Behind the green wire

[Aurel]

Thanks Andrew, I will look into that.
At least I realized all my coils are good. I was only measuring a low secondary resistance because the multimeter battery was low, duh…

[Aurel]

Originally Posted by PeteKz

CDI box failures at this age are common.

I am using newer MSD streetfire CDI boxes (and I have a spare one).

[Aurel]

I just checked the AC voltage at the green wire while cranking, and it is 0.6V AC, so it confirms the dizzy coil is good. The plot thickens…I will get the little oscilloscope to check the signal shape.

[AndrewCologne]

Next steps:
1. Check inductive signal (as shown above) at pin 7 of the CDI
2. Check for proper Voltage (12v min) at pin 15 of the CDI
3. With a high voltage probe (x100) measure the signal between the CDI and the CDI-Coil (pin A).
And this waveform should result :
https://nineelevenheaven.wordpress.com/wp-content/uploads/2023/12/bildschirmfoto-2020-11-27-um-00.34.35.jpg
Part of this site:
https://nineelevenheaven.wordpress.com/the-ignition-of-the-911-3-0-and-3-2/

[PeteKz]

Originally Posted by Aurel

I am using newer MSD streetfire CDI boxes (and I have a spare one).

Sorry I missed that in your first post.

[mysocal911]

Originally Posted by PeteKz

Sorry I missed that in your first post.

Good point! Then, most likely the typical distributor signal for the Bosch CDI is not compatible with the "newer MSD".

[Aurel]

Originally Posted by mysocal911

Good point! Then, most likely the typical distributor signal for the Bosch CDI is not compatible with the "newer MSD".

The car has run flawlessly for over 10 years with the MSD CDI box and the Bosch distributor. Something else happened…

[LJ851]

Are you getting spark at the coil wire before it enters the distributor cap ? A worn or defective carbon button could keep the spark from getting to the rotor.

[mysocal911]

Originally Posted by LJ851

Are you getting spark at the coil wire before it enters the distributor cap ? A worn or defective carbon button could keep the spark from getting to the rotor.

Right, he needs to use a scope and check for a signal (~ 350-400V peak) at the coil!

[Aurel]

Originally Posted by LJ851

Are you getting spark at the coil wire before it enters the distributor cap ? A worn or defective carbon button could keep the spark from getting to the rotor.

Yes. And I have also seen spark at the cylinder wires once when I crank, but it does not hold. I see one spark and then no spark.
Btw, my little scope just arrived, so I will be able to check the signal at the green wire, if I find time this WE.

[LJ851]

Originally Posted by Aurel

Yes. And I have also seen spark at the cylinder wires once when I crank, but it does not hold. I see one spark and then no spark.
Btw, my little scope just arrived, so I will be able to check the signal at the green wire, if I find time this WE.

I guess I could have asked my question more specifically. Does the coil output all the sparks it is supposed to but the sparks don’t continue downstream ?

If that is not the case , is the MSD box is getting 12v while you are cranking ?

[Aurel]

Originally Posted by LJ851

I guess I could have asked my question more specifically. Does the coil output all the sparks it is supposed to but the sparks don’t continue downstream ?

If that is not the case , is the MSD box is getting 12v while you are cranking ?

OK, I got busy this morning.
First, the signal at the green wire looks good, see the trace below:

Second, the voltage to the CDI box is 13.1V at rest, which is fine.

But when I crank, the voltage drops to 10.5V, which may be too low for the CDI box?

Also, if I trigger the CDI box with the white wire, I get a spark at the coil, but if I crank, I get no spark at the coil. This also suggests the voltage to the CDI box is too low when cranking.


So maybe a bad ground is causing too much voltage drop when I crank, or maybe the battery itself is bad, but it cranks the engine with no issues. At least, I think the distributor is not the source of the problem.

[mysocal911]

Originally Posted by Aurel

OK, I got busy this morning.
First, the signal at the green wire looks good, see the trace below:

Second, the voltage to the CDI box is 13.1V at rest, which is fine.

But when I crank, the voltage drops to 10.5V, which may be too low for the CDI box?

Also, if I trigger the CDI box with the white wire, I get a spark at the coil, but if I crank, I get no spark at the coil. This also suggests the voltage to the CDI box is too low when cranking.


So maybe a bad ground is causing too much voltage drop when I crank, or maybe the battery itself is bad, but it cranks the engine with no issues. At least, I think the distributor is not the source of the problem.

Or the battery voltage at the starter, where the MSD 12V source MUST be connected, is bad!

[targa72e]

If you are using the white and black wire from the MSD on the SC distributor to try and trigger the MSD you are using wrong connection. The white wire is for point or pertronics type signal. You need to use the purple and green wires labeled for mag pickup as that is the type of output from a SC distributor.

john

[Aurel]

Originally Posted by targa72e

If you are using the white and black wire from the MSD on the SC distributor to try and trigger the MSD you are using wrong connection. The white wire is for point or pertronics type signal. You need to use the purple and green wires labeled for mag pickup as that is the type of output from a SC distributor.

john

No, I only use the white wire for testing purposes. The box is connected to the distributor with the green and purple wires.

[PeteKz]

If you have enough battery charge to crank the engine, that's more than enough to power the CDI. A voltage drop to 10.5V during cranking is normal. The battery ain't your problem.

However, if you connect another battery in parallel, like when you jump start a car, and it still doesn't fire, then you can be certain the battery is not the problem.

[mysocal911]

Originally Posted by PeteKz

If you have enough battery charge to crank the engine, that's more than enough to power the CDI. A voltage drop to 10.5V during cranking is normal. The battery ain't your problem.

However, if you connect another battery in parallel, like when you jump start a car, and it still doesn't fire, then you can be certain the battery is not the problem.

Not necessarily! There's a reason why the MSD needs to be connected to the B+ at the starter, a good 12 volts.
Resolve the issue by connecting a jumper battery at the starter.

[Aurel]

Originally Posted by mysocal911

Not necessarily! There's a reason why the MSD needs to be connected to the B+ at the starter, a good 12 volts.
Resolve the issue by connecting a jumper battery at the starter.

I have always used the MSD by getting its power at the 3 fuse box in the engine compartment, without issues for over 10 years. Also, MSD datasheet says it will fire with as little as 7V, so I am not sure that will resolve the issue (but will try).

[mysocal911]

Originally Posted by Aurel

I have always used the MSD by getting its power at the 3 fuse box in the engine compartment, without issues for over 10 years. Also, MSD datasheet says it will fire with as little as 7V, so I am not sure that will resolve the issue (but will try).

Sounds like it's time to install a good Bosch OEM CDI, and junk the MSD!
Marginal if any benefit from a MSD, found this on Pelican Parts;

http://forums.pelicanparts.com/porsche-911-technical-forum/676385-msd-ignition-exposed.html

[Aurel]

Now is time for an update. I finally figured out what was preventing sparks, and now have ignition again.

I checked again the voltage at the CDI box, and even without cranking, it was only 6.5-7V.
The voltage at the front fuses was a solid 13V, and the voltage at the engine fuses was only 6.5-7V.
I checked my ground connection but that was fine.
I opened the 14 pin connector, and sure enough there was a connection holding by a thread that broke immediately. I resoldered it and voila, my ignition is back.