ignition coil failures on 3.0 engines

[groovydude]

I've just had my second coil failure on my SC, each with very few miles on the coil. The first was a new silver Bosch unit, which I assumed at the time was the culprit. Now the second failure, yesterday, was a Parts Klassic unit. I got an interesting response from Parts Klassic about this:

Bosch has discontinued the correct ignition rotor for the 911SC and 930 models as February 2016. Bosch is now suppling the 930.602.902.01 rotor as a replacement rotor for these cars. The issue is the replacement rotor is the one used in the 1984-89 Porsche 3.2L Carrera that no longer uses a CDI ignition system. The rotor used in that system only has a 1000 Ohm resistor installed in it.

Another problem with the 911SC and 930 models is that the current replacement ignition wires supplied by Beru have a fitment issue. The 1K resistor on the connector that is supposed to plug into the cap has a nominal size of 8 mm. This is now too big to fit into the distributor cap. This results in the wire not contacting the brass connector in the cap, and the spark energy must jump from the cap to the plug wire. This in turn results in energy ringing back through the transformer and into the CDI box. Both issues if not corrected can and will result in transformer and / or CDI box failure.

To avoid ignition system failures:
1. Maintain factory correct resistance values, including correct location of resistance
2. Check for correct contact of secondary wiring to prevent spark jumps within the system
3. Ensure replacement components comply with factory specifications and fit for secondary resistance
4. When building/servicing twin plug vehicles check for correct resistance in the secondary system
5. When using non-resistance rotors, make sure to add resistance to the coil wire

I use Clewett wires so I wasn't worried about the second part, but the rotor resistance comment did ring a bell, as I'm pretty sure I replaced my rotor around 2016. I inquired about #5, and here was their response:

If you or your mechanic check the resistance on your rotor, you may find that it’s 0 to 1K ohms even though the factory part was about 5K ohms. Because of this, we sell a 5K resistor that attaches to the cap-coil wire. We also offer a cap-coil wire with the 5K resistor already installed. Here are the links:

https://www.************.com/p-3363-ignition-wire-connector-5k-resistance.aspx

https://www.************.com/p-3591-coil-wire-with-5k-connector.aspx

I measured my resistance and it is indeed 1k. I've ordered the 5K wire of course.

[Zuffenwerker]

Yeah this is all true, it can kill the coil and the cdi box. Ideally you want to use a shielded set of solid core spark plug wires like came on the cars originally. Using the wrong wires can also result in failure

Daniel

[mysocal911]

Your vendor relies are total hyperbole!

So where's the original black Bosch coil, that coil rarely if ever fails?

[groovydude]

I can verify that the newer rotor I have has 1 ohm of resistance. If someone with an older rotor could measure theirs we could verify that what the vendor is saying is true.

For now, I’ll believe them. I’ve got two burned out coils since replacing the rotor which supports their hypothesis.

[Jonny H]

The SC rotor is 5k, each lead is about 1k and there is 3k of resistors in the plug cap making a total of about 9k ohms for the secondary coil circuit.

The PK reply is all 'good practice' but is not the primary reason why the Bosch black coils last a long time and the others don't.

The Bosch black coil uses a oil/tar like filler material that is impregnated into the winding to insulate from flash over between coil windings and/or the case which is grounded. The oil/tar melts as it heats so the insulation is kind of self-healing to some extent. E.g. if there is an air pocket, it will eventually be filled with oil/tar. That is also the reason why the coils are mounted upside down, since there is a gap at the base (top) where a void of insulation is of no concern as the windings are closer to the nose end of the coil.

The Bosch silver and other coils are filled with an epoxy compound which sets hard. For this reason, before the epoxy sets, the coil must be held under vacuum so that the epoxy can penetrate the windings and all air bubbles can be removed. This is very difficult in practice due to epoxy setting time, temperature, thickness, pot life and a host of other factors. For example, if the vacuum is too great, the epoxy will boil which introduces bubbles again. I do not believe any vendor (including Bosch) has managed to get repeatability in this process, mainly because epoxy resin is not the correct material for this function. Back in the day both Cosworth and Lucas had similar problems. Cosworth actually used a clear resin as a insulator which could be optically inspected for bubbles!

We have cut open failed Porsche silver and aftermarket CDI coils and air pockets and bubbles are clearly visible.

For our own coil development we are working with a transformer winding specialist to select a material that has the same self-healing effect as the original tar. Even so, the filling/vacuum 'recipe' and process has to be repeatable every time to get a long lasting product. This is what is taking so long to get our coils to market.

[ahh911]

I replaced mine 20k miles ago when I bought the car. I was concerned and checked today not having heard that the 911sc required 5k resistance. Checked, my new one is 1.2k Ohm, and the one I took off with date code 1988 is 1.2k Ohm. Also, when measuring, the carbon buildup on the air side was far beyond 20k Ohm, probably in the 100k range, so what is 4k relative to that?

If I've not understood something, please help me out.

Phil

[Dave Kost]

If you look at the back/bottom of the Bosch rotor you will find the resistance of the rotor.

R1 in a circle is 1k resistance
R5 in a circle is 5K resistance

Ill send some pictures.

[Dave Kost]

Sorry I don't know how to make the pictures smaller.

These are from spares I collect over the years.








The first two pictures are of the 1K rotors
The third picture is the bottom of the 5K rotor- see the R5 in the circle.
The forth picture is the top of the 5K rotor
The fifth picture confirms the resistance of 5K
The sixth picture is the box of one of the 5K rotors came,The box on the left is the 1K box.

[mysocal911]

And for those without access to the original Bosch black CDI coil, there's this for reliability and not having to check for rotor resistance, & etc.;

https://www.pelicanparts.com/search/?q=MSD+8222+coil&redir=yes&host=search

[Jonny H]

Originally Posted by mysocal911

And for those without access to the original Bosch black CDI coil, there's this for reliability and not having to check for rotor resistance, & etc.;

https://www.pelicanparts.com/search/?q=MSD+8222+coil&redir=yes&host=search

Since you are always asking for 'proof' and 'data' on every other thread, where is your evidence to show that this coil is reliable?

I am not saying that coil is good or bad, I just happen to know where and how it is made and it's the same as all the others!

[Jonny H]

Originally Posted by Dave Kost

Sorry I don't know how to make the pictures smaller.

These are from spares I collect over the years.

The first two pictures are of the 1K rotors
The third picture is the bottom of the 5K rotor- see the R5 in the circle.
The forth picture is the top of the 5K rotor
The fifth picture confirms the resistance of 5K
The sixth picture is the box of one of the 5K rotors came,The box on the left is the 1K box.

Very useful. I didn't know there was a 5k version of the non-limiter rotor. Do you know what car it is from?

[mysocal911]

Originally Posted by Jonny H

Since you are always asking for 'proof' and 'data' on every other thread, where is your evidence to show that this coil is reliable?

I am not saying that coil is good or bad, I just happen to know where and how it is made and it's the same as all the others!

Do a Pelican & Rennlist search over the last 18+ years!

Too much B.S. here about secondary resistance values & its relationship to the coil failures!!!

[husky2033]

What are symptoms of coil failing? I have a intermittent problem I've been chasing. The car runs great then suddenly stumbles for a few seconds & then is fine. Car is a 1983cs

thanks

[mysocal911]

Originally Posted by husky2033

What are symptoms of coil failing? I have a intermittent problem I've been chasing. The car runs great then suddenly stumbles for a few seconds & then is fine. Car is a 1983cs

thanks

That's one.

[husky2033]

Thanks for reply. Is there a way to check coil or do you just change & hope it fixes problem? At this point I don't even know what coil to buy. I have the made in Brazil silver Bosch on it.

Thanks

Jeff

[Tremelune]

If Classic Retrofit can get the coils right, they'll have a market in most air-cooled Porsche motors on the road that haven't already moved to EFI, COP and the like...

Originally Posted by mysocal911

Do a Pelican & Rennlist search over the last 18+ years!

Few parts are made the same way they were 20 years ago. The cost and difficulty of sourcing high-quality wear items in our ancient ignition systems is a big art of the reason I yearn to ditch them for crank triggers and modern coil packs and what not.

I have notes on which Bosch coil serial numbers are the "good" ones, which are black vs silver or blue, which ones are marked "Brazil" or something. They all have the same part number. It's tiresome. When new parts are worse than old parts, I have no strong conviction to stick the system.

[Jonny H]

^ Truth.

[Jonny042]

It seems a bit of a reach to conclude that having 4K less secondary resistance (don't forget the 5Kohms+ in each plug, too?) when using the 1K rotor would cause coil failures?

[mysocal911]

Originally Posted by Jonny042

It seems a bit of a reach to conclude that having 4K less secondary resistance (don't forget the 5Kohms+ in each plug, too?) when using the 1K rotor would cause coil failures?

Yes, it's not that complex!

[mysocal911]

Originally Posted by Tremelune

If Classic Retrofit can get the coils right, they'll have a market in most air-cooled Porsche motors on the road that haven't already moved to EFI, COP and the like...

We're waiting!

Originally Posted by Tremelune

Few parts are made the same way they were 20 years ago. The cost and difficulty of sourcing high-quality wear items in our ancient ignition systems is a big art of the reason I yearn to ditch them for crank triggers and modern coil packs and what not.

I have notes on which Bosch coil serial numbers are the "good" ones, which are black vs silver or blue, which ones are marked "Brazil" or something. They all have the same part number. It's tiresome. When new parts are worse than old parts, I have no strong conviction to stick the system.

My post references this recent (>10 yrs) part which is very reliable;

https://www.pelicanparts.com/search/?q=MSD+8222+coil&redir=yes&host=search