thread in the long run.
Repair kits can be bought that contain a special tap and a set of threaded sleeves. The tap is used to bore out the existing thread, and then the steel threaded sleeve (NOT a helicoil*) is screwed in, which bonds to the metal on one side, and gives you a fresh strong thread on the other for your spark plug.
The trick is to disconnect the coil and injectors, wind the engine by hand so that the relevant piston is at BDC, and fill the chamber with shaving foam. Then, manually use the supplied tap, liberally coated with grease, going in a quarter-turn at a time. After each turn, remove the tap completely, wipe off the grease and swarf, and go in again. If you take your time, you'll avoid having to remove the head. When you're done, thread your insert into the hole, and just spin the engine to blow all the foam (and any swarf you didn't catch with the grease) out of the piston. Then, just fit your spark plug. Piece of cake.
- A helicoil is not considered strong enough for this application. Also, it has a number of failings over solid-sleeve inserts; firstly, fitting the helicoil involves snapping off a shard of metal at the end which, if dropped into the chamber, is very difficult to fish out and can do irreperable engine damage. Secondly, it is not robust enough to survive another spark plug change, and will require regular replacement, exacerbating the first issue regarding that shard.
Coil On Plug
For M42 owners, a Coil On Plug conversion can yield some subtle increases in ignition performance. By moving the coil packs to sit on top of the spark plugs, the HT leads are removed which reduces voltage loss, keeping the spark pure and strong. Simple to fit and cheap to purchase, their only downside is a slight increase in complexity when changing spark plugs.
Learn more about installing a Coil On Plug system.