I think one is power to charge the coil, there's a ground, and the third goes to the ignition switch.

 

Pin 1 is the signal from the DME.
Pin 2 is ground.
Pin 3 is battery voltage from the ignition switch.

 

The coil needs 3 wires because there is not a shared ground in the configuration that BMW uses.

Coils have 3 connections.

If the diagram is labeled correctly.

#1 is the DME ground side primary trigger
#2 is the primary 12 Volt feed
#3 is the secondary ground wire.

This is why the ground wire is very important to be properly connected with this configuration.

Also the DME can monitor the primary ground current and behavior to pretty accurately determine if there is a coil related problem. But there are some secondary coil problems the DME may not be able to accurately report.

 

Power, ground, and trigger. The coil is powered and grounded with two wires, and there is a trigger on the third that invokes the spark.

 

Thank you for the info, everyone, but there is different data from different members.

From Strickland, I read it as: 12v, battery ground, ignition trigger. My understand is that these coils on plug are just a fancier form of the classic single coil on the old car: primary coil : one end connected to switched 12v and the other end to breaker point or ignition AMP. Secondary coil: one end connected to the same switched 12v internally and the other end to plug. There is no GROUND connected to the old coil. Why the new coil on plug need to be connected to Ground?

Jfoj said:
1. primary ground (means primary switched to ground by DME to charge the coil and open to fire the spark?)
2. 12v (switched on when key is on)
3. secondary ground.
This configuration makes more sense than Strickland's, but why secondary end needs to be connected to ground instead of the same 12V as primary, as in those old day coils?

 

someone posted a schematic for the coil wiring, and this is what I see:

pin 3 connected to 12v via a relay (turn on by ignition key I think)
pin 1 connected to a transistor for switching the primary coil current to create spark, as the old breaker point
pin 2 connected the secondary coil one end to ground. On the old single coil system, this end was connected to 12v internally of the coil. I wonder why bmw needed to connect it to ground instead of the internal 12v as everyone had done in the past 100 years, which would save bmw one pin and one wire for each coil, which mean lots of money saving. I think I have an answer why it was designed to connect to ground expensively instead of to 12v, but I want to hear from others.

sapote

 

I never heard of sharing 12V and secondary high voltage in the old distributor ignition system. Can you imagine the damage to the electrical system from high voltage generated on the secondary coil?
They may share the ground. Secondary coil should not be powered what ever the system was. It operates on the principle of inductance.

I want to check the coil with DVM. How do I know which is pin 1,2,3? The numbers on the coil are not 1,2,3.

Any two that have continuity are for primary coils. BMW online wiring diagram doesn't show pin assignments.

 

I want to check the coil with DVM. How do I know which is pin 1,2,3? The numbers on the coil are not 1,2,3.

 

***8220;I never heard of sharing 12V and secondary high voltage in the old distributor ignition system. Can you imagine the damage to the electrical system from high voltage generated on the secondary coil?
They may shared the ground. Secondary coil should not be powered what ever the system was. It operates on the principle of inductance.***8221;

This sounds like music to my ears ***61514; Sorry to say this. But, I bet after taking a few minutes to think you will agree that there is no ground terminal on the old 2-terminal round coil (+ and -).
So, If you don***8217;t think secondary end connected to 12v (the + terminal) then it must be connected to the other terminal which is the breaker point. Was it? Please don***8217;t say it was connected to the metal body because it***8217;s not. The old round coil still works fine laying on a piece of wood, not grounded.

 

Old style coils were technically grounded by the capacitor.

See the schematic in this link, I think this is be best representation.

https://www.google.com/search?q=ign...%2Fcirc_ic.gif;http%3A%2F%2Fmgaguru.com%2Fmgtech%2Fignition%2Fig104.htm;275;185

The issue is the newer engines have much larger spark gaps and much Leaner mixtures that requires more current/Voltage for the current day secondary ignition systems. This along with all the computers on board these days, you would not want the high oscillating high Voltage and current pumping through the vehicle electrical system.

So modern day ignition coils are primary and secondary sections are isolated.

 

Pins 2 & 3 have BRN and GRN wires (respectively). BRN is ground, GRN is system voltage. Pin 1 is the trigger for each coil, and is BLK/xxx, where xxx is the color of the wire stripe, which is different for each coil.

If you would break down and buy a Bentley Manual, you could fix your car. Well, you could know the answer to questions like this. You would also know which pins to read across with your meter, and what the outcome of the testing should be.

 

Jfoj,

From the link you posted about the old coil car, the combination of capacitor (condenser) and break point is equivalent to the E46's transistor switch (inside the DME box) or trigger signal. connecting the secondary to 12v or to ground doesn't affect how it operates or the rest electrical system. Then why E46 connected it to ground instead of the 12v? The reason: the old cars have it connected to 12v because it's convenience and cheap (no extra 3rd terminal and wire). If the coil is damaged and shorted secondary ground, the car is death. However on coil on plug system, the designer wanted the car to continue running even one or two coils are bad. To do this the secondary coil needs to be isolated from primary. If not, one damaged coil could take out the 12v common coil supply and the car will die.

 

I had checked all of my working coils, and all of them had the same measurement: my DVM ohm meter read infinite resistance (open circuit) between the spark plug terminal (boot removed) and any of the 3 pins. This means the secondary coil turns have a mysterious connection inside the coil, because I don't believe it has infinite resistance. Anyone have any ideas why?

 

I double checked and for some reason I had 2 of the numbers mixed up that I posted.

1. Primary Ground Control connected to DME
2. Secondary coil ground path
3. 12 Volt via relay and 30 Amp fuse.

You should, based on the diagram, have resistance between Pin #1 & #2 of a fairly low resistance, probably 10 Ohms or less.

You should, based on the diagram, have resistance between Pin #2 the spark plug terminal post. This resistance will be pretty high, likely somewhere in the 1-2k Ohm range. You may have a hard time making a proper connection inside the coil for the spark plug end, may require a paper clip or scrap piece of wire.

The more common failure on coils is an open primary winding or a cracked coil casing allowing flash over or arching.

The DME is actually pretty good at detecting a bad coil under some conditions and flagging a specific problem for a coil or for spark duration.

Not sure I would waste a lot of time and effort on measuring the coil resistance, you will not likely find an obvious problem checking the coils this way.

Given the DME can effectively measure or monitor the primary winding current draw, the DME will likely limit or shut a defective coil down that might put the 30 Amp fuse in jeopardy.

 

"you can not use resistance measurement to check circuit because a high voltage diode (to suppress pre-spark) is in between them. "

Dslboomer, many thanks for this information. Is there any article you can point me to about this? This makes lot of sense in explaining why my DVM read. This must be a special diode because the DVM's diode check didn't verify it. It's must be something acts like a high voltage Zener to block the pre-spark not very high voltage from discharging too soon.

Sapote

 

Here is one of reference. You can google "high voltage diode for ignition coil"

http://www.vishay.com/docs/49337/49337.pdf

 

Thanks again, Dslboomer for the link. So the diode works as a high voltage rectifier, not a Zener as I first thought. The diode is to prevent a spark during the primary winding is being charged (yes, the secondary winding generates quite high voltage during the breaker point closed, and of course it also generates much higher voltage during the moment the point is open, if I can borrow the old point to explain this). A spark during this time is bad because the fuel mixture is being pulled in and compressed, but too early for optimum ignition time. I don't think the old single coils have these diodes inside.

Sapote